https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80917
Bug ID: 80917
Summary: missed bit information propagation
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: drepper.fsp+rhbz at gmail dot com
Target Milestone: ---
Take the following code:
int f(unsigned a)
{
if ((a & 2) == 0)
return 0;
a += 4;
return (a & 2) != 0;
}
The addition clearly cannot affect the repeat of the test of the second bit.
Still, with the current trunk compiler I get with -O3 on x86-64:
xorl %eax, %eax
testb $2, %dil
je .L1
shrl %edi
movl %edi, %eax
andl $1, %eax
.L1:
ret
It might get quickly expansive but the bit set/unset information could be
tracked passed the test and through arithmetic operations like additions.
