The last post explored compile time hashing a bit and at the end showed a way to use compile time code to assist a string to enum function, by having a minimal perfect hash that was guaranteed at compile time not to have any hash collisions.

That was pretty interesting to me, and made me wonder what else was possible with compile time (or compile time assisted) data structures. This would be mainly for static data structures that were read only. We could get creative and allow for deletes, inserts and data edits, but we’ll probably come back to that in a future post.

In pursuing other compile time data structures, there is one big question that would be easy to make assumptions about: Is custom code for compile time data structure lookup functionality faster than data?

In other words, could we hard code a hash table key to value lookup that was more performant than a general case hash table which was meant to be used by any data?

The answer to this is “usually it can be” IMO, since we could do lots of things specific to the data, like finding cheaper / better hash functions for the specific data input we were expecting. Since we aren’t doing inserts or deletes, or data modification, we could also allocate just the right amount of memory, in one shot, which would reduce memory waste and memory fragmentation.

But lets do some experiments and see. Let’s start off by comparing how a switch statement performs compared to some simple array types.

Testing Details

I ran these tests in x86/x64 debug/release in visual studio 2015.

I made 5000 random uint32’s and stored them in these containers:

• std::vector
• std::array
• C style array
• dynamic memory
• Switch statement function
• Switch statement function with __assume(0) in default case

The __assume(0) in the switch default case is a hint to the optimizer to not do the usual bounds checking needed for handling a value not present in the switch statement. It’s microsoft specific, but there are equivelants in other compilers. Note that if you switch a value that would usually be handled by the default case, you would then have undefined behavior. It’s a speed up, but comes at a price. You can read about it here: msdn: __assume. This test code never passes values that would trigger the default case, so is arguably a good candidate for __assume, if you believe there are any good usages for __assume.

For a neat read on some details of undefined behavior check this out: Undefined behavior can result in time travel (among other things, but time travel is the funkiest)

I did the following tests.

• Sequential Sum: Add up the values at the indices 0-4999, in order. For the switch function, it was called with indices 0-4999 in order
• Shuffle Sum: Same as sequential sum, but in a shuffled order.
• Sparse Sum: Same as shuffle sum, but only doing the first 1000 indices. An alternate switch function was used which only contained values for those first 1000 indices, simulating the ability for us to strip out values never actually needed, to see if that has an impact on performance.

Timing was measured using std::chrono::high_resolution_clock, timing each test done 1,000 times in a row to make timing differences more apparent. I did this 50 times for each test to get an average and a standard deviation for those 50 samples.

The source code for the tests is at:
Github: Atrix256/RandomCode/ArrayAccessSpeeds

Results

The results are below. Times are in milliseconds. The number in parentheses is the standard deviation, also in milliseconds.

Sequential Sum: Sum 5,000 numbers 1,000 times, in order.

Shuffle Sum: Sum 5,000 numbers 1,000 times, in random order.

Sparse Sum: Sum 1,000 of the 5,000 numbers 1,000 times, in random order.

Observations

It’s interesting to see that std::array is 10 times slower than a c array or dynamic memory, in debug win32, and that std::vector is 20 times slower. In debug x64 std::array is only about 5 times slower, and std::vector is only a little bit slower than std::array. In release, std::vector and std::array are more in line with the cost of a c array, or dynamic memory.

It’s expected that std::array and std::vector are slower in debug because they do things like check indices for being out of bounds. They also have a lot more code associated with them which may not “optimize away” in debug, like it does in release (including inlining functions). It’s nice to see that they become basically free abstractions in release though. It gives us more confidence that they can be used in performance critical areas without too much extra worry, as long as we are ok with the debug slowdowns and want or need the debug functionality the types give us.

It’s also interesting to see that adding the __assume(0) to the default case in the switch statements actually improves performance a measurable amount in most tests.

But, onto the sad news…

The switch function IS NOT faster than the simple array access, and in fact is quite a bit slower!

Why is it so much slower?

One reason is because of the function call overhead associated with each lookup. When I marked the function as inline, the compiler refused to inline it. When i marked it as __forceinline to force it to inline, the compiler took A LONG TIME in the “generating code” section. I left it for 10 minutes and it still wasn’t done so killed it since it is unacceptably long. I tried a bit to get apples to apples comparisons by putting the test code into the function, but the optimizer ended up realizing it could run the function once and multiply it’s result by the number of test repeats I wanted to do. Trying to fool the optimizer meant doing things that weren’t quite an apples to apples comparison anymore, and made the results get slower. So, I’m leaving it at this: In the release assembly, you can see the function call to the switch function, where you don’t have one with eg. dynamic memory and function calls are not free, so no, the function call is not faster than not having a function call!

The second reason is how switch statements work compared to how arrays work. The switch statement first does a comparison to see if the value is within range of the maximum valued case (in other words, it does a range check, like what std::array and std::vector do in debug! It does this in debug and release both.), and then it does a jmp to a location of code where it then does a return with a constant value.

An array on the other hand just calculates the offset of a memory value based on the index passed in and the object size, and then reads that memory address.

Arrays have more direct access to the values than switch statements do.

It’s true that the switch statements could be optimized to behave more like arrays, and the whole function could go away in lieu of a memory lookup, but the optimizer doesn’t make that optimization here.

Here’s a good read on details of how switch statements are actually implemented in msvc under various conditions:
Something You May Not Know About the Switch Statement in C/C++

On the memory usage front, the simplest array (not worrying about things like geometric dynamic growth policies or memory padding) takes 20,000 bytes to store 5000 uint32’s.

In win32 (didn’t try x64 but I’m sure it’s similar), gutting the switch functions to only have a single case made a huge difference in executable size which shows us that switch statements are bad for memory efficiency too:
Debug went from 340 KB (348,160 bytes) to 244 KB (249,856 bytes).
Release went from 191 KB (196,096 bytes) to 133 KB (136,192 bytes).

More Questions

I’d like to see how switch statements compare to hash tables. I think switch statements will still be slower, but it would be good to investigate the details and confirm.

This also makes me think that the “String To Enum” code in the last post, which makes use of a switch statement, is probably not the ideal code for doing that, as far as performance and memory usage go.

I think that instead of being switch statement based, it should probably be array based.

However, the idea that we can have compile time assurance that our hash has no collisions is really nice and we get that from doing case statements on run time hashes of our full set of valid inputs. It would be nice to find a way to leverage that while still getting the speed we want, as automatically as possible. At worst, code generation could be used, but it would be nicer to do something less rigid / heavy handed.

Lastly, I still believe that “ad hoc” custom fit code for specific data lookups have the best potential for performance. I want to keep looking into it to see about finding some good avenues for success.

Have any info to share? Please do!

Thanks for reading and I hope you find this useful for at least interesting.

Never put off til run time what can be done at compile time.

C++11 gave us constexpr which lets us make C++ code that the compiler can run during compilation, instead of at runtime.

This is great because now we can use C++ to do some things that we previously had to use macros or templates for.

As with many of the newish C++ features, it feels like there are some rough edges to work out with constexpr, but it adds a lot of new exciting capabilities.

In this post we will explore some new possibilities, and get a better feel for this new world of compile time code execution. There are going to be some unexpected surprises along the way 😛

Testing Details

The code in this post will be using some compile time crc code I found at Github Gist: oktal/compile-time-crc32.cc. I haven’t tested it for correctness or speed, but it serves the purpose of being a compile time hash implementation that allows us to explore things a bit.

I’ve compiled and analyzed the code in this post in visual studio 2015, in both debug/release and x86/x64. There are differences in behavior between debug and release of course, but x86 and x64 behaved the same. If you have different results with different compilers or different code, please share!

With that out of the way, onto the fun!

We are going to be looking at:

1. Simple Compile Time Hashing Behavior
2. Compile Time Hash Switching
3. Leveraging Jump Tables
4. Perfect Hashing
5. Minimally Perfect Hashing
6. Compile Time Assisted String To Enum

Simple Compile Time Hashing Behavior

Let’s analyze some basic cases of trying to do some compile time hashing

    const char *hello1String = "Hello1";
    unsigned int hashHello1 = crc32(hello1String);  // 1) Always Run Time.
    unsigned int hashHello2 = crc32("Hello2");      // 2) Always Run Time.

