https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125811
Bug ID: 125811
Summary: Possible Missed Optimization at O3
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: apowenq at gmail dot com
Target Milestone: ---
cat prog.c
#include <stdint.h>
uint8_t src(uint8_t v0_u8) {
if (!((0 <= v0_u8) && (v0_u8 <= 2))) __builtin_unreachable();
uint8_t i0_u8 = 1 << v0_u8;
uint8_t i1_u8 = v0_u8 * i0_u8;
return i1_u8;
}
GCC truck at O3 cannot optimize the C program, but Clang truck at O3 can.
Reproducer: https://godbolt.org/z/WqhqoEKb3
GCC at O3:
src:
mov ecx, edi
mov edx, 1
sal edx, cl
mov eax, edx
imul eax, edi
ret
Clang at O3:
src:
mov eax, edi
mov ecx, eax
shl al, cl
ret
If change `uint8_t` to `int`, GCC O3 can optimize the program like Clang.
