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* [Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value
@ 2012-10-22 16:39 mans at mansr dot com
2012-10-22 16:41 ` [Bug target/55026] " pinskia at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: mans at mansr dot com @ 2012-10-22 16:39 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026
Bug #: 55026
Summary: [Multiple targets] Inefficient code with structs
passed by value
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: mans@mansr.com
Multiple targets (including ARM, HPPA, and MIPS) generate inefficient code in
functions taking struct arguments by value. Consider this code:
struct foo {
int a;
int b;
};
int f(struct foo x)
{
return x.a + x.b;
}
On ARM, this compiles to the following at -O3 optimisation:
f:
@ args = 0, pretend = 0, frame = 8
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
sub sp, sp, #8
add r3, sp, #8
stmdb r3, {r0, r1}
ldmia sp, {r0, r3}
add r0, r0, r3
add sp, sp, #8
@ sp needed
bx lr
Note the entirely unnecessary (and inefficiently done to boot) storing and
loading of the argument registers to/from the stack.
The x86_64 and SH4 targets do not show this behaviour.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/55026] [Multiple targets] Inefficient code with structs passed by value
2012-10-22 16:39 [Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value mans at mansr dot com
@ 2012-10-22 16:41 ` pinskia at gcc dot gnu.org
2012-10-22 19:08 ` [Bug target/55026] Useless stores generated for structures " ebotcazou at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2012-10-22 16:41 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-10-22 16:41:09 UTC ---
I think I already filed this bug about 4 years ago or so.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/55026] Useless stores generated for structures passed by value
2012-10-22 16:39 [Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value mans at mansr dot com
2012-10-22 16:41 ` [Bug target/55026] " pinskia at gcc dot gnu.org
@ 2012-10-22 19:08 ` ebotcazou at gcc dot gnu.org
2012-10-22 19:34 ` mans at mansr dot com
2012-10-22 19:38 ` mans at mansr dot com
3 siblings, 0 replies; 5+ messages in thread
From: ebotcazou at gcc dot gnu.org @ 2012-10-22 19:08 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026
Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2012-10-22
CC| |ebotcazou at gcc dot
| |gnu.org
Summary|[Multiple targets] |Useless stores generated
|Inefficient code with |for structures passed by
|structs passed by value |value
Ever Confirmed|0 |1
Severity|normal |enhancement
--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-10-22 19:08:10 UTC ---
This depends on the internal argument passing mechanism (BLKmode registers vs
PARALLEL) and on the strict-alignment requirements of the architecture. Try:
struct __attribute__((aligned (8))) foo {
int a;
int b;
};
int f(struct foo x)
{
return x.a + x.b;
}
to watch the effects of the latter.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/55026] Useless stores generated for structures passed by value
2012-10-22 16:39 [Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value mans at mansr dot com
2012-10-22 16:41 ` [Bug target/55026] " pinskia at gcc dot gnu.org
2012-10-22 19:08 ` [Bug target/55026] Useless stores generated for structures " ebotcazou at gcc dot gnu.org
@ 2012-10-22 19:34 ` mans at mansr dot com
2012-10-22 19:38 ` mans at mansr dot com
3 siblings, 0 replies; 5+ messages in thread
From: mans at mansr dot com @ 2012-10-22 19:34 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026
--- Comment #3 from Mans Rullgard <mans at mansr dot com> 2012-10-22 19:34:25 UTC ---
It has actually got worse over time. With 4.3 I get this:
f:
sub sp, sp, #8
mov r2, r0
stmia sp, {r0, r1}
add r0, r1, r2
add sp, sp, #8
bx lr
Here it's only doing the stores, not the loads, and it does them using sp
directly.
With 4.4 and 4.5 it gets slightly worse:
f:
sub sp, sp, #8
mov r3, sp
stmia r3, {r0, r1}
mov r3, r0
add r0, r1, r3
add sp, sp, #8
bx lr
Now it's copying sp to another register used as base address in the store. The
load is still absent.
4.6 and later produce the code I quoted originally. I can't say I like the
trend.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/55026] Useless stores generated for structures passed by value
2012-10-22 16:39 [Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value mans at mansr dot com
` (2 preceding siblings ...)
2012-10-22 19:34 ` mans at mansr dot com
@ 2012-10-22 19:38 ` mans at mansr dot com
3 siblings, 0 replies; 5+ messages in thread
From: mans at mansr dot com @ 2012-10-22 19:38 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026
--- Comment #4 from Mans Rullgard <mans at mansr dot com> 2012-10-22 19:37:58 UTC ---
For the record, clang/llvm gets this right.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2012-10-22 19:38 UTC | newest]
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2012-10-22 16:39 [Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value mans at mansr dot com
2012-10-22 16:41 ` [Bug target/55026] " pinskia at gcc dot gnu.org
2012-10-22 19:08 ` [Bug target/55026] Useless stores generated for structures " ebotcazou at gcc dot gnu.org
2012-10-22 19:34 ` mans at mansr dot com
2012-10-22 19:38 ` mans at mansr dot com
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