------- Comment #10 from rguenth at gcc dot gnu dot org 2008-03-14 20:06
-------
This is related to PR27799. It is also fixed with -fstrict-aliasing on the
tree level:
bar ()
{
struct example * ex1.0;
<bb 2>:
ex1.0 = ex1;
ex1.0->a = 1;
ex1.0->b = 2;
ex1.0->c = 3;
return;
}
Without -fstrict-aliasing we get (see PR27799):
bar ()
{
struct example * ex1.0;
<bb 2>:
# VUSE <ex1_4(D)>
ex1.0_1 = ex1;
# ex1_6 = VDEF <ex1_4(D)>
# SMT.5_7 = VDEF <SMT.5_5(D)>
ex1.0_1->a = 1;
# VUSE <ex1_6>
ex1.0_2 = ex1;
# ex1_8 = VDEF <ex1_6>
# SMT.5_9 = VDEF <SMT.5_7>
ex1.0_2->b = 2;
# VUSE <ex1_8>
ex1.0_3 = ex1;
# ex1_10 = VDEF <ex1_8>
# SMT.5_11 = VDEF <SMT.5_9>
ex1.0_3->c = 3;
return;
while with -fstrict-aliasing we have
bar ()
{
struct example * ex1.0;
<bb 2>:
# VUSE <ex1_4(D)>
ex1.0_1 = ex1;
# SMT.5_6 = VDEF <SMT.5_5(D)>
ex1.0_1->a = 1;
# VUSE <ex1_4(D)>
ex1.0_2 = ex1;
# SMT.5_7 = VDEF <SMT.5_6>
ex1.0_2->b = 2;
# VUSE <ex1_4(D)>
ex1.0_3 = ex1;
# SMT.5_8 = VDEF <SMT.5_7>
ex1.0_3->c = 3;
return;
}
If you disable all tree optimizations that do the optimization you still
have the missed optimization on the RTL level and get
bar:
pushl %ebp
movl %esp, %ebp
movl ex1, %eax
movb $1, (%eax)
movl ex1, %eax
movl $2, 4(%eax)
movb $3, 8(%eax)
popl %ebp
ret
without strict-aliasing it gets even
bar:
pushl %ebp
movl %esp, %ebp
movl ex1, %eax
movb $1, (%eax)
movl ex1, %eax
movl $2, 4(%eax)
movl ex1, %eax
movb $3, 8(%eax)
popl %ebp
ret
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu dot
| |org
OtherBugsDependingO| |27799
nThis| |
Last reconfirmed|2006-02-16 21:27:01 |2008-03-14 20:06:36
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=2480
