https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88603
Bug ID: 88603
Summary: optimization missed for saturation arithmetic add
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: SztfG at yandex dot ru
Target Milestone: ---
Target: x86_64-linux-gnu
example:
#include <inttypes.h>
uint32_t saturation_add(uint32_t a, uint32_t b)
{
const uint64_t tmp = (uint64_t)a + b;
if (tmp > UINT32_MAX)
{
return UINT32_MAX;
}
return tmp;
}
output:
mov edx, esi
mov eax, edi
add edi, esi # Why need to add two times here
add rax, rdx # and here ?
mov edx, 4294967295
cmp rax, rdx
mov eax, -1 # Why? edx already have this value. -1 and 4294967295
are same
cmovbe eax, edi
ret
better do something like this:
add edi, esi
mov eax, -1
cmovae eax, edi
ret
