Re: A problem about loop store motion

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

On Tue, Feb 21, 2012 at 10:57 AM, Jiangning Liu <jiangning.liu@arm.com> wrote:
>
>
>> -----Original Message-----
>> From: Richard Guenther [mailto:richard.guenther@gmail.com]
>> Sent: Tuesday, February 21, 2012 5:40 PM
>> To: Jiangning Liu
>> Cc: gcc@gcc.gnu.org
>> Subject: Re: A problem about loop store motion
>>
>> On Tue, Feb 21, 2012 at 9:54 AM, Jiangning Liu <jiangning.liu@arm.com>
>> wrote:
>> >> The MEM form is more "canonical", so the loop SM machinery to detect
>> >> equality should be adjusted accordingly. ?Alternatively you can
>> teach
>> >> PRE insertion to strip off the MEM if possible (though
>> >> fold_stmt_inplace should
>> >> arelady do this if possible).
>> >
>> > Richard,
>> >
>> > Thank you! You are right. I noticed on latest trunk the problem in
>> PRE was
>> > already fixed by invoking fold_stmt_inplace.
>> >
>> > Unfortunately for this small case, the latest trunk code still can't
>> do SM
>> > for variable pos, because refs_may_alias_p(*D.4074_10, pos) is true,
>> that
>> > is, pos has alias with l[pos].
>> >
>> > I think alias analysis should be able to know they don't have alias
>> with
>> > each other, unless there is an assignment statement like "l=&pos;".
>> >
>> > Can alias analysis fix the problem?
>>
>> The problem is that 'pos' is marked TREE_ADDRESSABLE, so we think
>> its address got taken. ?'l' points to any global possibly address-taken
>> variable. ?It get's the TREE_ADDRESSABLE flag via PRE insertion which
>> re-gimplifies the tree it creates which marks the variable as
>> addressable.
>>
>> I'll have a look.
>
> Terrific! :-) Could you please let me know when you have a patch to fix
> this, because I want to double check the big case, and there might be other
> hidden problems?

PR52324, I am testing the following patch (in reality the gimplifier should not
mark things addressable unless it itself makes them so, but frontends are
broken, so we do that ... ugh).

Index: gcc/gimplify.c
===================================================================
--- gcc/gimplify.c      (revision 184428)
+++ gcc/gimplify.c      (working copy)
@@ -7061,15 +7061,20 @@ gimplify_expr (tree *expr_p, gimple_seq
              ret = GS_OK;
              break;
            }
-         ret = gimplify_expr (&TREE_OPERAND (*expr_p, 0), pre_p, post_p,
-                              is_gimple_mem_ref_addr, fb_rvalue);
-         if (ret == GS_ERROR)
-           break;
+         /* Avoid re-gimplifying the address operand if it is already
+            in suitable form.  */
+         if (!is_gimple_mem_ref_addr (TREE_OPERAND (*expr_p, 0)))
+           {
+             ret = gimplify_expr (&TREE_OPERAND (*expr_p, 0), pre_p, post_p,
+                                  is_gimple_mem_ref_addr, fb_rvalue);
+             if (ret == GS_ERROR)
+               break;
+           }
          recalculate_side_effects (*expr_p);
          ret = GS_ALL_DONE;
          break;

-         /* Constants need not be gimplified.  */
+       /* Constants need not be gimplified.  */
        case INTEGER_CST:
        case REAL_CST:
        case FIXED_CST: