While I shouldn't list out the entire 4 line sample I'm given, (since this is a homework question) I'm confused how this should be read and translated into C.
cmovge %edi, %eax
What I understand so far is that the instruction is a conditional move for when the result is >=. It's comparing the first parameter of a function %edi to the integer register %eax (which was assigned the other parameter value %esi in the previous line of assembly code). However, I don't understand its result.
My problem is interpreting the optimized code. It doesn't manipulate the stack, and I'm not sure how to write this in C (or at least the gcc switch I could even use to generate the same result when compiling).
Could someone please give a few small examples of how the cmovge instruction might translate into C code? If it doesn't make sense as its own line of code, feel free to make something up with it.
This is in x86-64 assembly through a virtualized Linux operating system (CentOS 7).
I'm probably giving you the whole solution here:
int
doit(int a, int b) {
return a >= b ? a : b;
}
With gcc -O3 -masm=intel becomes:
doit:
.LFB0:
.cfi_startproc
cmp edi, esi
mov eax, esi
cmovge eax, edi
ret
.cfi_endproc