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* [Bug tree-optimization/19131] New: alloca returning unnecessarily aligned pointer and uses too much memory
@ 2004-12-22 14:39 rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-22 15:06 ` [Bug target/19131] " pinskia at gcc dot gnu dot org
` (3 more replies)
0 siblings, 4 replies; 6+ messages in thread
From: rguenth at tat dot physik dot uni-tuebingen dot de @ 2004-12-22 14:39 UTC (permalink / raw)
To: gcc-bugs
The testcase
int foo(int bar)
{
int i, res = 0;
for (i=0; i<bar; ++i) {
int *x = __builtin_alloca(4);
res += *x;
}
return res;
}
uses a lot more memory than necessary because of __builtin_alloca
apparently returning 16-byte aligned stack space (-O2 -fomit-frame-pointer):
.L5:
subl $32, %esp #,
incl %edx # i
leal 15(%esp), %eax #, tmp64
andl $-16, %eax #, tmp64
addl (%eax), %ecx #, res
cmpl %edx, %ebx # i, bar
jg .L5 #,
I cannot find anything in the C standard about alloca, but alignment
bigger than the requested storage size should be not needed (though
this may be architecture dependent).
At least playing with alignment should be moved outside of the loop,
so we could save half of the wasted memory (I know the testcase is
dumb, but it's at least simple).
Btw. the same problem applies to 3.4, 3.3 doesn't play alignment
games but still allocates 16 bytes each time (that won't be aligned
to 16 bytes this way anyway?). 2.95 beats all of them again in
simplicity:
.L6:
addl $-16,%esp
addl (%esp),%eax
decl %edx
jnz .L6
--
Summary: alloca returning unnecessarily aligned pointer and uses
too much memory
Product: gcc
Version: 4.0.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: rguenth at tat dot physik dot uni-tuebingen dot de
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug target/19131] alloca returning unnecessarily aligned pointer and uses too much memory
2004-12-22 14:39 [Bug tree-optimization/19131] New: alloca returning unnecessarily aligned pointer and uses too much memory rguenth at tat dot physik dot uni-tuebingen dot de
@ 2004-12-22 15:06 ` pinskia at gcc dot gnu dot org
2004-12-22 18:16 ` rguenth at tat dot physik dot uni-tuebingen dot de
` (2 subsequent siblings)
3 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2004-12-22 15:06 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22 15:06 -------
The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid
We need to keep the stack aligned sorry.
--
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Component|tree-optimization |target
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug target/19131] alloca returning unnecessarily aligned pointer and uses too much memory
2004-12-22 14:39 [Bug tree-optimization/19131] New: alloca returning unnecessarily aligned pointer and uses too much memory rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-22 15:06 ` [Bug target/19131] " pinskia at gcc dot gnu dot org
@ 2004-12-22 18:16 ` rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-23 22:23 ` rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-23 22:27 ` hubicka at ucw dot cz
3 siblings, 0 replies; 6+ messages in thread
From: rguenth at tat dot physik dot uni-tuebingen dot de @ 2004-12-22 18:16 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From rguenth at tat dot physik dot uni-tuebingen dot de 2004-12-22 18:16 -------
Subject: Re: alloca returning unnecessarily aligned pointer
and uses too much memory
pinskia at gcc dot gnu dot org wrote:
> ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22 15:06 -------
> The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid
>
> We need to keep the stack aligned sorry.
Inside a function!? Or just at function callsites? Humm, the Intel
compiler produces
..B1.3: # Preds ..B1.2 ..B1.4
movl $4, %eax #5.12
subl %eax, %esp #5.12
andl $-16, %esp #5.12
movl %esp, %eax #5.12
# LOE eax ebx ebp esi edi
..B1.4: # Preds ..B1.3
addl (%eax), %ebx #6.3
addl $1, %esi #4.21
cmpl %edi, %esi #4.2
jl ..B1.3
which looks like it aligns the stack after alloca, but it manages to
waste less space by subtracting $4, not $32.
Also if the ABI says the stack is aligned, why do we not make use of
this and avoid the andl $-16, %esp -- or is the alignment only about
alloca?
I'm a bit confused.
