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* [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
@ 2023-01-11 13:47 dhowells at redhat dot com
2023-01-11 14:04 ` [Bug c/108370] " rguenth at gcc dot gnu.org
` (7 more replies)
0 siblings, 8 replies; 9+ messages in thread
From: dhowells at redhat dot com @ 2023-01-11 13:47 UTC (permalink / raw)
To: gcc-bugs
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>
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c/108370] gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
@ 2023-01-11 14:04 ` rguenth at gcc dot gnu.org
2023-01-11 14:24 ` rguenth at gcc dot gnu.org
` (6 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-01-11 14:04 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2023-01-11
Ever confirmed|0 |1
Keywords| |missed-optimization
Status|UNCONFIRMED |NEW
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The good case is handled by ifcombine:
optimizing bits or bits test to _5 & T != 0
with temporary T = 2 | 1
Merging blocks 2 and 3
<bb 2> [local count: 1073741824]:
_5 = bio_4(D)->bi_flags;
_8 = _5 & 1;
if (_8 != 0)
goto <bb 4>; [33.00%]
else
goto <bb 3>; [67.00%]
<bb 3> [local count: 719407025]:
_9 = _5 & 2;
if (_9 != 0)
goto <bb 4>; [50.00%]
else
goto <bb 5>; [50.00%]
<bb 4> [local count: 714038313]:
_1 = (int) mark_dirty_6(D);
__bio_release_pages (bio_4(D), _1);
but the bad case is not handled:
<bb 2> [local count: 1073741824]:
_6 = bio_4(D)->bi_flags;
_5 = (unsigned int) _6;
_9 = (_Bool) _6;
if (_9 != 0)
goto <bb 4>; [33.00%]
else
goto <bb 3>; [67.00%]
<bb 3> [local count: 719407025]:
_10 = _5 >> 1;
_11 = (_Bool) _10;
if (_11 != 0)
goto <bb 4>; [50.00%]
else
goto <bb 5>; [50.00%]
<bb 4> [local count: 714038313]:
_1 = (int) mark_dirty_7(D);
__bio_release_pages (bio_4(D), _1);
it looks like some premature optimization triggered (not) there.
.original is (good)
;; Function bio_flagged (null)
;; enabled by -tree-original
{
return ((unsigned int) bio->bi_flags & 1 << bit) != 0;
}
vs (bad)
;; Function bio_flagged (null)
;; enabled by -tree-original
{
return ((unsigned int) bio->bi_flags >> bit & 1) != 0;
}
I suppose one variant is folded after the promotion to bool and one
before only.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c/108370] gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
2023-01-11 14:04 ` [Bug c/108370] " rguenth at gcc dot gnu.org
@ 2023-01-11 14:24 ` rguenth at gcc dot gnu.org
2023-01-11 14:42 ` dhowells at redhat dot com
` (5 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-01-11 14:24 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
ifcombine seems to assume that
D.1987_7 = op0 & 1;
if (D.1987_7 != 0)
is canonical but we see
_9 = (_Bool) _6;
if (_9 != 0)
instead. That's already the form introduced by inlining from
_Bool bio_flagged (struct bio * bio, unsigned int bit)
{
short unsigned int _1;
unsigned int _2;
unsigned int _3;
unsigned int _4;
_Bool _8;
<bb 2> :
_1 = bio_6(D)->bi_flags;
_2 = (unsigned int) _1;
_3 = _2 >> bit_7(D);
_4 = _3 & 1;
_8 = _4 != 0;
return _8;
and
<bb 2> :
_1 = bio_flagged (bio_7(D), 0);
if (_1 != 0)
goto <bb 4>; [INV]
else
goto <bb 3>; [INV]
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c/108370] gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
2023-01-11 14:04 ` [Bug c/108370] " rguenth at gcc dot gnu.org
2023-01-11 14:24 ` rguenth at gcc dot gnu.org
@ 2023-01-11 14:42 ` dhowells at redhat dot com
2023-02-08 20:30 ` [Bug middle-end/108370] " pinskia at gcc dot gnu.org
` (4 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: dhowells at redhat dot com @ 2023-01-11 14:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
--- Comment #3 from dhowells at redhat dot com <dhowells at redhat dot com> ---
We don't want to do:
return ((unsigned int) bio->bi_flags >> bit & 1) != 0;
if we can avoid it as "bit" is usually constant - though I'm guessing the
optimiser should handle that?