    // 3) error C2131: expression did not evaluate to a constant
    //const char *hello3String = "Hello3";
    //constexpr unsigned int hashHello3 = crc32(hello3String);
    constexpr unsigned int hashHello4 = crc32("Hello4");  // 4) Debug: Run Time.  Release: Compile Time

    printf("%X %X %X %Xn", hashHello1, hashHello2, hashHello4, crc32("hello5"));  // 5) Always Run Time. (!!!)

Let’s take a look at the assembly for the above code when compiled in debug. The assembly line calls to crc32 are highlighted for clarity.

    const char *hello1String = "Hello1";
00007FF717B71C3E  lea         rax,[string "Hello1" (07FF717B7B124h)]  
00007FF717B71C45  mov         qword ptr [hello1String],rax  
    unsigned int hashHello1 = crc32(hello1String);  // 1) Always Run Time.
00007FF717B71C49  xor         edx,edx  
00007FF717B71C4B  mov         rcx,qword ptr [hello1String]  
00007FF717B71C4F  call        crc32 (07FF717B710C3h)  
00007FF717B71C54  mov         dword ptr [hashHello1],eax  
    unsigned int hashHello2 = crc32("Hello2");      // 2) Always Run Time.
00007FF717B71C57  xor         edx,edx  
00007FF717B71C59  lea         rcx,[string "Hello2" (07FF717B7B12Ch)]  
00007FF717B71C60  call        crc32 (07FF717B710C3h)  
00007FF717B71C65  mov         dword ptr [hashHello2],eax  

    // 3) error C2131: expression did not evaluate to a constant
    //const char *hello3String = "Hello3";
    //constexpr unsigned int hashHello3 = crc32(hello3String);
    constexpr unsigned int hashHello4 = crc32("Hello4");  // 4) Debug: Run Time.  Release: Compile Time
00007FF717B71C68  xor         edx,edx  
00007FF717B71C6A  lea         rcx,[string "Hello4" (07FF717B7B134h)]  
00007FF717B71C71  call        crc32 (07FF717B710C3h)  
00007FF717B71C76  mov         dword ptr [hashHello4],eax  

    printf("%X %X %X %Xn", hashHello1, hashHello2, hashHello4, crc32("hello5"));  // 5) Always Run Time. (!!!)
00007FF717B71C79  xor         edx,edx  
00007FF717B71C7B  lea         rcx,[string "hello5" (07FF717B7B13Ch)]  
00007FF717B71C82  call        crc32 (07FF717B710C3h)  
00007FF717B71C87  mov         dword ptr [rsp+20h],eax  
00007FF717B71C8B  mov         r9d,0BECA76E1h  
00007FF717B71C91  mov         r8d,dword ptr [hashHello2]  
00007FF717B71C95  mov         edx,dword ptr [hashHello1]  
00007FF717B71C98  lea         rcx,[string "%X %X %X %Xn" (07FF717B7B150h)]  
00007FF717B71C9F  call        printf (07FF717B711FEh)  

As you can see, there is a “call crc32” in the assembly for every place that we call crc32 in the c++ code – 4 crc32 calls in the c++, and 4 crc32 calls in the asm. That means that all of these crc32 calls happen at run time while in debug mode.

I can sort of see the reasoning for always doing constexpr at runtime in debug mode, since you probably want to be able to step through constexpr functions to see how they operate. I’d bet that is the reasoning here.

Let’s see what it compiles to in release. Release is a little bit harder to understand since the optimizations make it difficult/impossible to pair the c++ lines with the asm.

00007FF68DC010BA  lea         rdx,[string "Hello1"+1h (07FF68DC02211h)]  
00007FF68DC010C1  mov         ecx,7807C9A2h  
00007FF68DC010C6  call        crc32_rec (07FF68DC01070h)  
00007FF68DC010CB  lea         rdx,[string "Hello2"+1h (07FF68DC02219h)]  
00007FF68DC010D2  mov         ecx,7807C9A2h  
00007FF68DC010D7  mov         edi,eax  
00007FF68DC010D9  call        crc32_rec (07FF68DC01070h)  
00007FF68DC010DE  lea         rdx,[string "hello5"+1h (07FF68DC02221h)]  
00007FF68DC010E5  mov         ecx,4369E96Ah  
00007FF68DC010EA  mov         ebx,eax  
00007FF68DC010EC  call        crc32_rec (07FF68DC01070h)  
00007FF68DC010F1  mov         r9d,0BECA76E1h  
00007FF68DC010F7  mov         dword ptr [rsp+20h],eax  
00007FF68DC010FB  mov         r8d,ebx  
00007FF68DC010FE  lea         rcx,[string "%X %X %X %Xn" (07FF68DC02228h)]  
00007FF68DC01105  mov         edx,edi  
00007FF68DC01107  call        printf (07FF68DC01010h)  

We can see that in release, there are still 3 calls to crc32 which means that only one hash actually happens at compile time.

From the assembly we can easily see that “Hello1”, “Hello2” and “Hello5” are hashed at runtime. The assembly shows those strings as parameters to the function.

That leaves only “Hello4” remaining, which means that is the one that got hashed at compile time. You can actually see that on line 12, the value 0x0BECA76E1 is being moved into register r9d. If you step through the code in debug mode, you can see that the value of hashHello4 is actually 0x0BECA76E1, so that “move constant into register” on line 12 is the result of our hash happening at compile time. Pretty neat right?

I was actually surprised to see how many hashes remained happening at run time though, especially the one that is a parameter to printf. There really is no reason I can think of why they would need to remain happening at run time, versus happening at compile time, other than (this?) compiler not aggressively moving whatever it can to compile time. I really wish it worked more like that though, and IMO I think it should. Maybe in the future we’ll see compilers move more that direction.

Compile Time Hash Switching

Something neat about being able to do hashing at compile time, is that you can use the result of a compile time hash as a case value in a switch statement!

Let’s explore that a bit:

    unsigned int hash = crc32("Hello1");  // 1) Run Time.
    constexpr unsigned int hashTestHello2 = crc32("Hello2"); // 2) Debug: Run Time. Release: Not calculated at all.
    switch (hash) { // 3) Uses variable on stack
        case hashTestHello2: {  // 4) Compile Time Constant.
            printf("An");
            break;
        }
        case crc32("Hello3"): {  // 5) Compile Time Constant.
            printf("Bn");
            break;
        }
        // 6) error C2196: case value '1470747604' already used
        /*
        case crc32("Hello2"): { 
            printf("Cn");
            break;
        }
        */
        default: {
            printf("Cn");
            break;
        }
    }

Something interesting to note is that if you have duplicate cases in your switch statement, due to things hashing to the same value (either duplicates, or actual hash collisions) that you will get a compile time error. This might come in handy, let’s come back to that later.

Let’s look at the assembly code in debug:

    unsigned int hash = crc32("Hello1");  // 1) Run Time.
00007FF77B4119FE  xor         edx,edx  
00007FF77B411A00  lea         rcx,[string "Hello1" (07FF77B41B124h)]  
00007FF77B411A07  call        crc32 (07FF77B4110C3h)  
00007FF77B411A0C  mov         dword ptr [hash],eax  
    constexpr unsigned int hashTestHello2 = crc32("Hello2"); // 2) Debug: Run Time. Release: Not calculated at all.
00007FF77B411A0F  xor         edx,edx  
00007FF77B411A11  lea         rcx,[string "Hello2" (07FF77B41B12Ch)]  
00007FF77B411A18  call        crc32 (07FF77B4110C3h)  
00007FF77B411A1D  mov         dword ptr [hashTestHello2],eax  
    switch (hash) { // 3) Uses variable on stack
00007FF77B411A20  mov         eax,dword ptr [hash]  
00007FF77B411A23  mov         dword ptr [rbp+0F4h],eax  
00007FF77B411A29  cmp         dword ptr [rbp+0F4h],20AEE342h  
00007FF77B411A33  je          Snippet_CompileTimeHashSwitching1+71h (07FF77B411A51h)  
00007FF77B411A35  cmp         dword ptr [rbp+0F4h],57A9D3D4h  
00007FF77B411A3F  je          Snippet_CompileTimeHashSwitching1+63h (07FF77B411A43h)  
00007FF77B411A41  jmp         Snippet_CompileTimeHashSwitching1+7Fh (07FF77B411A5Fh)  
        case hashTestHello2: {  // 4) Compile Time Constant.
            printf("An");
00007FF77B411A43  lea         rcx,[string "An" (07FF77B41B144h)]  
00007FF77B411A4A  call        printf (07FF77B4111FEh)  
            break;
00007FF77B411A4F  jmp         Snippet_CompileTimeHashSwitching1+8Bh (07FF77B411A6Bh)  
        }
        case crc32("Hello3"): {  // 5) Compile Time Constant.
            printf("Bn");
00007FF77B411A51  lea         rcx,[string "Bn" (07FF77B41B160h)]  
00007FF77B411A58  call        printf (07FF77B4111FEh)  
            break;
00007FF77B411A5D  jmp         Snippet_CompileTimeHashSwitching1+8Bh (07FF77B411A6Bh)  
        }
        // 6) error C2196: case value '1470747604' already used
        /*
        case crc32("Hello2"): { 
            printf("Cn");
            break;
        }
        */
        default: {
            printf("Cn");
00007FF77B411A5F  lea         rcx,[string "Cn" (07FF77B41B164h)]  
00007FF77B411A66  call        printf (07FF77B4111FEh)  
            break;
        }
    }

We can see that the hash for “Hello1” and “Hello2” are both calculated at run time, and that the switch statement uses the stack variable [hash] to move the value into a register to do the switch statement.

