Very often in computer code integers are divided by powers of two, such as 2, 4, 8, 16, 32, 64, and so on). These divisions are much faster than dividing by other numbers (since computers represent the underlying number in binary). In C/C++, there are two ways this type of division is typically expressed: normal division (x/256), or a shift (x<<8). These two methods produce the same results for non-negative values, and depending on the meaning of the code in question, (x/256) is often more readable, and thus I use it regularly.

If x is signed and is negative, division rounds towards 0, whereas the shift rounds towards negative infinity, but in situations where rounding of negative values is not important, I had generally assumed that modern compilers would generate similarly efficient code (reducing x/256 into a single x86 sar instruction).

It turns out, testing with a two modern compilers (and one slightly out of date compiler), this is not the case.

Here is the C code:

  void b(int r);
  void f(int v)
  {
    int i;
    for (i=0;i<v/256;i++) b(v > 0 ? v/65536 : 0);
  }

  void f2(int v)
  {
    int i;
    for (i=0;i<(v>>8);i++) b(v > 0 ? v >> 16 : 0);
  }

Ideally, a compiler should generate identical code for each of these functions. In the case of the loop counter, if v is less than 0, how it is rounded makes no difference. In the case of the parameter to b(), the code v >> 16 is only evaluated if v is known to be above 0.

Let's look at the output of some compilers (removing decoration and unrelated code). I've marked some code as bold to signify instructions that could be eliminated (with slight changes to the surrounding instructions):

• Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
  Xcode 5.1.1 on OSX 10.9.4
  Target: x86_64-apple-darwin13.3.0
  Command line flags: -O2 -fomit-frame-pointer

  f():                              # using divides
  	cmpl	$256, %edi
  	jl	LBB0_3            # if v is less than 256, skip loop
  
          # v is known to be 256 or greater. 
  
  	movl	%edi, %ebx
  	sarl	$31, %ebx         # ebx=0xffffffff if negative, 0 if non-negative
  	movl	%ebx, %r14d    
  	shrl	$24, %r14d        # r14d=0xff if negative, 0 if non-negative
  	addl	%edi, %r14d       # r14d = (v+255) if v negative, v if v non-negative
  	sarl	$8, %r14d         # r14d = v/256
  
  	shrl	$16, %ebx         # this will make ebx 65535 if v negative, 0 if v non-negative
  	addl	%edi, %ebx        # ebx = (v+65535) if v negative, v if v non-negative
  	sarl	$16, %ebx         # ebx = v/65536
  
          # interestingly, the optimizer used its knowledge of v being greater than 0 to remove the ternary conditional expression completely.
  
  	xorl	%ebp, %ebp
  LBB0_2:
  	movl	%ebx, %edi
  	callq	_b
  	incl	%ebp
  	cmpl	%r14d, %ebp
  	jl	LBB0_2
  LBB0_3:
  
  
  
  f2():                             # using shifts
  	movl	%edi, %ebp
  	sarl	$8, %ebp          # ebp = v>>8
  	testl	%ebp, %ebp
  	jle	LBB1_3            # if less than or equal to 0, skip
  
  	movl	%edi, %eax
  	sarl	$16, %eax         # eax = v>>16
  	xorl	%ebx, %ebx
  	testl	%edi, %edi
  	cmovgl	%eax, %ebx        # if v is greater than 0, set ebx to eax
  
          # the optimizer could also have removed the xorl/testl/cmovgl sequence as well
  
  LBB1_2:
  	movl	%ebx, %edi
  	callq	_b
  	decl	%ebp
  	jne	LBB1_2
  LBB1_3:
  
  

  In the first function (division), the LLVM optimizer appears to have removed the ternary expression (checking to see if v was greater than 0), likely because it knew that if the loop was running, v was greater than 0. Unfortunately, it didn't apply this knowledge to the integer divisions of v, which would have allowed it to not generate (substantial) rounding code.
  
  In the second function (shifts), LLVM wasn't required to generate rounding code (as C's >> maps to x86 sar directly), but it also didn't use the knowledge that v would be greater than 0.

• Microsoft (R) C/C++ Optimizing Compiler Version 18.00.30501 for x64
  Visual Studio Express 2013 for Windows Desktop on Windows 7
  Command line flags: /O2 (/Os produced different results, but nothing related to rounding)

  
  f():                              # using divides
  
          mov     eax, ecx
          mov     ebx, ecx
          cdq                       # set edx to 0xffffffff if v negative, 0 otherwise
          movzx   edx, dl           # set edx to 0xff if v negative, 0 otherwise
          add     eax, edx          # eax = v+255 if v negative, v otherwise
          sar     eax, 8            # eax = v/256
          test    eax, eax
          jle     SHORT $LN1@f      # skip loop if v/256 is less than or equal to 0
          mov     QWORD PTR [rsp+48], rdi
  
          mov     edi, eax          # edi is loop counter
  $LL3@f:
          test    ebx, ebx
          jle     SHORT $LN6@f      # if v is less than or equal to 0, jump to set eax to 0
          mov     eax, ebx
          cdq                       # set edx to 0xffffffff if v negative, 0 otherwise
          movzx   edx, dx           # set edx to 0xffff if v negative, 0 otherwise
          add     eax, edx          # eax = v+65535 if v negative, v otherwise
          sar     eax, 16           # eax = v/65536
          jmp     SHORT $LN7@f
  $LN6@f:
          xor     eax, eax
  $LN7@f:
          mov     ecx, eax
          call    b
          dec     rdi
          jne     SHORT $LL3@f
  
