I tested with gcc version 4.9.2 (Debian 4.9.2-10) targeting x86_64.
The snippet you posted compiles into a single 'ret' instruction under
most optimization levels since sum has no visible effect.
When I change the return type of sum to int and return s, most
optimization levels (even just -O) seem to do the hoisting you desire.
The loop became
.L3:
addl (%rdx), %eax
addq $4, %rdx
cmpq %rcx, %rdx
jne .L3
rep ret
which I read roughly as
do { eax += *rdx; rdx += 4; } while(rcx != rdx)
I do not have an aarch64 compiler handy, but I do have an arm compiler,
so I checked it. Again under -Os,
.L3:
ldr r2, [r3], #4
add r0, r0, r2
cmp r3, r1
bne .L3
bx lr
which I read the same as the x86_64 code above.
Jeff
