https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96938
Bug ID: 96938
Summary: Failure to optimize bit-setting pattern when not using
temporary
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
void g(char *f, int offset, char value)
{
*f = (int)(*f & ~(1 << (offset & 0x1F))) | (value << (offset & 0x1F));
}
This has much worse code generation than this:
void g(char *f, int offset, char value)
{
int tmp = *f & ~(1 << (offset & 0x1F));
*f = tmp | (value << (offset & 0x1F));
}
Which should be equivalent to the first example.
Example of the worse code generation, on x86 the first example compiles to
this:
g(char*, int, char):
movzx ecx, BYTE PTR [rdi]
mov eax, 1
movsx edx, dl
shlx eax, eax, esi
shlx edx, edx, esi
andn eax, eax, ecx
or eax, edx
mov BYTE PTR [rdi], al
ret
Whereas the second example compiles to this:
g(char*, int, char):
movsx eax, BYTE PTR [rdi]
movsx edx, dl
shlx edx, edx, esi
btr eax, esi
or eax, edx
mov BYTE PTR [rdi], al
ret