Interestingly though, on lines 14 and 16 we can see it moving a constant value into registers to use in a cmp (compare) operation. 0x20AEE342 is the hash value of “Hello3” and 0x57A9D3D4 is the hash value of “Hello2” so it ended up doing those hashes at compile time, even though we are in debug mode. This is because case values must be known at compile time.

It’s interesting to see though that the compiler calculates hashTestHello2 at runtime, even though the only place we use it (in the case statement), it puts a compile time constant from a compile time hash. Odd.

Let’s see what happens in release:

00007FF7FA9B10B4  lea         rdx,[string "Hello1"+1h (07FF7FA9B2211h)]  
00007FF7FA9B10BB  mov         ecx,7807C9A2h  
00007FF7FA9B10C0  call        crc32_rec (07FF7FA9B1070h)  
00007FF7FA9B10C5  cmp         eax,20AEE342h  
00007FF7FA9B10CA  je          main+49h (07FF7FA9B10F9h)  
00007FF7FA9B10CC  cmp         eax,57A9D3D4h  
00007FF7FA9B10D1  je          main+36h (07FF7FA9B10E6h)  
00007FF7FA9B10D3  lea         rcx,[string "Cn" (07FF7FA9B2220h)]  
00007FF7FA9B10DA  call        printf (07FF7FA9B1010h)  
// ... More asm code below, but not relevant

Release is a little more lean which is nice. On line 3 we calculate the hash of “Hello1” at runtime, then on lines 4 and 6, we compare it against the constant values of our compile time hashes for “Hello2” and “Hello3”. That is all that is done at runtime, which is more in line with what we’d like to see the compiler do. It’s still a little bit lame that it didn’t see that “Hello1” was a compile time constant that was being hashed, and did it at runtime, but at least it got most of the hashing to happen at compile time.

What if we change the “hash” variable to be constexpr?

    // Release: this function just prints "Cn" and exits.  All code melted away at compile time!
    constexpr unsigned int hash = crc32("Hello1");  // 1) Debug: Run Time
    constexpr unsigned int hashTestHello2 = crc32("Hello2"); // 2) Debug: Run Time
    switch (hash) { // 3) Debug: Compile Time Constant.
        case hashTestHello2: {   // 4) Debug: Compile Time Constant.
            printf("An");
            break;
        }
        case crc32("Hello3"): {   // 5) Debug: Compile Time Constant.
            printf("Bn");
            break;
        }
        default: {
            printf("Cn");
            break;
        }
    }

Let’s check it out in debug first:

constexpr unsigned int hash = crc32("Hello1");  // 1) Debug: Run Time
00007FF71F7A1ABE  xor         edx,edx  
00007FF71F7A1AC0  lea         rcx,[string "Hello1" (07FF71F7AB124h)]  
00007FF71F7A1AC7  call        crc32 (07FF71F7A10C3h)  
00007FF71F7A1ACC  mov         dword ptr [hash],eax  
    constexpr unsigned int hashTestHello2 = crc32("Hello2"); // 2) Debug: Run Time
00007FF71F7A1ACF  xor         edx,edx  
00007FF71F7A1AD1  lea         rcx,[string "Hello2" (07FF71F7AB12Ch)]  
00007FF71F7A1AD8  call        crc32 (07FF71F7A10C3h)  
00007FF71F7A1ADD  mov         dword ptr [hashTestHello2],eax  
    switch (hash) { // 3) Debug: Compile Time Constant.
00007FF71F7A1AE0  mov         dword ptr [rbp+0F4h],0CEA0826Eh  
00007FF71F7A1AEA  cmp         dword ptr [rbp+0F4h],20AEE342h  
00007FF71F7A1AF4  je          Snippet_CompileTimeHashSwitching2+72h (07FF71F7A1B12h)  
00007FF71F7A1AF6  cmp         dword ptr [rbp+0F4h],57A9D3D4h  
00007FF71F7A1B00  je          Snippet_CompileTimeHashSwitching2+64h (07FF71F7A1B04h)  
00007FF71F7A1B02  jmp         Snippet_CompileTimeHashSwitching2+80h (07FF71F7A1B20h)  
        case hashTestHello2: {   // 4) Debug: Compile Time Constant.
            printf("An");
00007FF71F7A1B04  lea         rcx,[string "An" (07FF71F7AB144h)]  
        case hashTestHello2: {   // 4) Debug: Compile Time Constant.
            printf("An");
00007FF71F7A1B0B  call        printf (07FF71F7A11FEh)  
            break;
00007FF71F7A1B10  jmp         Snippet_CompileTimeHashSwitching2+8Ch (07FF71F7A1B2Ch)  
        }
        case crc32("Hello3"): {   // 5) Debug: Compile Time Constant.
            printf("Bn");
00007FF71F7A1B12  lea         rcx,[string "Bn" (07FF71F7AB160h)]  
00007FF71F7A1B19  call        printf (07FF71F7A11FEh)  
            break;
00007FF71F7A1B1E  jmp         Snippet_CompileTimeHashSwitching2+8Ch (07FF71F7A1B2Ch)  
        }
        default: {
            printf("Cn");
00007FF71F7A1B20  lea         rcx,[string "Cn" (07FF71F7AB164h)]  
00007FF71F7A1B27  call        printf (07FF71F7A11FEh)  
            break;
        }
    }

The code does a runtime hash of “Hello1” and “Hello2” on lines 4 and 9. Then, on line 12, it moves the compile time hash value of “Hello1” into memory. On line 13 it compares that against the compile time hash value of “Hello3”. On line 15 it compares it against the compile time hash value of “Hello2”.

Now let’s check release:

00007FF7B7B91074  lea         rcx,[string "Cn" (07FF7B7B92210h)]  
00007FF7B7B9107B  call        printf (07FF7B7B91010h)  

Awesome! It was able to do ALL calculation at compile time, and only do the printf at run time. Neat!

As one final test, let’s see what happens when we put a crc32 call straight into the switch statement.

    constexpr unsigned int hashTestHello2 = crc32("Hello2"); // 1) Debug: Run Time. Release: Not calculated at all.
    switch (crc32("Hello1")) {  // 2) Always Run Time (!!!)
        case hashTestHello2: {  // 3) Compile Time Constant.
            printf("An");
            break;
        }
        case crc32("Hello3"): {  // 4) Compile Time Constant.
            printf("Bn");
            break;
        }
        default: {
            printf("Cn");
            break;
        }
    }