Richard.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131
^ permalink raw reply [flat|nested] 6+ messages in thread* [Bug target/19131] alloca returning unnecessarily aligned pointer and uses too much memory
2004-12-22 14:39 [Bug tree-optimization/19131] New: alloca returning unnecessarily aligned pointer and uses too much memory rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-22 15:06 ` [Bug target/19131] " pinskia at gcc dot gnu dot org
2004-12-22 18:16 ` rguenth at tat dot physik dot uni-tuebingen dot de
@ 2004-12-23 22:23 ` rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-23 22:27 ` Jan Hubicka
2004-12-23 22:27 ` hubicka at ucw dot cz
3 siblings, 1 reply; 6+ messages in thread
From: rguenth at tat dot physik dot uni-tuebingen dot de @ 2004-12-23 22:23 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From rguenth at tat dot physik dot uni-tuebingen dot de 2004-12-23 22:23 -------
Subject: Re: alloca returning unnecessarily aligned pointer
and uses too much memory
pinskia at gcc dot gnu dot org wrote:
> ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22 15:06 -------
> The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid
Where is the ABI specified? Is it the "System V ABI, Intel386
Architecture Processor Supplement" document I found at
http://www.caldera.com/developers/devspecs/abi386-4.pdf?
This one talks about word-alignment of the stack, not 16 byte alignment.
> We need to keep the stack aligned sorry.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Bug target/19131] alloca returning unnecessarily aligned pointer and uses too much memory
2004-12-23 22:23 ` rguenth at tat dot physik dot uni-tuebingen dot de
@ 2004-12-23 22:27 ` Jan Hubicka
0 siblings, 0 replies; 6+ messages in thread
From: Jan Hubicka @ 2004-12-23 22:27 UTC (permalink / raw)
To: rguenth at tat dot physik dot uni-tuebingen dot de; +Cc: gcc-bugs
>
> ------- Additional Comments From rguenth at tat dot physik dot uni-tuebingen dot de 2004-12-23 22:23 -------
> Subject: Re: alloca returning unnecessarily aligned pointer
> and uses too much memory
>
> pinskia at gcc dot gnu dot org wrote:
> > ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22 15:06 -------
> > The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid
>
> Where is the ABI specified? Is it the "System V ABI, Intel386
> Architecture Processor Supplement" document I found at
> http://www.caldera.com/developers/devspecs/abi386-4.pdf?
> This one talks about word-alignment of the stack, not 16 byte alignment.
We keep the stack 16 byte aligned as an extension to support SSE....
Honza
>
> > We need to keep the stack aligned sorry.
>
>
> --
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug target/19131] alloca returning unnecessarily aligned pointer and uses too much memory
2004-12-22 14:39 [Bug tree-optimization/19131] New: alloca returning unnecessarily aligned pointer and uses too much memory rguenth at tat dot physik dot uni-tuebingen dot de
` (2 preceding siblings ...)
2004-12-23 22:23 ` rguenth at tat dot physik dot uni-tuebingen dot de
@ 2004-12-23 22:27 ` hubicka at ucw dot cz
3 siblings, 0 replies; 6+ messages in thread
From: hubicka at ucw dot cz @ 2004-12-23 22:27 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From hubicka at ucw dot cz 2004-12-23 22:27 -------
Subject: Re: alloca returning unnecessarily aligned pointer and uses too much memory
>
> ------- Additional Comments From rguenth at tat dot physik dot uni-tuebingen dot de 2004-12-23 22:23 -------
> Subject: Re: alloca returning unnecessarily aligned pointer
> and uses too much memory
>
> pinskia at gcc dot gnu dot org wrote:
> > ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22 15:06 -------
> > The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid
>
> Where is the ABI specified? Is it the "System V ABI, Intel386
> Architecture Processor Supplement" document I found at
> http://www.caldera.com/developers/devspecs/abi386-4.pdf?
> This one talks about word-alignment of the stack, not 16 byte alignment.
We keep the stack 16 byte aligned as an extension to support SSE....
Honza
>
> > We need to keep the stack aligned sorry.
>
>
> --
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2004-12-23 22:27 UTC | newest]
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-- links below jump to the message on this page --
2004-12-22 14:39 [Bug tree-optimization/19131] New: alloca returning unnecessarily aligned pointer and uses too much memory rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-22 15:06 ` [Bug target/19131] " pinskia at gcc dot gnu dot org
2004-12-22 18:16 ` rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-23 22:23 ` rguenth at tat dot physik dot uni-tuebingen dot de
2004-12-23 22:27 ` Jan Hubicka
2004-12-23 22:27 ` hubicka at ucw dot cz
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