https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111698
Bug ID: 111698
Summary: Narrow memory access of compare to byte width
Product: gcc
Version: 12.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: ubizjak at gmail dot com
Target Milestone: ---
Following testcase:
--cut here--
int m;
_Bool foo (void)
{
return m & 0x0f0000;
}
--cut here--
compiles to:
0: f7 05 00 00 00 00 00 testl $0xf0000,0x0(%rip)
7: 00 0f 00
The test instruction can be demoted to byte test from addr+2.
Currently, the demotion works for lowest byte, so the testcase:
--cut here--
int m;
_Bool foo (void)
{
return m & 0x0f;
}
--cut here--
compiles to:
0: f6 05 00 00 00 00 0f testb $0xf,0x0(%rip)
which is three bytes shorter.
Any half-way modern Intel and AMD cores will forward any fully contained load,
so there is no danger of forwarding stall with recent CPU cores.
