Re: Allow union variables to share stack slots wwith -fno-strict-aliasing (issue4444051)

[Richard Guenther <richard dot guenther at gmail dot com>]

On Wed, Apr 20, 2011 at 2:42 AM, Easwaran Raman <eraman@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 ?<eraman@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
>