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* [Bug rtl-optimization/50938] New: x86 alloca adds 15 twice
@ 2011-10-31 17:37 ian at airs dot com
2011-10-31 17:44 ` [Bug rtl-optimization/50938] " ppluzhnikov at google dot com
2011-10-31 20:41 ` ebotcazou at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: ian at airs dot com @ 2011-10-31 17:37 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50938
Bug #: 50938
Summary: x86 alloca adds 15 twice
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: ian@airs.com
Using alloca on x86 requires that the return value be aligned to a 16-byte
(boundary). So code needs to add 15 to the size and then do & ~15. However,
compile this file:
extern void bar (char *);
void
foo (int n)
{
bar (__builtin_alloca (n));
}
The generated code adds 30, not 15. That is, the adjustment is made twice.
This works but wastes stack space.
The code in allocate_dynamic_stack_space in explow.c has become rather
convoluted and needs to be cleaned up to make this work. E.g., it does #if
defined (STACK_POINTER_OFFSET), but defaults.h ensures that
STACK_POINTER_OFFSET is always defined.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug rtl-optimization/50938] x86 alloca adds 15 twice
2011-10-31 17:37 [Bug rtl-optimization/50938] New: x86 alloca adds 15 twice ian at airs dot com
@ 2011-10-31 17:44 ` ppluzhnikov at google dot com
2011-10-31 20:41 ` ebotcazou at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: ppluzhnikov at google dot com @ 2011-10-31 17:44 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50938
Paul Pluzhnikov <ppluzhnikov at google dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |ppluzhnikov at google dot
| |com
--- Comment #1 from Paul Pluzhnikov <ppluzhnikov at google dot com> 2011-10-31 17:43:50 UTC ---
This bug prevent glibc's extend_alloca from working as it is intended.
The macro (for _STACK_GROWS_DOWN) is:
# define extend_alloca(buf, len, newlen) \
(__typeof (buf)) ({ size_t __newlen = (newlen); \
char *__newbuf = __alloca (__newlen); \
if (__newbuf + __newlen == (char *) buf) \
len += __newlen; \
else \
len = __newlen; \
__newbuf; })
However due to the +30 adjustment, __newbuf + __newlen == (char *) buf is never
true.
This leads to excessive stack use in glibc under some conditions.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug rtl-optimization/50938] x86 alloca adds 15 twice
2011-10-31 17:37 [Bug rtl-optimization/50938] New: x86 alloca adds 15 twice ian at airs dot com
2011-10-31 17:44 ` [Bug rtl-optimization/50938] " ppluzhnikov at google dot com
@ 2011-10-31 20:41 ` ebotcazou at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: ebotcazou at gcc dot gnu.org @ 2011-10-31 20:41 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50938
Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2011-10-31
CC| |ebotcazou at gcc dot
| |gnu.org
Ever Confirmed|0 |1
--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2011-10-31 20:40:55 UTC ---
> The code in allocate_dynamic_stack_space in explow.c has become rather
> convoluted and needs to be cleaned up to make this work. E.g., it does #if
> defined (STACK_POINTER_OFFSET), but defaults.h ensures that
> STACK_POINTER_OFFSET is always defined.
Yep, this is PR middle-end/47353.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2011-10-31 20:41 UTC | newest]
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2011-10-31 17:37 [Bug rtl-optimization/50938] New: x86 alloca adds 15 twice ian at airs dot com
2011-10-31 17:44 ` [Bug rtl-optimization/50938] " ppluzhnikov at google dot com
2011-10-31 20:41 ` ebotcazou at gcc dot gnu.org
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