Here’s the debug assembly:

    constexpr unsigned int hashTestHello2 = crc32("Hello2"); // 1) Debug: Run Time. Release: Not calculated at all.
00007FF72ED51B7E  xor         edx,edx  
00007FF72ED51B80  lea         rcx,[string "Hello2" (07FF72ED5B12Ch)]  
00007FF72ED51B87  call        crc32 (07FF72ED510C3h)  
00007FF72ED51B8C  mov         dword ptr [hashTestHello2],eax  
    switch (crc32("Hello1")) {  // 2) Always Run Time (!!!)
00007FF72ED51B8F  xor         edx,edx  
00007FF72ED51B91  lea         rcx,[string "Hello1" (07FF72ED5B124h)]  
00007FF72ED51B98  call        crc32 (07FF72ED510C3h)  
00007FF72ED51B9D  mov         dword ptr [rbp+0D4h],eax  
00007FF72ED51BA3  cmp         dword ptr [rbp+0D4h],20AEE342h  
00007FF72ED51BAD  je          Snippet_CompileTimeHashSwitching3+6Bh (07FF72ED51BCBh)  
00007FF72ED51BAF  cmp         dword ptr [rbp+0D4h],57A9D3D4h  
00007FF72ED51BB9  je          Snippet_CompileTimeHashSwitching3+5Dh (07FF72ED51BBDh)  
00007FF72ED51BBB  jmp         Snippet_CompileTimeHashSwitching3+79h (07FF72ED51BD9h)  
        case hashTestHello2: {  // 3) Compile Time Constant.
            printf("An");
00007FF72ED51BBD  lea         rcx,[string "An" (07FF72ED5B144h)]  
00007FF72ED51BC4  call        printf (07FF72ED511FEh)  
            break;
00007FF72ED51BC9  jmp         Snippet_CompileTimeHashSwitching3+85h (07FF72ED51BE5h)  
        }
        case crc32("Hello3"): {  // 4) Compile Time Constant.
            printf("Bn");
00007FF72ED51BCB  lea         rcx,[string "Bn" (07FF72ED5B160h)]  
00007FF72ED51BD2  call        printf (07FF72ED511FEh)  
            break;
00007FF72ED51BD7  jmp         Snippet_CompileTimeHashSwitching3+85h (07FF72ED51BE5h)  
        }
        default: {
            printf("Cn");
00007FF72ED51BD9  lea         rcx,[string "Cn" (07FF72ED5B164h)]  
00007FF72ED51BE0  call        printf (07FF72ED511FEh)  
            break;
        }
    }

It was able to do the case value crc’s at compile time, but the other two it did at runtime. Not surprising for debug. Let’s check release:

00007FF6A46D10B4  lea         rdx,[string "Hello1"+1h (07FF6A46D2211h)]  
00007FF6A46D10BB  mov         ecx,7807C9A2h  
00007FF6A46D10C0  call        crc32_rec (07FF6A46D1070h)  
00007FF6A46D10C5  cmp         eax,20AEE342h  
00007FF6A46D10CA  je          main+49h (07FF6A46D10F9h)  
00007FF6A46D10CC  cmp         eax,57A9D3D4h  
00007FF6A46D10D1  je          main+36h (07FF6A46D10E6h)  
00007FF6A46D10D3  lea         rcx,[string "Cn" (07FF6A46D2220h)]  
00007FF6A46D10DA  call        printf (07FF6A46D1010h)  
// ... More asm code below, but not relevant

It did the hash for “Hello1” at run time (why??), but it did the others at release. A bit disappointing that it couldn’t make the “Hello1” hash be compile time, but we saw this behavior before so nothing new there.

Leveraging Jump Tables

In the above switch statements, the tests for hashes were very much “If hash is a, do this, else if hash is b, do that”. It was an if/else if/else if style chain.

Switch statements actually have the ability to become jump tables though, which let them get to the right case value with fewer comparisons.

Check out this code:

    // Debug: Jump Table
    // Release: Just does the constant case, everything else goes away
    unsigned int i = 3;
    switch (i) {
        case 0: printf("An"); break;
        case 1: printf("Bn"); break;
        case 2: printf("Cn"); break;
        case 3: printf("Dn"); break;
        case 4: printf("En"); break;
        case 5: printf("Fn"); break;
        case 6: printf("Gn"); break;
        case 7: printf("Hn"); break;
        default: printf("Nonen"); break;
    }

Here’s the debug assembly:

    // Debug: Jump Table
    // Release: Just does the constant case, everything else goes away
    unsigned int i = 3;
00007FF69CF9238E  mov         dword ptr [i],3  
    switch (i) {
00007FF69CF92395  mov         eax,dword ptr [i]  
00007FF69CF92398  mov         dword ptr [rbp+0D4h],eax  
00007FF69CF9239E  cmp         dword ptr [rbp+0D4h],7  
00007FF69CF923A5  ja          $LN11+0Eh (07FF69CF92434h)  
00007FF69CF923AB  mov         eax,dword ptr [rbp+0D4h]  
00007FF69CF923B1  lea         rcx,[__ImageBase (07FF69CF80000h)]  
00007FF69CF923B8  mov         eax,dword ptr [rcx+rax*4+1244Ch]  
00007FF69CF923BF  add         rax,rcx  
00007FF69CF923C2  jmp         rax  
        case 0: printf("An"); break;
00007FF69CF923C4  lea         rcx,[string "An" (07FF69CF9B144h)]  
00007FF69CF923CB  call        printf (07FF69CF911FEh)  
00007FF69CF923D0  jmp         $LN11+1Ah (07FF69CF92440h)  
        case 1: printf("Bn"); break;
00007FF69CF923D2  lea         rcx,[string "Bn" (07FF69CF9B160h)]  
00007FF69CF923D9  call        printf (07FF69CF911FEh)  
00007FF69CF923DE  jmp         $LN11+1Ah (07FF69CF92440h)  
        case 2: printf("Cn"); break;
00007FF69CF923E0  lea         rcx,[string "Cn" (07FF69CF9B164h)]  
00007FF69CF923E7  call        printf (07FF69CF911FEh)  
00007FF69CF923EC  jmp         $LN11+1Ah (07FF69CF92440h)  
        case 3: printf("Dn"); break;
00007FF69CF923EE  lea         rcx,[string "Dn" (07FF69CF9B168h)]  
00007FF69CF923F5  call        printf (07FF69CF911FEh)  
00007FF69CF923FA  jmp         $LN11+1Ah (07FF69CF92440h)  
        case 4: printf("En"); break;
00007FF69CF923FC  lea         rcx,[string "En" (07FF69CF9B16Ch)]  
00007FF69CF92403  call        printf (07FF69CF911FEh)  
00007FF69CF92408  jmp         $LN11+1Ah (07FF69CF92440h)  
        case 5: printf("Fn"); break;
00007FF69CF9240A  lea         rcx,[string "Fn" (07FF69CF9B170h)]  
00007FF69CF92411  call        printf (07FF69CF911FEh)  
00007FF69CF92416  jmp         $LN11+1Ah (07FF69CF92440h)  
        case 6: printf("Gn"); break;
00007FF69CF92418  lea         rcx,[string "Gn" (07FF69CF9B174h)]  
00007FF69CF9241F  call        printf (07FF69CF911FEh)  
00007FF69CF92424  jmp         $LN11+1Ah (07FF69CF92440h)  
        case 7: printf("Hn"); break;
00007FF69CF92426  lea         rcx,[string "Hn" (07FF69CF9B178h)]  
00007FF69CF9242D  call        printf (07FF69CF911FEh)  
00007FF69CF92432  jmp         $LN11+1Ah (07FF69CF92440h)  
        default: printf("Nonen"); break;
00007FF69CF92434  lea         rcx,[string "Nonen" (07FF69CF9AFDCh)]  
00007FF69CF9243B  call        printf (07FF69CF911FEh)  
    }

On line 8 and 9 it checks to see if the value we are switching on is greater than 7, and if so, jumps to 07FF69CF92434h, which is the “default” case where it prints “None”.

If the number is not greater than 7, it calculates an address based on the value we are switching on, and then jumps to it, on line 14.

Instead of testing for every possible value in an if/else if/else if/else if type setup, it can go IMMEDIATELY to the code associated with the particular case statement.

This is a jump table and can be a big speed improvement if you have a lot of cases, or if the code is called a lot.

I’ll spare you the release assembly. The compiler can tell that this is a compile time known result and only has the printf for the “D” without any of the other logic.

Let’s go back to our crc32 code and try and leverage a jump table:

    // Debug / Release: Does hash and jump table.
    // Note: It AND's against 7 and then tests to see if i is greater than 7 (!!!)
    unsigned int i = crc32("Hello") & 7;
    switch (i) {
        case 0: printf("An"); break;
        case 1: printf("Bn"); break;
        case 2: printf("Cn"); break;
        case 3: printf("Dn"); break;
        case 4: printf("En"); break;
        case 5: printf("Fn"); break;
        case 6: printf("Gn"); break;
        case 7: printf("Hn"); break;
        default: printf("Nonen"); break;
    }

I’ll show you just the debug assembly for brevity. It does the hash and jump table at runtime for both debug and release. Interestingly, even though we do &7 on the hash value, the switch statement STILL makes sure that the value being switched on is not greater than 7. It can never be greater than 7, and that can be known at compile time, but it still checks. This is true even of the release assembly!

