https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108477
Bug ID: 108477
Summary: fwprop over-optimizes conversion from + to |
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ubizjak at gmail dot com
Target Milestone: ---
In the unsigned int case (baz) fwprop over-optimizes the addition to a logical
or:
--cut here--
int lock;
int bar (int old)
{
int val = (old >> 1) & 0x1;
int new = (old & ~0x3) + 0x2 + val;
lock = new;
return val ? 0 : -1;
}
int ulock;
int baz (unsigned int old)
{
unsigned int val = (old >> 1) & 0x1;
unsigned int new = (old & ~0x3) + 0x2 + val;
ulock = new;
return val ? 0 : -1;
}
--cut here--
resulting in:
bar:
movl %edi, %eax
andl $-4, %edi
sarl %eax
andl $1, %eax
leal 2(%rax,%rdi), %edx <---- here
subl $1, %eax
movl %edx, lock(%rip)
ret
baz:
movl %edi, %eax
andl $-4, %edi
shrl %eax
andl $1, %eax
orl %eax, %edi <--- here ...
subl $1, %eax
addl $2, %edi <--- ... and here
movl %edi, ulock(%rip)
ret
Please note the three-operand addition, implemented with LEAL instruction in
the signed case, which is not emitted in the unsigned case. The reason is
fwprop pass that substitutes addition with the equivalent or operation.
