Difficulties to measure C/C++ performance

Question

I wrote a piece of C code to show a point in a discussion about optimizations and branch prediction. Then I noticed even more diverse outcome than I did expect. My goal was to w

Answer 1

With -O1, gcc-4.7.1 calls unpredictableIfs only once and resuses the result, since it recognizes that it's a pure function, so the result will be the same every time it's called. (Mine did, verified by looking at the generated assembly.)

With higher optimisation level, the functions are inlined, and the compiler doesn't recognize that it's the same code anymore, so it is run each time a function call appears in the source.

Apart from that, my gcc-4.7.1 deals best with unpredictableIfs when using -O1 or -O2 (apart from the reuse issue, both produce the same code), while noIfs is treated much better with -O3. The timings between the different runs of the same code are however consistent here - equal or differing by 10 milliseconds (granularity of clock), so I have no idea what could cause the substantially different times for unpredictableIfs you reported for -O3.

With -O2, the loop for unpredictableIfs is identical to the code generated with -O1 (except for register swapping):

.L12:
    movl    %eax, %ecx
    andl    $1073741826, %ecx
    cmpl    $1, %ecx
    adcl    $0, %edx
    addl    $1, %eax
    cmpl    $1000000000, %eax
    jne .L12

and for noIfs it's similar:

.L15:
    xorl    %ecx, %ecx
    testl   $1073741826, %eax
    sete    %cl
    addl    $1, %eax
    addl    %ecx, %edx
    cmpl    $1000000000, %eax
    jne .L15

where it was

.L7:
    testl   $1073741826, %edx
    sete    %cl
    movzbl  %cl, %ecx
    addl    %ecx, %eax
    addl    $1, %edx
    cmpl    $1000000000, %edx
    jne .L7

with -O1. Both loops run in similar time, with unpredictableIfs a bit faster.

With -O3, the loop for unpredictableIfs becomes worse,

.L14:
    leal    1(%rdx), %ecx
    testl   $1073741826, %eax
    cmove   %ecx, %edx
    addl    $1, %eax
    cmpl    $1000000000, %eax
    jne     .L14

and for noIfs (including the setup-code here), it becomes better:

    pxor    %xmm2, %xmm2
    movq    %rax, 32(%rsp)
    movdqa  .LC3(%rip), %xmm6
    xorl    %eax, %eax
    movdqa  .LC2(%rip), %xmm1
    movdqa  %xmm2, %xmm3
    movdqa  .LC4(%rip), %xmm5
    movdqa  .LC5(%rip), %xmm4
    .p2align 4,,10
    .p2align 3
.L18:
    movdqa  %xmm1, %xmm0
    addl    $1, %eax
    paffffd   %xmm6, %xmm1
    cmpl    $250000000, %eax
    pand    %xmm5, %xmm0
    pcmpeqd %xmm3, %xmm0
    pand    %xmm4, %xmm0
    paffffd   %xmm0, %xmm2
    jne .L18

.LC2:
    .long   0
    .long   1
    .long   2
    .long   3
    .align 16
.LC3:
    .long   4
    .long   4
    .long   4
    .long   4
    .align 16
.LC4:
    .long   1073741826
    .long   1073741826
    .long   1073741826
    .long   1073741826
    .align 16
.LC5:
    .long   1
    .long   1
    .long   1
    .long   1

it computes four iterations at once, and accordingly, noIfs runs almost four times as fast then.