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* [Bug target/96176] New: Failure to omit extraneous movzx in atomic compare exchange with unsigned char
@ 2020-07-12 23:49 gabravier at gmail dot com
2020-07-13 10:48 ` [Bug target/96176] " jakub at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: gabravier at gmail dot com @ 2020-07-12 23:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96176
Bug ID: 96176
Summary: Failure to omit extraneous movzx in atomic compare
exchange with unsigned char
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
void f(unsigned char *addr, unsigned char old_val, unsigned char new_val)
{
__atomic_compare_exchange_n(addr, &old_val, new_val, 0, 0, 0);
}
On x86 with -O3, LLVM generates this :
f(unsigned char*, unsigned char, unsigned char): # @f(unsigned char*, unsigned
char, unsigned char)
mov eax, esi
lock cmpxchg byte ptr [rdi], dl
ret
GCC generates this :
f(unsigned char*, unsigned char, unsigned char):
mov eax, esi
movzx edx, dl
lock cmpxchg BYTE PTR [rdi], dl
ret
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug target/96176] Failure to omit extraneous movzx in atomic compare exchange with unsigned char
2020-07-12 23:49 [Bug target/96176] New: Failure to omit extraneous movzx in atomic compare exchange with unsigned char gabravier at gmail dot com
@ 2020-07-13 10:48 ` jakub at gcc dot gnu.org
2020-07-13 10:48 ` jakub at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2020-07-13 10:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96176
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The zero extension is initially there because the builtin has the arguments
promoted. Normally combine would fix this up:
(insn 10 7 11 2 (set (reg:SI 88 [ new_val ])
(zero_extend:SI (subreg:QI (reg:SI 93) 0))) "pr96176.c":3:5 143
{*zero_extendqisi2}
(expr_list:REG_DEAD (reg:SI 93)
(nil)))
(insn 11 10 0 2 (parallel [
(set (reg:QI 89)
(unspec_volatile:QI [
(mem/v:QI (reg/v/f:DI 83 [ addr ]) [-1 S1 A8])
(reg/v:QI 84 [ old_val ])
(subreg:QI (reg:SI 88 [ new_val ]) 0)
(const_int 0 [0])
] UNSPECV_CMPXCHG))
(set (mem/v:QI (reg/v/f:DI 83 [ addr ]) [-1 S1 A8])
(unspec_volatile:QI [
(const_int 0 [0])
] UNSPECV_CMPXCHG))
(set (reg:CCZ 17 flags)
(unspec_volatile:CCZ [
(const_int 0 [0])
] UNSPECV_CMPXCHG))
]) "pr96176.c":3:5 6114 {atomic_compare_and_swapqi_1}
(expr_list:REG_DEAD (reg:SI 88 [ new_val ])
(expr_list:REG_DEAD (reg/v:QI 84 [ old_val ])
(expr_list:REG_DEAD (reg/v/f:DI 83 [ addr ])
(expr_list:REG_UNUSED (reg:QI 89)
(expr_list:REG_UNUSED (reg:CCZ 17 flags)
(nil)))))))
but it fails:
Trying 10 -> 11:
10: r88:SI=zero_extend(r93:SI#0)
REG_DEAD r93:SI
11: {r89:QI=unspec/v[[r83:DI],r84:QI,r88:SI#0,0] 90;[r83:DI]=unspec/v[0]
90;flags:CCZ=unspec/v[0] 90;}
REG_DEAD r88:SI
REG_DEAD r84:QI
REG_DEAD r83:DI
REG_UNUSED r89:QI
REG_UNUSED flags:CCZ
Failed to match this instruction:
(parallel [
(set (reg:QI 89)
(unspec_volatile:QI [
(mem/v:QI (reg/v/f:DI 83 [ addr ]) [-1 S1 A8])
(reg/v:QI 84 [ old_val ])
(subreg:QI (reg:SI 93) 0)
(const_int 0 [0])
] UNSPECV_CMPXCHG))
(set (mem/v:QI (reg/v/f:DI 83 [ addr ]) [-1 S1 A8])
(unspec_volatile:QI [
(const_int 0 [0])
] UNSPECV_CMPXCHG))
(set (reg:CCZ 17 flags)
(unspec_volatile:CCZ [
(const_int 0 [0])
] UNSPECV_CMPXCHG))
])
due to the MEM_VOLATILE_P and volatile_ok not being true during combine.
But we can do something about it during expansion.
