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* [Bug c/110567] New: GCC fails using a register to initialize multiple variables with a constant
@ 2023-07-05 22:09 vincent.riviere at freesbee dot fr
2023-07-05 22:20 ` [Bug target/110567] " pinskia at gcc dot gnu.org
2023-07-05 22:54 ` pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: vincent.riviere at freesbee dot fr @ 2023-07-05 22:09 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110567
Bug ID: 110567
Summary: GCC fails using a register to initialize multiple
variables with a constant
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: vincent.riviere at freesbee dot fr
Target Milestone: ---
Target: m68k-elf
This is about a missed optimization for an obvious case.
GCC fails to use a register to initialize several global variables with the
same constant value.
$ cat foo.c
extern long init;
extern long g1, g2, g3, g4, g5;
void f(void)
{
long val = 12345678;
g1 = val;
g2 = val;
g3 = val;
g4 = val;
g5 = val;
}
$ m68k-elf-gcc -S foo.c -o - -Os
#NO_APP
.file "foo.c"
.text
.align 2
.globl f
.type f, @function
f:
move.l #12345678,g1
move.l #12345678,g2
move.l #12345678,g3
move.l #12345678,g4
move.l #12345678,g5
rts
.size f, .-f
.ident "GCC: (GNU) 13.1.0"
We can see that 12345678 is used many times. This is a waste of space and time.
On the other hand, if I replace 12345678 by "init" (an external global
variable) then GCC produces the expected optimized output:
$ m68k-elf-gcc -S foo.c -o - -Os
#NO_APP
.file "foo.c"
.text
.align 2
.globl f
.type f, @function
f:
move.l init,%d0
move.l %d0,g1
move.l %d0,g2
move.l %d0,g3
move.l %d0,g4
move.l %d0,g5
rts
.size f, .-f
.ident "GCC: (GNU) 13.1.0"
GCC should be able to optimize a constant initializer, just as it does with a
variable initializer.
I can reproduce the same wrong behaviour with GCC 11.4.0 for x86_64-pc-cygwin
host with -m32:
$ gcc -m32 -S foo.c -o - -Os
.file "foo.c"
.text
.globl _f
.def _f; .scl 2; .type 32; .endef
_f:
movl $12345678, _g1
movl $12345678, _g2
movl $12345678, _g3
movl $12345678, _g4
movl $12345678, _g5
ret
.ident "GCC: (GNU) 11.4.0"
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/110567] GCC fails using a register to initialize multiple variables with a constant
2023-07-05 22:09 [Bug c/110567] New: GCC fails using a register to initialize multiple variables with a constant vincent.riviere at freesbee dot fr
@ 2023-07-05 22:20 ` pinskia at gcc dot gnu.org
2023-07-05 22:54 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-07-05 22:20 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110567
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|c |target
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is a target issue where the cost model for -Os is still saying constants
are "free".
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/110567] GCC fails using a register to initialize multiple variables with a constant
2023-07-05 22:09 [Bug c/110567] New: GCC fails using a register to initialize multiple variables with a constant vincent.riviere at freesbee dot fr
2023-07-05 22:20 ` [Bug target/110567] " pinskia at gcc dot gnu.org
@ 2023-07-05 22:54 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-07-05 22:54 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110567
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=44073
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
x86 bug is PR 44073 .
^ permalink raw reply [flat|nested] 3+ messages in thread
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2023-07-05 22:09 [Bug c/110567] New: GCC fails using a register to initialize multiple variables with a constant vincent.riviere at freesbee dot fr
2023-07-05 22:20 ` [Bug target/110567] " pinskia at gcc dot gnu.org
2023-07-05 22:54 ` pinskia at gcc dot gnu.org
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