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* [Bug target/49421] New: [arm] suboptimal choice of working regs
@ 2011-06-15 12:02 philb at gnu dot org
2011-06-29 20:47 ` [Bug target/49421] " ramana at gcc dot gnu.org
2024-03-28 6:23 ` pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: philb at gnu dot org @ 2011-06-15 12:02 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49421
Summary: [arm] suboptimal choice of working regs
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: philb@gnu.org
If a leaf function requires one more working register than can be accomodated
in the call-clobbered set, gcc currently tends to push r4 and use that next.
However, in the specific case of a leaf function, it would be better to push lr
and use that as the working register, since then the return can be done with a
single pop. Consider the made-up example:
int f(int *a, int *b, int *c, int *d)
{
int i;
for (i = 0; i < 4; i++)
if (a[i] || b[i] || c[i] || d[i])
return 1;
return 0;
}
which compiles (-march=armv6 -mtune=arm1136jf-s -O2) to:
f:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
mov ip, #0
str r4, [sp, #-4]!
.L3:
ldr r4, [r0, ip]
cmp r4, #0
bne .L7
ldr r4, [r1, ip]
cmp r4, #0
bne .L7
ldr r4, [r2, ip]
cmp r4, #0
bne .L7
ldr r4, [r3, ip]
add ip, ip, #4
cmp r4, #0
bne .L7
cmp ip, #16
bne .L3
mov r0, r4
.L2:
ldmfd sp!, {r4}
bx lr
.L7:
mov r0, #1
b .L2
If lr had been pushed instead of r4 then the return could have simply been "pop
{lr}".
Also, since this is arm11, it is no more expensive to push two words than one.
If the compiler had stacked both r4 and lr, it would have freed up an extra
register for the loop which would probably have allowed the loads to be
scheduled better.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/49421] [arm] suboptimal choice of working regs
2011-06-15 12:02 [Bug target/49421] New: [arm] suboptimal choice of working regs philb at gnu dot org
@ 2011-06-29 20:47 ` ramana at gcc dot gnu.org
2024-03-28 6:23 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: ramana at gcc dot gnu.org @ 2011-06-29 20:47 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49421
Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Status|UNCONFIRMED |NEW
Last reconfirmed| |2011.06.29 20:47:18
CC| |ramana at gcc dot gnu.org
Ever Confirmed|0 |1
--- Comment #1 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2011-06-29 20:47:18 UTC ---
confirmed -
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/49421] [arm] suboptimal choice of working regs
2011-06-15 12:02 [Bug target/49421] New: [arm] suboptimal choice of working regs philb at gnu dot org
2011-06-29 20:47 ` [Bug target/49421] " ramana at gcc dot gnu.org
@ 2024-03-28 6:23 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-03-28 6:23 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49421
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |FIXED
Status|NEW |RESOLVED
Known to work| |5.4.0
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed since at least GCC 5.
lr is the only thing pushed/poped:
str lr, [sp, #-4]!
...
ldr pc, [sp], #4
.L7:
mov r0, #1
ldr pc, [sp], #4
^ permalink raw reply [flat|nested] 3+ messages in thread
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2011-06-15 12:02 [Bug target/49421] New: [arm] suboptimal choice of working regs philb at gnu dot org
2011-06-29 20:47 ` [Bug target/49421] " ramana at gcc dot gnu.org
2024-03-28 6:23 ` pinskia at gcc dot gnu.org
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