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug target/96176] Failure to omit extraneous movzx in atomic compare exchange with unsigned char
2020-07-12 23:49 [Bug target/96176] New: Failure to omit extraneous movzx in atomic compare exchange with unsigned char gabravier at gmail dot com
2020-07-13 10:48 ` [Bug target/96176] " jakub at gcc dot gnu.org
@ 2020-07-13 10:48 ` jakub at gcc dot gnu.org
2020-07-15 9:29 ` cvs-commit at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2020-07-13 10:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96176
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2020-07-13
Assignee|unassigned at gcc dot gnu.org |jakub at gcc dot gnu.org
Status|UNCONFIRMED |ASSIGNED
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 48868
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48868&action=edit
gcc11-pr96176.patch
Untested fix.
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug target/96176] Failure to omit extraneous movzx in atomic compare exchange with unsigned char
2020-07-12 23:49 [Bug target/96176] New: Failure to omit extraneous movzx in atomic compare exchange with unsigned char gabravier at gmail dot com
2020-07-13 10:48 ` [Bug target/96176] " jakub at gcc dot gnu.org
2020-07-13 10:48 ` jakub at gcc dot gnu.org
@ 2020-07-15 9:29 ` cvs-commit at gcc dot gnu.org
2020-07-15 10:00 ` jakub at gcc dot gnu.org
2024-01-20 17:16 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2020-07-15 9:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96176
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:
https://gcc.gnu.org/g:410675cb63466d8de9ad590521f0766b012d2475
commit r11-2103-g410675cb63466d8de9ad590521f0766b012d2475
Author: Jakub Jelinek <jakub@redhat.com>
Date: Wed Jul 15 11:26:22 2020 +0200
builtins: Avoid useless char/short -> int promotions before atomics
[PR96176]
As mentioned in the PR, we generate a useless movzbl insn before lock
cmpxchg.
The problem is that the builtin for the char/short cases has the arguments
promoted to int and combine gives up, because the instructions have
MEM_VOLATILE_P arguments and recog in that case doesn't recognize anything
when volatile_ok is false, and nothing afterwards optimizes the
(reg:SI a) = (zero_extend:SI (reg:QI a))
... (subreg:QI (reg:SI a) 0) ...
The following patch fixes it at expansion time, we already have a function
that is meant to undo the promotion, so this just adds the very common case
to that.
2020-07-15 Jakub Jelinek <jakub@redhat.com>
PR target/96176
* builtins.c: Include gimple-ssa.h, tree-ssa-live.h and
tree-outof-ssa.h.
(expand_expr_force_mode): If exp is a SSA_NAME with different mode
from MODE and get_gimple_for_ssa_name is a cast from MODE, use the
cast's rhs.
* gcc.target/i386/pr96176.c: New test.
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug target/96176] Failure to omit extraneous movzx in atomic compare exchange with unsigned char
2020-07-12 23:49 [Bug target/96176] New: Failure to omit extraneous movzx in atomic compare exchange with unsigned char gabravier at gmail dot com
` (2 preceding siblings ...)
2020-07-15 9:29 ` cvs-commit at gcc dot gnu.org
@ 2020-07-15 10:00 ` jakub at gcc dot gnu.org
2024-01-20 17:16 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2020-07-15 10:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96176
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |FIXED
Status|ASSIGNED |RESOLVED
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed for 11+.
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug target/96176] Failure to omit extraneous movzx in atomic compare exchange with unsigned char
2020-07-12 23:49 [Bug target/96176] New: Failure to omit extraneous movzx in atomic compare exchange with unsigned char gabravier at gmail dot com
` (3 preceding siblings ...)
2020-07-15 10:00 ` jakub at gcc dot gnu.org
@ 2024-01-20 17:16 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-01-20 17:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96176
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |11.0
^ permalink raw reply [flat|nested] 6+ messages in thread
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2020-07-12 23:49 [Bug target/96176] New: Failure to omit extraneous movzx in atomic compare exchange with unsigned char gabravier at gmail dot com
2020-07-13 10:48 ` [Bug target/96176] " jakub at gcc dot gnu.org
2020-07-13 10:48 ` jakub at gcc dot gnu.org
2020-07-15 9:29 ` cvs-commit at gcc dot gnu.org
2020-07-15 10:00 ` jakub at gcc dot gnu.org
2024-01-20 17:16 ` pinskia at gcc dot gnu.org
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