https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102392
Bug ID: 102392
Summary: Failure to optimize out sign extension when input is
non-negative
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
#include <stdint.h>
void f(int64_t x);
void g(int32_t x)
{
if (x < 0)
__builtin_unreachable();
f(x);
}
This can be optimized to avoid the sign extension since x can't be under 0.
This optimization is done by LLVM, but not by GCC.
Sample resulting assembly from GCC:
g:
movsx rdi, edi
jmp f
from LLVM:
g:
mov edi, edi
jmp f
(PS: I originally found this while looking at the code that led me to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102391: an error check earlier in
the code (not in the example cited there) wound up making this assumption
possible, and slightly changed the assembly code emitted by LLVM there to be
even more efficient)