    // Debug / Release: Does hash and jump table.
    // Note: It AND's against 7 and then tests to see if i is greater than 7 (!!!)
    unsigned int i = crc32("Hello") & 7;
00007FF7439C24CE  xor         edx,edx  
00007FF7439C24D0  lea         rcx,[string "Hello" (07FF7439CB180h)]  
00007FF7439C24D7  call        crc32 (07FF7439C10C3h)  
00007FF7439C24DC  and         eax,7  
00007FF7439C24DF  mov         dword ptr [i],eax  
    switch (i) {
00007FF7439C24E2  mov         eax,dword ptr [i]  
00007FF7439C24E5  mov         dword ptr [rbp+0D4h],eax  
00007FF7439C24EB  cmp         dword ptr [rbp+0D4h],7  
00007FF7439C24F2  ja          $LN11+0Eh (07FF7439C2581h)  
00007FF7439C24F8  mov         eax,dword ptr [rbp+0D4h]  
00007FF7439C24FE  lea         rcx,[__ImageBase (07FF7439B0000h)]  
00007FF7439C2505  mov         eax,dword ptr [rcx+rax*4+12598h]  
00007FF7439C250C  add         rax,rcx  
00007FF7439C250F  jmp         rax  
        case 0: printf("An"); break;
00007FF7439C2511  lea         rcx,[string "An" (07FF7439CB144h)]  
        case 0: printf("An"); break;
00007FF7439C2518  call        printf (07FF7439C11FEh)  
00007FF7439C251D  jmp         $LN11+1Ah (07FF7439C258Dh)  
        case 1: printf("Bn"); break;
00007FF7439C251F  lea         rcx,[string "Bn" (07FF7439CB160h)]  
00007FF7439C2526  call        printf (07FF7439C11FEh)  
00007FF7439C252B  jmp         $LN11+1Ah (07FF7439C258Dh)  
        case 2: printf("Cn"); break;
00007FF7439C252D  lea         rcx,[string "Cn" (07FF7439CB164h)]  
00007FF7439C2534  call        printf (07FF7439C11FEh)  
00007FF7439C2539  jmp         $LN11+1Ah (07FF7439C258Dh)  
        case 3: printf("Dn"); break;
00007FF7439C253B  lea         rcx,[string "Dn" (07FF7439CB168h)]  
00007FF7439C2542  call        printf (07FF7439C11FEh)  
00007FF7439C2547  jmp         $LN11+1Ah (07FF7439C258Dh)  
        case 4: printf("En"); break;
00007FF7439C2549  lea         rcx,[string "En" (07FF7439CB16Ch)]  
00007FF7439C2550  call        printf (07FF7439C11FEh)  
00007FF7439C2555  jmp         $LN11+1Ah (07FF7439C258Dh)  
        case 5: printf("Fn"); break;
00007FF7439C2557  lea         rcx,[string "Fn" (07FF7439CB170h)]  
00007FF7439C255E  call        printf (07FF7439C11FEh)  
00007FF7439C2563  jmp         $LN11+1Ah (07FF7439C258Dh)  
        case 6: printf("Gn"); break;
00007FF7439C2565  lea         rcx,[string "Gn" (07FF7439CB174h)]  
00007FF7439C256C  call        printf (07FF7439C11FEh)  
00007FF7439C2571  jmp         $LN11+1Ah (07FF7439C258Dh)  
        case 7: printf("Hn"); break;
00007FF7439C2573  lea         rcx,[string "Hn" (07FF7439CB178h)]  
00007FF7439C257A  call        printf (07FF7439C11FEh)  
00007FF7439C257F  jmp         $LN11+1Ah (07FF7439C258Dh)  
        default: printf("Nonen"); break;
00007FF7439C2581  lea         rcx,[string "Nonen" (07FF7439CAFDCh)]  
00007FF7439C2588  call        printf (07FF7439C11FEh)  
    }

If we make “i” be a constexpr variable, it doesn’t affect debug, but in release, it is able to melt away all the code and just prints the correct case.

00007FF6093F1074  lea         rcx,[string "An" (07FF6093F2210h)]  
00007FF6093F107B  call        printf (07FF6093F1010h)  

Perfect Hashing

Perfect hashing is when you have a known set of inputs, and a hash function such that there is no collision between any of the inputs.

Perfect hashing can be great for being able to turn complex objects into IDs for faster lookup times, but the IDs are not usually going to be contiguous. One ID might be 3, and another might be 45361. Perfect hashing can still be useful though.

The code below shows show some compile time perfect hashing could be achieved.

    // Debug: Does have some sort of jump table setup, despite the cases not being continuous.
    // Release: prints "An".  All other code melts away at compile time.
    static const unsigned int c_numBuckets = 16;
    static const unsigned int c_salt = 1337;

    constexpr unsigned int i = crc32("Identifier_A", c_salt) % c_numBuckets;
    switch (i) {
        case (crc32("Identifier_A", c_salt) % c_numBuckets): printf("An"); break;
        case (crc32("Identifier_B", c_salt) % c_numBuckets): printf("Bn"); break;
        case (crc32("Identifier_C", c_salt) % c_numBuckets): printf("Cn"); break;
        case (crc32("Identifier_D", c_salt) % c_numBuckets): printf("Dn"); break;
        case (crc32("Identifier_E", c_salt) % c_numBuckets): printf("En"); break;
        case (crc32("Identifier_F", c_salt) % c_numBuckets): printf("Fn"); break;
        case (crc32("Identifier_G", c_salt) % c_numBuckets): printf("Gn"); break;
        case (crc32("Identifier_H", c_salt) % c_numBuckets): printf("Hn"); break;
        default: printf("Nonen"); break;
    }

One nice thing about doing perfect hashing at compile time like this is that if you ever have a hash collision, you’ll have a duplicate case in your switch statement, and will get a compile error. This means that you are guaranteed that your perfect hash is valid at runtime. With non compile time perfect hashes, you could easily get into a situation where you added some more valid inputs and may now have a hash collision, which would make hard to track subtle bugs as two input values would be sharing the same index for whatever read and/or write data they wanted to interact with.

You might notice that i am doing %16 on the hash instead of %8, even though there are only 8 items I want to test against.

The reason for that is hash collisions.

When you hash a string you are effectively getting a pseudo random number back that will always be the same for when that string is given as input.

For the above to work correctly and let me modulus against 8, i would have to roll an 8 sided dice 8 times and get no repeats.

There are 8! (8 factorial) different ways to roll that 8 sided dice 8 times and not get any collisions.

There are 8^8 different ways to roll the 8 sided dice 8 times total.

To get the chance that we roll an 8 sided dice 8 times and get no repeats (no hash collisions), the probability is 8! / 8^8 or 0.24%.

The salt in the crc32 function allows us to effectively re-roll our dice. Each salt value is a different roll of the dice.

How that fits in is that 0.24% of all salt values should let us be able to do %8 on our hashes and not have a hash collision.

Those odds are pretty bad, but they aren’t SUPER bad. We could just brute force search to find a good salt value to use and then hard code the salt in, like i did in the example above.

Unfortunately, the probability I calculated above only works if the hash function gives uniformly distributed output and is “truly random”.

In practice no hash function or pseudo random number generator is, and in fact this crc32 function has NO salt values which make for no collisions! I brute force tested all 2^32 (4.2 billion) possible salt values and came up with no salt that worked!

To get around that problem, instead of trying to get a perfect fit of 8 hashes with values 0-7 without collisions, i opted to go for 8 hashes with values 0-15 with no collisions. That changes my odds for the better, and there are in fact many salts that satisfy that.

It’s the equivalent of rolling a 16 sided dice 8 times without repeats.

Thinking about things a bit differently than before, the first roll has a 100% chance of not being a duplicate. The next roll has a 15/16 chance of not being duplicate. The next has a 14/16 chance and so on until the 8th roll which has a 9/16 chance.

Multiplying those all together, we end up with a 12.08% chance of rolling a 16 sided dice 8 times and not getting a duplicate. That means 12% of the salts (about 1 in 9) won’t produce collisions when we use 16 buckets, which makes it much easier for us to find a salt to use.

Looking at the disassembly in debug, we can see the jump table is miraculously in tact! This is great because now we can get compile time assured perfect hashing of objects, and can use a jump table to convert a runtime object into a perfect hash result.

