c/2480: aliasing problem with global structures
Dan Nicolaescu
dann@godzilla.ics.uci.edu
Tue Apr 3 16:56:00 GMT 2001
>Number: 2480
>Category: c
>Synopsis: aliasing problem with global structures
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: pessimizes-code
>Submitter-Id: net
>Arrival-Date: Tue Apr 03 16:55:59 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator: Dan Nicolaescu <dann@godzilla.ics.uci.edu>
>Release: gcc version 3.1 20010402 (experimental)
>Organization:
>Environment:
sun-sparc-solaris2.7
>Description:
The following code:
struct example
{
char a;
int b;
char c;
} *ex1;
void
bar (void)
{
ex1->a = 1;
ex1->b = 2;
ex1->c = 3;
}
void
foo (struct example *ex2)
{
ex2->a = 1;
ex2->b = 2;
ex2->c = 3;
}
When compiled with -O2 -fstrict-aliasing -S on sun-sparc-solaris2.7
with a GCC mainline snapshot from 2000-04-02
(but the same problem occurs on the GCC-3.0 branch and 2.95.2)
bar:
!#PROLOGUE# 0
!#PROLOGUE# 1
sethi %hi(ex1), %o2
ld [%o2+%lo(ex1)], %o1
mov 1, %o0
stb %o0, [%o1]
ld [%o2+%lo(ex1)], %o3
^^^^^
after the store ex1 is reloaded.
true_dependence returns true for these last 2 instructions, it
seems that the ex1->a is put in the alias set 0. That is a
mistake, but I couldn't find where that is done....
It looks like GCC treats ex1->a as a char*, but that is
incorrect, a store to ex1->a cannot alias ex1
mov 3, %o0
mov 2, %o1
st %o0, [%o3+8]
^^^
After this store ex1 is not reloaded, but in this case the
struct member is an "int"
retl
st %o1, [%o3+4]
foo:
!#PROLOGUE# 0
!#PROLOGUE# 1
mov 3, %o1
st %o1, [%o0+8]
mov 1, %o2
mov 2, %o1
stb %o2, [%o0]
retl
st %o1, [%o0+4]
nothing like that happens here, when the pointer to the structure is
passed as a parameter.
This is an important pessimization.
GCC itself contains a lot of global pointers to structures...
>How-To-Repeat:
Compile the code in the description with -O2 -S -fstrict-aliasing
and look at the resulting assembly
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted:
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