On Wed, Apr 20, 2011 at 2:42 AM, Easwaran Raman <era...@google.com> wrote:
> Hi,
> This patch allows variables whose type transitively contains a union to
> share stack slots if -fno-strict-aliasing is used.
>
> Bootstraps on x86_64 with no test regressions. Also tested by changing
> flag_strict_aliasing to 0 by default. Bootstraps and no test regressions when
> compared to flag_strict_aliasing=0 without this patch.
>
> OK for trunk?
Ok.
Thanks,
Richard.
> -Easwaran
>
> 2011-04-19 Easwaran Raman <era...@google.com>
>
> * gcc/testsuite/gcc.dg/stack-layout-1.c: New
> * gcc/cfgexpand.c (add_alias_set_conflicts): Add conflicts
> with a variable containing union type only with
> -fstrict-aliasing.
>
> Index: gcc/testsuite/gcc.dg/stack-layout-1.c
> ===================================================================
> --- gcc/testsuite/gcc.dg/stack-layout-1.c (revision 0)
> +++ gcc/testsuite/gcc.dg/stack-layout-1.c (revision 0)
> @@ -0,0 +1,25 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fno-strict-aliasing -fdump-rtl-expand" } */
> +union U {
> + int a;
> + float b;
> +};
> +struct A {
> + union U u1;
> + char a[100];
> +};
> +void bar (struct A *);
> +void foo ()
> + {
> + {
> + struct A a;
> + bar (&a);
> + }
> + {
> + struct A a;
> + bar (&a);
> + }
> + }
> +
> +/* { dg-final { scan-rtl-dump-times "Partition" 1 "expand" } } */
> +/* { dg-final { cleanup-rtl-dump "expand" } } */
> Index: gcc/cfgexpand.c
> ===================================================================
> --- gcc/cfgexpand.c (revision 172657)
> +++ gcc/cfgexpand.c (working copy)
> @@ -373,8 +373,9 @@ add_alias_set_conflicts (void)
> to elements will conflict. In case of unions we have
> to be careful as type based aliasing rules may say
> access to the same memory does not conflict. So play
> - safe and add a conflict in this case. */
> - || contains_union)
> + safe and add a conflict in this case when
> + -fstrict-aliasing is used. */
> + || (contains_union && flag_strict_aliasing))
> add_stack_var_conflict (i, j);
> }
> }
>
> --
> This patch is available for review at http://codereview.appspot.com/4444051
>