Note that in release, the code melts away and just does the “correct printf”.

    // Debug: Does have some sort of jump table setup, despite the cases not being continuous.
    // Release: prints "An".  All other code melts away at compile time.
    static const unsigned int c_numBuckets = 16;
    static const unsigned int c_salt = 1337;

    constexpr unsigned int i = crc32("Identifier_A", c_salt) % c_numBuckets;
00007FF7CE651E5E  mov         edx,539h  
00007FF7CE651E63  lea         rcx,[string "Identifier_A" (07FF7CE65B190h)]  
00007FF7CE651E6A  call        crc32 (07FF7CE6510C3h)  
00007FF7CE651E6F  xor         edx,edx  
00007FF7CE651E71  mov         ecx,10h  
00007FF7CE651E76  div         eax,ecx  
00007FF7CE651E78  mov         eax,edx  
00007FF7CE651E7A  mov         dword ptr [i],eax  
    switch (i) {
00007FF7CE651E7D  mov         dword ptr [rbp+0D4h],4  
00007FF7CE651E87  cmp         dword ptr [rbp+0D4h],0Eh  
00007FF7CE651E8E  ja          $LN11+0Eh (07FF7CE651F1Dh)  
00007FF7CE651E94  mov         eax,dword ptr [rbp+0D4h]  
00007FF7CE651E9A  lea         rcx,[__ImageBase (07FF7CE640000h)]  
00007FF7CE651EA1  mov         eax,dword ptr [rcx+rax*4+11F34h]  
00007FF7CE651EA8  add         rax,rcx  
00007FF7CE651EAB  jmp         rax  
        case (crc32("Identifier_A", c_salt) % c_numBuckets): printf("An"); break;
00007FF7CE651EAD  lea         rcx,[string "An" (07FF7CE65B144h)]  
00007FF7CE651EB4  call        printf (07FF7CE6511FEh)  
00007FF7CE651EB9  jmp         $LN11+1Ah (07FF7CE651F29h)  
        case (crc32("Identifier_B", c_salt) % c_numBuckets): printf("Bn"); break;
00007FF7CE651EBB  lea         rcx,[string "Bn" (07FF7CE65B160h)]  
00007FF7CE651EC2  call        printf (07FF7CE6511FEh)  
00007FF7CE651EC7  jmp         $LN11+1Ah (07FF7CE651F29h)  
        case (crc32("Identifier_C", c_salt) % c_numBuckets): printf("Cn"); break;
00007FF7CE651EC9  lea         rcx,[string "Cn" (07FF7CE65B164h)]  
00007FF7CE651ED0  call        printf (07FF7CE6511FEh)  
00007FF7CE651ED5  jmp         $LN11+1Ah (07FF7CE651F29h)  
        case (crc32("Identifier_D", c_salt) % c_numBuckets): printf("Dn"); break;
00007FF7CE651ED7  lea         rcx,[string "Dn" (07FF7CE65B168h)]  
00007FF7CE651EDE  call        printf (07FF7CE6511FEh)  
00007FF7CE651EE3  jmp         $LN11+1Ah (07FF7CE651F29h)  
        case (crc32("Identifier_E", c_salt) % c_numBuckets): printf("En"); break;
00007FF7CE651EE5  lea         rcx,[string "En" (07FF7CE65B16Ch)]  
00007FF7CE651EEC  call        printf (07FF7CE6511FEh)  
00007FF7CE651EF1  jmp         $LN11+1Ah (07FF7CE651F29h)  
        case (crc32("Identifier_F", c_salt) % c_numBuckets): printf("Fn"); break;
00007FF7CE651EF3  lea         rcx,[string "Fn" (07FF7CE65B170h)]  
00007FF7CE651EFA  call        printf (07FF7CE6511FEh)  
00007FF7CE651EFF  jmp         $LN11+1Ah (07FF7CE651F29h)  
        case (crc32("Identifier_G", c_salt) % c_numBuckets): printf("Gn"); break;
00007FF7CE651F01  lea         rcx,[string "Gn" (07FF7CE65B174h)]  
00007FF7CE651F08  call        printf (07FF7CE6511FEh)  
00007FF7CE651F0D  jmp         $LN11+1Ah (07FF7CE651F29h)  
        case (crc32("Identifier_H", c_salt) % c_numBuckets): printf("Hn"); break;
00007FF7CE651F0F  lea         rcx,[string "Hn" (07FF7CE65B178h)]  
00007FF7CE651F16  call        printf (07FF7CE6511FEh)  
00007FF7CE651F1B  jmp         $LN11+1Ah (07FF7CE651F29h)  
        default: printf("Nonen"); break;
00007FF7CE651F1D  lea         rcx,[string "Nonen" (07FF7CE65AFDCh)]  
00007FF7CE651F24  call        printf (07FF7CE6511FEh)  
    }

Minimally Perfect Hashing

Minimally perfect hashing is like perfect hashing, except the results are contiguous values.

If you have 8 possible inputs to your minimally perfect hash function, you are going to get as output 0-7. The order of what inputs map to which outputs isn’t strictly defined unless you want to go through a lot of extra effort to make it be that way though.

This is even more useful than perfect hashing, as you can hash a (known good) input and use the result as an index into an array, or similar!

For more info on MPH, check out my post on it: O(1) Data Lookups With Minimal Perfect Hashing

The code below is a way of doing compile time assisted minimally perfect hashing:

    // Debug / Release:
    //   Runs crc32 at runtime only for "i".  The cases are compile time constants as per usual.
    //   Does a jumptable type setup for the switch and does fallthrough to do multiple increments to get the right ID.
    //
    // Release with constexpr on i:
    //   does the printf with a value of 2.  The rest of the code melts away.
    static const unsigned int c_numBuckets = 16;
    static const unsigned int c_salt = 1337;
    static const unsigned int c_invalidID = -1;

    unsigned int i = crc32("Identifier_F", c_salt) % c_numBuckets;
    unsigned int id = c_invalidID;
    switch (i) {
        case (crc32("Identifier_A", c_salt) % c_numBuckets): ++id;
        case (crc32("Identifier_B", c_salt) % c_numBuckets): ++id;
        case (crc32("Identifier_C", c_salt) % c_numBuckets): ++id;
        case (crc32("Identifier_D", c_salt) % c_numBuckets): ++id;
        case (crc32("Identifier_E", c_salt) % c_numBuckets): ++id;
        case (crc32("Identifier_F", c_salt) % c_numBuckets): ++id;
        case (crc32("Identifier_G", c_salt) % c_numBuckets): ++id;
        case (crc32("Identifier_H", c_salt) % c_numBuckets): ++id; 
        // the two lines below are implicit behavior of how this code works
        // break;
        // default: id = c_invalidID; break;
    }

    printf("id = %in", id);

Here’s the debug assembly for the above. The release does similar, so i’m not showing it.

    // Debug / Release:
    //   Runs crc32 at runtime only for "i".  The cases are compile time constants as per usual.
    //   Does a jumptable type setup for the switch and does fallthrough to do multiple increments to get the right ID.
    //
    // Release with constexpr on i:
    //   does the printf with a value of 2.  The rest of the code melts away.
    static const unsigned int c_numBuckets = 16;
    static const unsigned int c_salt = 1337;
    static const unsigned int c_invalidID = -1;

