https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
Bug ID: 108370
Summary: gcc doesn't merge bitwise-AND if an explicit
comparison against 0 is given
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: dhowells at redhat dot com
Target Milestone: ---
Created attachment 54245
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54245&action=edit
Demo code
If gcc sees a couple of calls to an inline function that does a bitwise-AND and
returns whether the result was zero or non-zero (e.g. a flag check helper), gcc
cannot merge them if the result of the AND is explicitly compared against 0,
even if the function's return type is a bool (which would do that anyway). For
example:
static inline bool bio_flagged(struct bio *bio, unsigned int bit)
{
return (bio->bi_flags & (1U << bit)) != 0;
}
void bio_release_pages(struct bio *bio, bool mark_dirty)
{
if (bio_flagged(bio, BIO_PAGE_REFFED) ||
bio_flagged(bio, BIO_PAGE_PINNED))
__bio_release_pages(bio, mark_dirty);
}
compiles bio_release_pages() to:
0: 66 8b 07 mov (%rdi),%ax
3: a8 01 test $0x1,%al
5: 75 04 jne b <bio_release_pages+0xb>
7: a8 02 test $0x2,%al
9: 74 09 je 14 <bio_release_pages+0x14>
b: 40 0f b6 f6 movzbl %sil,%esi
f: e9 00 00 00 00 jmp 14 <bio_release_pages+0x14>
14: c3 ret
but:
static inline bool bio_flagged(struct bio *bio, unsigned int bit)
{
return bio->bi_flags & (1U << bit);
}
gives:
0: f6 07 03 testb $0x3,(%rdi)
3: 74 09 je e <bio_release_pages+0xe>
5: 40 0f b6 f6 movzbl %sil,%esi
9: e9 00 00 00 00 jmp e <bio_release_pages+0xe>
e: c3 ret
Possibly the comparison against 0 could be optimised away.
I've attached some demo code that can be compiled with one of:
gcc -Os -c gcc-bool-demo.c
gcc -Os -c gcc-bool-demo.c -Dfix
The gcc I used above is the Fedora 37 system compiler:
gcc-12.2.1-4.fc37.x86_64
binutils-2.38-25.fc37.x86_64
but similar results can be seen with the Fedora arm cross-compiler:
0: e1d030b0 ldrh r3, [r0]
4: e3130001 tst r3, #1
8: 1a000001 bne 14 <bio_release_pages+0x14>
c: e3130002 tst r3, #2
10: 012fff1e bxeq lr
14: eafffffe b 0 <__bio_release_pages>
vs
0: e1d030b0 ldrh r3, [r0]
4: e3130003 tst r3, #3
8: 012fff1e bxeq lr
c: eafffffe b 0 <__bio_release_pages>
