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

Right, looking at the assembler code from gcc on 64-bit Linux, the first case, with -O1, the function UnpredictableIfs is indeed called only once, and the result reused.

With -O2 and -O3, the functions are inlined, and the time it takes should be identical. There is also no actual branches in either bit of code, but the translation for the two bits of code is somewhat different, I've cut out the lines that update "sum" [in %edx in both cases]

UnpredictableIfs:

movl    %eax, %ecx
andl    $1073741826, %ecx
cmpl    $1, %ecx
adcl    $0, %edx
addl    $1, %eax

NoIfs:

xorl    %ecx, %ecx
testl   $1073741826, %eax
sete    %cl
addl    $1, %eax
addl    %ecx, %edx

As you can see, it's not quite identical, but it does very similar things.