    unsigned int i = crc32("Identifier_F", c_salt) % c_numBuckets;
00007FF79E481D0E  mov         edx,539h  
00007FF79E481D13  lea         rcx,[string "Identifier_F" (07FF79E48B1E0h)]  
00007FF79E481D1A  call        crc32 (07FF79E4810C3h)  
00007FF79E481D1F  xor         edx,edx  
00007FF79E481D21  mov         ecx,10h  
00007FF79E481D26  div         eax,ecx  
00007FF79E481D28  mov         eax,edx  
00007FF79E481D2A  mov         dword ptr [i],eax  
    unsigned int id = c_invalidID;
00007FF79E481D2D  mov         dword ptr [id],0FFFFFFFFh  
    switch (i) {
00007FF79E481D34  mov         eax,dword ptr [i]  
00007FF79E481D37  mov         dword ptr [rbp+0F4h],eax  
00007FF79E481D3D  cmp         dword ptr [rbp+0F4h],0Eh  
00007FF79E481D44  ja          $LN11+8h (07FF79E481D9Fh)  
00007FF79E481D46  mov         eax,dword ptr [rbp+0F4h]  
00007FF79E481D4C  lea         rcx,[__ImageBase (07FF79E470000h)]  
00007FF79E481D53  mov         eax,dword ptr [rcx+rax*4+11DB8h]  
00007FF79E481D5A  add         rax,rcx  
00007FF79E481D5D  jmp         rax  
        case (crc32("Identifier_A", c_salt) % c_numBuckets): ++id;
00007FF79E481D5F  mov         eax,dword ptr [id]  
00007FF79E481D62  inc         eax  
00007FF79E481D64  mov         dword ptr [id],eax  
        case (crc32("Identifier_B", c_salt) % c_numBuckets): ++id;
00007FF79E481D67  mov         eax,dword ptr [id]  
00007FF79E481D6A  inc         eax  
00007FF79E481D6C  mov         dword ptr [id],eax  
        case (crc32("Identifier_C", c_salt) % c_numBuckets): ++id;
00007FF79E481D6F  mov         eax,dword ptr [id]  
00007FF79E481D72  inc         eax  
00007FF79E481D74  mov         dword ptr [id],eax  
        case (crc32("Identifier_D", c_salt) % c_numBuckets): ++id;
00007FF79E481D77  mov         eax,dword ptr [id]  
00007FF79E481D7A  inc         eax  
00007FF79E481D7C  mov         dword ptr [id],eax  
        case (crc32("Identifier_E", c_salt) % c_numBuckets): ++id;
00007FF79E481D7F  mov         eax,dword ptr [id]  
00007FF79E481D82  inc         eax  
00007FF79E481D84  mov         dword ptr [id],eax  
        case (crc32("Identifier_F", c_salt) % c_numBuckets): ++id;
00007FF79E481D87  mov         eax,dword ptr [id]  
00007FF79E481D8A  inc         eax  
00007FF79E481D8C  mov         dword ptr [id],eax  
        case (crc32("Identifier_G", c_salt) % c_numBuckets): ++id;
00007FF79E481D8F  mov         eax,dword ptr [id]  
00007FF79E481D92  inc         eax  
00007FF79E481D94  mov         dword ptr [id],eax  
        case (crc32("Identifier_H", c_salt) % c_numBuckets): ++id; 
00007FF79E481D97  mov         eax,dword ptr [id]  
00007FF79E481D9A  inc         eax  
00007FF79E481D9C  mov         dword ptr [id],eax  
        // the two lines below are implicit behavior of how this code works
        // break;
        // default: id = c_invalidID; break;
    }

    printf("id = %in", id);
00007FF79E481D9F  mov         edx,dword ptr [id]  
00007FF79E481DA2  lea         rcx,[string "id = %in" (07FF79E48B238h)]  
        // the two lines below are implicit behavior of how this code works
        // break;
        // default: id = c_invalidID; break;
    }

    printf("id = %in", id);
00007FF79E481DA9  call        printf (07FF79E4811FEh)  

As you can see, the jump table is still in tact, which is good, but it does a lot of repeated increments to get the right ID values. I wish the compiler were smart enough to “flatten” this and just give each case it’s proper ID value.

As is, this could be a performance issue if you had a very large number of inputs.

You could always just hard code a number there instead of relying on the fallthrough and increment, but then there is a lot of copy pasting. Maybe you could do something clever with macros or templates to help that though.

Compile Time Assisted String To Enum

Another interesting thing to think about is that we could actually use compile time hashing to convert a string to an enum.

In this case, let’s say that we don’t know if our input is valid or not. Since we don’t know that, we have to switch on the hash of our input string, but then do a string compare against whatever string has that hash, to make sure it matches. If it does match, it should take that enum value, else it should be invalid.

Since that would be a lot of error prone copy/pasting, I simplified things a bit by using a macro list:

    // Debug / Release:
    //   Runs crc32 at runtime only for "i".  The cases are compile time constants as per usual.
    //   Does a jumptable type setup for the switch and does a string comparison against the correct string.
    //   If strings are equal, sets the enum value.

    static const unsigned int c_numBuckets = 16;
    static const unsigned int c_salt = 1337;

    const char* testString = "Identifier_F";
    unsigned int i = crc32(testString, c_salt) % c_numBuckets;

    // This macro list is used for:
    //  * making the enum
    //  * making the cases in the switch statement
    // D.R.Y. - Don't Repeat Yourself.
    // Fewer moving parts = fewer errors, but admittedly is harder to understand vs redundant code.
    #define ENUM_VALUE_LIST 
        VALUE(Identifier_A) 
        VALUE(Identifier_B) 
        VALUE(Identifier_C) 
        VALUE(Identifier_D) 
        VALUE(Identifier_E) 
        VALUE(Identifier_F) 
        VALUE(Identifier_G) 
        VALUE(Identifier_H) 

    // Make the enum values.
    // Note these enum values are also usable as a contiguous ID if you needed one for an array index or similar.
    // You could define an array with size EIdentifier::count for instance and use these IDs to index into it.
    enum class EIdentifier : unsigned char {
        #define VALUE(x) x,
        ENUM_VALUE_LIST
        #undef VALUE
        count,
        invalid = (unsigned char)-1
    };

    // do a compile time hash assisted string comparison to convert string to enum
    EIdentifier identifier = EIdentifier::invalid;
    switch (i) {
        #define VALUE(x) case (crc32(#x, c_salt) % c_numBuckets) : if(!strcmp(testString, #x)) identifier = EIdentifier::x; else identifier = EIdentifier::invalid; break;
        ENUM_VALUE_LIST
        #undef VALUE
        default: identifier = EIdentifier::invalid;
    }
    
    // undefine the enum value list
    #undef ENUM_VALUE_LIST

    printf("string translated to enum value %i", identifier);

Here’s the debug assembly showing it working like it’s supposed to:

    // Debug / Release:
    //   Runs crc32 at runtime only for "i".  The cases are compile time constants as per usual.
    //   Does a jumptable type setup for the switch and does a string comparison against the correct string.
    //   If strings are equal, sets the enum value.

    static const unsigned int c_numBuckets = 16;
    static const unsigned int c_salt = 1337;

    const char* testString = "Identifier_F";
00007FF6B188179E  lea         rax,[string "Identifier_F" (07FF6B188B1E0h)]  
00007FF6B18817A5  mov         qword ptr [testString],rax  
    unsigned int i = crc32(testString, c_salt) % c_numBuckets;
00007FF6B18817A9  mov         edx,539h  
00007FF6B18817AE  mov         rcx,qword ptr [testString]  
00007FF6B18817B2  call        crc32 (07FF6B18810C3h)  
00007FF6B18817B7  xor         edx,edx  
00007FF6B18817B9  mov         ecx,10h  
00007FF6B18817BE  div         eax,ecx  
00007FF6B18817C0  mov         eax,edx  
00007FF6B18817C2  mov         dword ptr [i],eax  

    // This macro list is used for:
    //  * making the enum
    //  * making the cases in the switch statement
    // D.R.Y. - Don't Repeat Yourself.
    // Fewer moving parts = fewer errors, but admittedly is harder to understand vs redundant code.
    #define ENUM_VALUE_LIST 
        VALUE(Identifier_A) 
        VALUE(Identifier_B) 
        VALUE(Identifier_C) 
        VALUE(Identifier_D) 
        VALUE(Identifier_E) 
        VALUE(Identifier_F) 
        VALUE(Identifier_G) 
        VALUE(Identifier_H) 

    // Make the enum values.
    // Note these enum values are also usable as a contiguous ID if you needed one for an array index or similar.
    // You could define an array with size EIdentifier::count for instance and use these IDs to index into it.
    enum class EIdentifier : unsigned char {
        #define VALUE(x) x,
        ENUM_VALUE_LIST
        #undef VALUE
        count,
        invalid = (unsigned char)-1
    };

