This question is a followup to this one: Question on Undefined Behaviour (UB) and loop optimization
When using https://godbolt.org/ online compiler (g++ 7.2, x86_64) with -O2 setting, the loop condition gets optimized out when the loop contains std::cout but not with an identical loop using printf.
Any idea why? Both code versions are compiled with the C++ compiler. Both versions produce an UB warning, but no warning without -O2, even though the code is still in UB land.
FWIW, I also tried the MIPS g++ compiler, that one does not seem to optimize out the loop condition even with std::cout code version and -O2.
I can provide the compiler outputs if necessary, but the std::cout version is quite long.
#include <stdio.h>
int main()
{
for (int i = 0; i < 300; i++)
printf("%d %d", i, i*12345678);
}
/*#include <iostream>
int main()
{
for (int i = 0; i < 300; i++)
std::cout << i << " " << i * 12345678 << std::endl;
}*/
UPDATE: On suggestion from the comments, I removed the UB, then even the printf version removes the loop condition, instead jumping out of the loop when i is 11 (very unsurprising), see below:
#include <stdio.h>
int main()
{
for (int i = 0; i < 300; i++) {
printf("%d %d", i, i*123);
if (i * 123 > 1230) break;
}
}
// Generated assembly:
LC0:
.string "%d %d"
main:
push rbp
push rbx
xor edx, edx
xor esi, esi
mov edi, OFFSET FLAT:.LC0
xor eax, eax
sub rsp, 8
mov ebp, 123
xor ebx, ebx
call printf
.L2:
add ebx, 1
mov edx, ebp
xor eax, eax
mov esi, ebx
mov edi, OFFSET FLAT:.LC0
add ebp, 123
call printf
cmp ebx, 11
jne .L2
add rsp, 8
xor eax, eax
pop rbx
pop rbp
ret