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug middle-end/108370] gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
` (2 preceding siblings ...)
2023-01-11 14:42 ` dhowells at redhat dot com
@ 2023-02-08 20:30 ` pinskia at gcc dot gnu.org
2023-09-16 22:58 ` pinskia at gcc dot gnu.org
` (3 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-02-08 20:30 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
Component|c |middle-end
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug middle-end/108370] gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
` (3 preceding siblings ...)
2023-02-08 20:30 ` [Bug middle-end/108370] " pinskia at gcc dot gnu.org
@ 2023-09-16 22:58 ` pinskia at gcc dot gnu.org
2023-09-16 23:16 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-09-16 22:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Assignee|unassigned at gcc dot gnu.org |pinskia at gcc dot gnu.org
Status|NEW |ASSIGNED
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Mine.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug middle-end/108370] gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
` (4 preceding siblings ...)
2023-09-16 22:58 ` pinskia at gcc dot gnu.org
@ 2023-09-16 23:16 ` pinskia at gcc dot gnu.org
2023-09-16 23:48 ` pinskia at gcc dot gnu.org
2023-09-17 0:35 ` pinskia at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-09-16 23:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
_10 = _9 >> 1;
_11 = (bool) _10;
if (_11 != 0)
Should just be optimized to:
_t = _9 & 1
if (_t != 0)
Let me add that to match.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug middle-end/108370] gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
` (5 preceding siblings ...)
2023-09-16 23:16 ` pinskia at gcc dot gnu.org
@ 2023-09-16 23:48 ` pinskia at gcc dot gnu.org
2023-09-17 0:35 ` pinskia at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-09-16 23:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #5)
> _10 = _9 >> 1;
> _11 = (bool) _10;
> if (_11 != 0)
>
>
> Should just be optimized to:
> _t = _9 & 1
> if (_t != 0)
>
> Let me add that to match.
We do handle:
/* Fold ((X << C1) & C2) cmp C3 into (X & (C2 >> C1)) cmp (C3 >> C1)
((X >> C1) & C2) cmp C3 into (X & (C2 << C1)) cmp (C3 << C1). */
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug middle-end/108370] gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given
2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
` (6 preceding siblings ...)
2023-09-16 23:48 ` pinskia at gcc dot gnu.org
@ 2023-09-17 0:35 ` pinskia at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-09-17 0:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370
--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 55914
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55914&action=edit
Patch
Combined with the patch for PR 109960. We are able to optimize this correctly:
_5 = bio_4(D)->bi_flags;
_8 = _5 & 3;
if (_8 != 0)
goto <bb 3>; [66.50%]
else
goto <bb 4>; [33.50%]
<bb 3> [local count: 714038312]:
_1 = (int) mark_dirty_6(D);
__bio_release_pages (bio_4(D), _1); [tail call]
^ permalink raw reply [flat|nested] 9+ messages in thread
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2023-01-11 13:47 [Bug c/108370] New: gcc doesn't merge bitwise-AND if an explicit comparison against 0 is given dhowells at redhat dot com
2023-01-11 14:04 ` [Bug c/108370] " rguenth at gcc dot gnu.org
2023-01-11 14:24 ` rguenth at gcc dot gnu.org
2023-01-11 14:42 ` dhowells at redhat dot com
2023-02-08 20:30 ` [Bug middle-end/108370] " pinskia at gcc dot gnu.org
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