    // do a compile time hash assisted string comparison to convert string to enum
    EIdentifier identifier = EIdentifier::invalid;
00007FF6B18817C5  mov         byte ptr [identifier],0FFh  
    switch (i) {
00007FF6B18817C9  mov         eax,dword ptr [i]  
00007FF6B18817CC  mov         dword ptr [rbp+114h],eax  
00007FF6B18817D2  cmp         dword ptr [rbp+114h],0Eh  
    switch (i) {
00007FF6B18817D9  ja          $LN25+20h (07FF6B1881904h)  
00007FF6B18817DF  mov         eax,dword ptr [rbp+114h]  
00007FF6B18817E5  lea         rcx,[__ImageBase (07FF6B1870000h)]  
00007FF6B18817EC  mov         eax,dword ptr [rcx+rax*4+11924h]  
00007FF6B18817F3  add         rax,rcx  
00007FF6B18817F6  jmp         rax  
        #define VALUE(x) case (crc32(#x, c_salt) % c_numBuckets) : if(!strcmp(testString, #x)) identifier = EIdentifier::x; else identifier = EIdentifier::invalid; break;
        ENUM_VALUE_LIST
00007FF6B18817F8  lea         rdx,[string "Identifier_A" (07FF6B188B190h)]  
00007FF6B18817FF  mov         rcx,qword ptr [testString]  
00007FF6B1881803  call        strcmp (07FF6B18811CCh)  
00007FF6B1881808  test        eax,eax  
00007FF6B188180A  jne         Snippet_CompileTimeHashAssistedStringToEnum+92h (07FF6B1881812h)  
00007FF6B188180C  mov         byte ptr [identifier],0  
00007FF6B1881810  jmp         Snippet_CompileTimeHashAssistedStringToEnum+96h (07FF6B1881816h)  
00007FF6B1881812  mov         byte ptr [identifier],0FFh  
00007FF6B1881816  jmp         $LN25+24h (07FF6B1881908h)  
$LN7:
00007FF6B188181B  lea         rdx,[string "Identifier_B" (07FF6B188B1A0h)]  
00007FF6B1881822  mov         rcx,qword ptr [testString]  
00007FF6B1881826  call        strcmp (07FF6B18811CCh)  
00007FF6B188182B  test        eax,eax  
00007FF6B188182D  jne         Snippet_CompileTimeHashAssistedStringToEnum+0B5h (07FF6B1881835h)  
00007FF6B188182F  mov         byte ptr [identifier],1  
00007FF6B1881833  jmp         Snippet_CompileTimeHashAssistedStringToEnum+0B9h (07FF6B1881839h)  
00007FF6B1881835  mov         byte ptr [identifier],0FFh  
00007FF6B1881839  jmp         $LN25+24h (07FF6B1881908h)  
$LN10:
00007FF6B188183E  lea         rdx,[string "Identifier_C" (07FF6B188B1B0h)]  
00007FF6B1881845  mov         rcx,qword ptr [testString]  
00007FF6B1881849  call        strcmp (07FF6B18811CCh)  
00007FF6B188184E  test        eax,eax  
00007FF6B1881850  jne         Snippet_CompileTimeHashAssistedStringToEnum+0D8h (07FF6B1881858h)  
00007FF6B1881852  mov         byte ptr [identifier],2  
00007FF6B1881856  jmp         Snippet_CompileTimeHashAssistedStringToEnum+0DCh (07FF6B188185Ch)  
00007FF6B1881858  mov         byte ptr [identifier],0FFh  
00007FF6B188185C  jmp         $LN25+24h (07FF6B1881908h)  
$LN13:
00007FF6B1881861  lea         rdx,[string "Identifier_D" (07FF6B188B1C0h)]  
00007FF6B1881868  mov         rcx,qword ptr [testString]  
00007FF6B188186C  call        strcmp (07FF6B18811CCh)  
00007FF6B1881871  test        eax,eax  
00007FF6B1881873  jne         Snippet_CompileTimeHashAssistedStringToEnum+0FBh (07FF6B188187Bh)  
00007FF6B1881875  mov         byte ptr [identifier],3  
00007FF6B1881879  jmp         Snippet_CompileTimeHashAssistedStringToEnum+0FFh (07FF6B188187Fh)  
00007FF6B188187B  mov         byte ptr [identifier],0FFh  
00007FF6B188187F  jmp         $LN25+24h (07FF6B1881908h)  
$LN16:
00007FF6B1881884  lea         rdx,[string "Identifier_E" (07FF6B188B1D0h)]  
00007FF6B188188B  mov         rcx,qword ptr [testString]  
00007FF6B188188F  call        strcmp (07FF6B18811CCh)  
00007FF6B1881894  test        eax,eax  
00007FF6B1881896  jne         Snippet_CompileTimeHashAssistedStringToEnum+11Eh (07FF6B188189Eh)  
00007FF6B1881898  mov         byte ptr [identifier],4  
00007FF6B188189C  jmp         Snippet_CompileTimeHashAssistedStringToEnum+122h (07FF6B18818A2h)  
00007FF6B188189E  mov         byte ptr [identifier],0FFh  
00007FF6B18818A2  jmp         $LN25+24h (07FF6B1881908h)  
$LN19:
00007FF6B18818A4  lea         rdx,[string "Identifier_F" (07FF6B188B1E0h)]  
00007FF6B18818AB  mov         rcx,qword ptr [testString]  
00007FF6B18818AF  call        strcmp (07FF6B18811CCh)  
00007FF6B18818B4  test        eax,eax  
00007FF6B18818B6  jne         Snippet_CompileTimeHashAssistedStringToEnum+13Eh (07FF6B18818BEh)  
00007FF6B18818B8  mov         byte ptr [identifier],5  
00007FF6B18818BC  jmp         Snippet_CompileTimeHashAssistedStringToEnum+142h (07FF6B18818C2h)  
00007FF6B18818BE  mov         byte ptr [identifier],0FFh  
00007FF6B18818C2  jmp         $LN25+24h (07FF6B1881908h)  
$LN22:
00007FF6B18818C4  lea         rdx,[string "Identifier_G" (07FF6B188B1F0h)]  
00007FF6B18818CB  mov         rcx,qword ptr [testString]  
00007FF6B18818CF  call        strcmp (07FF6B18811CCh)  
00007FF6B18818D4  test        eax,eax  
00007FF6B18818D6  jne         Snippet_CompileTimeHashAssistedStringToEnum+15Eh (07FF6B18818DEh)  
00007FF6B18818D8  mov         byte ptr [identifier],6  
00007FF6B18818DC  jmp         Snippet_CompileTimeHashAssistedStringToEnum+162h (07FF6B18818E2h)  
00007FF6B18818DE  mov         byte ptr [identifier],0FFh  
00007FF6B18818E2  jmp         $LN25+24h (07FF6B1881908h)  
$LN25:
00007FF6B18818E4  lea         rdx,[string "Identifier_H" (07FF6B188B200h)]  
00007FF6B18818EB  mov         rcx,qword ptr [testString]  
00007FF6B18818EF  call        strcmp (07FF6B18811CCh)  
00007FF6B18818F4  test        eax,eax  
00007FF6B18818F6  jne         $LN25+1Ah (07FF6B18818FEh)  
00007FF6B18818F8  mov         byte ptr [identifier],7  
00007FF6B18818FC  jmp         $LN25+1Eh (07FF6B1881902h)  
00007FF6B18818FE  mov         byte ptr [identifier],0FFh  
00007FF6B1881902  jmp         $LN25+24h (07FF6B1881908h)  
        #undef VALUE
        default: identifier = EIdentifier::invalid;
00007FF6B1881904  mov         byte ptr [identifier],0FFh  
    }
    
    // undefine the enum value list
    #undef ENUM_VALUE_LIST

    printf("string translated to enum value %i", identifier);
00007FF6B1881908  movzx       eax,byte ptr [identifier]  
00007FF6B188190C  mov         edx,eax  
00007FF6B188190E  lea         rcx,[string "string translated to enum value "... (07FF6B188B248h)]  
00007FF6B1881915  call        printf (07FF6B18811FEh)  

Other Possibilities

Besides the above, I think there is a lot of other really great uses for constexpr functions.

For instance, i’d like to see how it’d work to have compile time data structures to do faster read access of constant data.

I want to see compile time trees, hash tables, sparse arrays, bloom filters, and more.

I believe they have the potential to be a lot more performant than even static data structures, since empty or unaccessed sections of the data structure could possibly melt away when the optimizer does it’s pass.

It may not turn out like that, but I think it’d be interesting to investigate it deeper and see how it goes.

I’d also like to see compilers get more aggressive about doing whatever it can at compile time. If there’s no reason for it to happen at runtime, why make it happen then? It is only going to make things slower.

Thanks for reading!

Links

You can find the source code for the code snippets in this post here: Github Atrix256/RandomCode/CompileTimeCRC/

Here’s a couple interesting discussions on constexpr on stack overflow:
detecting execution time of a constexpr function
How to ensure constexpr function never called at runtime?