https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89163
Bug ID: 89163
Summary: Missed optimization: sar and shr equivalent for
non-negative numbers
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: tspiteri at ieee dot org
Target Milestone: ---
For the minimized test case below, shift is equivalent to same_as, because shr
and sar are equivalent for non-negative numbers. The generated code is not the
same.
int shift(int i) {
return i >= 0 ? (unsigned)i >> 8 : i >> 8;
}
int same_as(int i) {
return i >> 8;
}
shift:
movl %edi, %edx
movl %edi, %eax
shrl $8, %edx
sarl $8, %eax
testl %edi, %edi
cmovns %edx, %eax
ret
same_as:
movl %edi, %eax
sarl $8, %eax
ret
