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David LeBlanc

David LeBlanc David LeBlanc

Niner since 2009

  • David LeBlanc: Inside SafeInt

    Oh - I forgot to edit a couple of things - the suggested approach adds up to 5 clocks, not 6, which makes it 2.5x slower, not 3x.

  • David LeBlanc: Inside SafeInt

    Interesting question. I am not as good with perf as I would like, but in a situation like this, we're essentially counting instructions. Here's how it breaks down:

     

    Method 1 (suggested) (11 instructions) 32-bit, not optimized

       unsigned char tmp = (unsigned char)i;
    009A143E  mov         al,byte ptr [i]
    009A1441  mov         byte ptr [tmp],al
       if(tmp != i)
    009A1444  movzx       eax,byte ptr [tmp]
    009A1448  cmp         eax,dword ptr [i]
    009A144B  je          IntToUchar2+31h (9A1451h)
          return false;
    009A144D  xor         al,al
    009A144F  jmp         IntToUchar2+3Bh (9A145Bh)

       uc = (unsigned char)i;
    009A1451  mov         eax,dword ptr [uc]
    009A1454  mov         cl,byte ptr [i]
    009A1457  mov         byte ptr [eax],cl
       return true;
    009A1459  mov         al,1

     

    Method 2 (original) (10 instructions)
       if(i < 0 || i > 255)
    009A13DE  cmp         dword ptr [i],0
    009A13E2  jl          IntToUchar+2Dh (9A13EDh)
    009A13E4  cmp         dword ptr [i],0FFh
    009A13EB  jle         IntToUchar+31h (9A13F1h)
          return false;
    009A13ED  xor         al,al
    009A13EF  jmp         IntToUchar+3Bh (9A13FBh)

       uc = (unsigned char)i;
    009A13F1  mov         eax,dword ptr [uc]
    009A13F4  mov         cl,byte ptr [i]
    009A13F7  mov         byte ptr [eax],cl
       return true;
    009A13F9  mov         al,1

     

    Suggested method in release build:

    01041050 0F B6 C8         movzx       ecx,al  (3 clock)
    01041053 3B C8            cmp         ecx,eax  (1 clock)
    01041055 75 03            jne         main+5Ah (104105Ah) (1 clock)


    Current method in release build:

    01041018 3D FF 00 00 00   cmp         eax,0FFh (1 clock)
    0104101D 77 06            ja          main+25h (1041025h)  (1 clock)

     

    I also checked this in 64-bit, and it is the same instructions. In the suggested approach, we have an extra movzx, and it consumes a register, which would tend to cause surrounding code to optimize less efficiently on x86 (probably negligble on x64, but same effect). Due to the movzx being expensive (relative to cmp), the suggested approach is 6 cycles to the current approach's 2 cycles, or 3x worse. If the surrounding code were register constrained, as x86 often is, then you might need to push something on the stack to free a register, which would be more overhead.

     

    To be fair, I chalk this up to pure, blind luck. While I did try to write the code to be as efficient as possible, readability and correctness were of higher importance. It is entirely possible that other operations may not be as efficient as possible. I think I did try to avoid temporary variables where possible, for just this reason, though in some cases it couldn't be avoided, and the effect would often be diluted by how expensive large multiplication and division operations are.

     

    A large chunk of work that has not yet been done is to take all 64 combinations for each and every operation type and see exactly how they optimize, and dink with it to see if it can be better. As an aside, we considered using intrinsics for x64 multiplication, which would have been a lot faster, but decided not to in this release due to schedule constraints.

     

    In addition, unless you happened to be using SafeInt inside of a rendering engine, or something else where a couple of cycles matter, a couple of cycles here and there are not going to add up to anything, considering that an allocation could cost several 10's of thousands of cycles, loading a COM object is much, much bigger, and so on.

     

    Thanks for the question - I got to learn a bit about assembly that I didn't know before today.