          mov     rdi, QWORD PTR [rsp+48]
  $LN1@f:
  
  f2():                             # using shifts
          mov     eax, ecx
          mov     ebx, ecx
          sar     eax, 8            # eax = v>>8
          test    eax, eax
          jle     SHORT $LN1@f2     # skip loop if v>>8 is less than or equal to 0
  
          mov     QWORD PTR [rsp+48], rdi
          mov     edi, eax
  $LL3@f2:
          test    ebx, ebx
          jle     SHORT $LN6@f2     # if v is less than or equal to 0, jump to set ecx to 0
          mov     ecx, ebx
          sar     ecx, 16           # ecx = v>>16
          jmp     SHORT $LN7@f2
  $LN6@f2:
          xor     ecx, ecx
  $LN7@f2:
          call    b
          dec     rdi
          jne     SHORT $LL3@f2
          mov     rdi, QWORD PTR [rsp+48]
  $LN1@f2:
  
  

  VS 2013 generates different rounding code for the division, using cdq/movzx (or cdq/and if shifting by something other than 8 or 16 bits).
  
  Also worth noting is that VS 2013 doesn't even bother moving the invariant ternary operator and (v/65536) or (v>>16) out of the loop. Ideally, it could move that calculation out of the loop, or remove the ternary operator completely. Ouch. I have to say, VS 2013 does seem to produce pretty good code overall, but I guess most of ours is heavily in floating point these days.

• gcc 4.4.5
  Linux x86_64
  Command line flags: -O2 -fomit-frame-pointer

  f():                              # using divides
          testl   %edi, %edi
          movl    %edi, %ebp        # ebp = edi = v
          leal    255(%rbp), %r12d  # r12d = v+255
  
          cmovns  %edi, %r12d       # set r12d to v, if v is non-negative (otherwise r12d was v+255)
          sarl    $8, %r12d         # r12d = v/256
          testl   %r12d, %r12d
          jle     .L14              # if r12d is less than or equal to 0, skip
          movl    %edi, %r14d
          xorl    %ebx, %ebx        # ebx is loop counter
          xorl    %r13d, %r13d
          sarl    $16, %r14d        # r14d = v>>16
  .L13:
          testl   %ebp, %ebp
          movl    %r13d, %edi
          cmovg   %r14d, %edi       # if v is greater than 0, use v>>16 instead of 0
          addl    $1, %ebx
          call    b
          cmpl    %r12d, %ebx
          jl      .L13
  .L14:
  
  f2():                             # using shifts
          movl    %edi, %r12d
          sarl    $8, %r12d
          testl   %r12d, %r12d
          movl    %edi, %ebp
          jle     .L6               # skip loop if (v>>8) is less than or equal to 0
          movl    %edi, %r14d
          xorl    %ebx, %ebx        # ebx is loop counter
          xorl    %r13d, %r13d
          sarl    $16, %r14d        # r14d = (v>>16)
  .L5:
          testl   %ebp, %ebp
          movl    %r13d, %edi
          cmovg   %r14d, %edi       # if v is greater than 0, use v>>16 instead of 0
          addl    $1, %ebx
          call    b
          cmpl    %r12d, %ebx
          jl      .L5
  .L6:
  
  
  

  gcc 4.4 does an interesting job, using lea to generate v+255, and then cmovns to replace it with v if v is non-negative. It doesn't bother generating rounding code for v/65536, but it does still generate rounding code for v/256, even though any non-positive result for v/256 is treated the same way throughout. Also, gcc doesn't eliminate the non-varying ternary expression, nor put the constant v/65536 or v>>16 outside of the loop.

Conclusions?

I'm not sure what to say here -- modern compilers can generate a lot of really good code, especially looking at floating point and SSE, but this makes me feel as though some of the basics have been neglected. If I were a better programmer I'd go dig into LLVM and GCC and submit patches.

I should have also tested ICC, but I've spent enough time on this, and the only ICC version we use is old enough that I would just regret not using the latest.

For comparison, here is what I would like to see LLVM generate for f():

f():                              # using divides
	cmpl	$256, %edi
	jl	LBB0_3            # if v is less than 256, skip loop

	movl	%edi, %ebx
	movl	%edi, %ebp
	shrl	$8, %ebx          # ebx = v/256, since v is non-negative
	shrl	$16, %ebp         # ebp = v/65536, since v is non-negative

LBB0_2:
	movl	%ebp, %edi
	callq	_b
	decl    %ebx
	jnz	LBB0_2
LBB0_3:

Performance-wise, I'm sure they wouldn't differ in any meaningful way, but the decrease in size would be nice.

Finally: write for the compiler you have, not the compiler you wish you had. When performance is important, use shifts instead of divides, or use unsigned types (which really should generate the same code for (x/256) vs (x>>8)). Move as much logic out of the loop as you can -- yes, the compiler might be able to do it for you, but why depend on that? But most important of all: test your assumptions.