[Bug tree-optimization/28807] New: [4.2 Regression] wrong code with may_alias and structs

[Bug tree-optimization/28807] New: [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

Testcase:
#include <stdio.h>

struct S { short x; };
typedef struct S __attribute__((__may_alias__)) test;

int f() {
  int a=10;
  test *p=(test *)&a;
  test s={1};
  *p=s;
  return a;
}

int main() {
  if (f() == 10)
    __builtin_abort()
  return 0;
}

-------
Even though the struct is not the same size as int, this should still work as
the struct is marked as may_alias so we get an aliasing set of 0.


-- 
           Summary: [4.2 Regression] wrong code with may_alias and structs
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |4.2.0
      Known to work|                            |4.1.2
   Target Milestone|---                         |4.2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[dberlin at gcc dot gnu dot org]

------- Comment #1 from dberlin at gcc dot gnu dot org  2006-08-22 18:18 -------
Add an alias dump so we can see what things think of all this?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-08-22 21:49 -------
Better testcase which shows the problem before SRA:
struct S { short x; };
typedef struct S __attribute__((__may_alias__)) test;

int f() {
  int a=10;
  test *p=(test *)&a;
  p->x = 1;
  return a;
}

int main() {
  if (f() == 10)
    __builtin_abort();
  return 0;
}

------
I will attach the aliasing dump after this.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2006-08-22 21:50 -------
Created an attachment (id=12116)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=12116&action=view)
aliasing dump

aliasing dump for the newest testcase.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[dberlin at dberlin dot org]

------- Comment #4 from dberlin at gcc dot gnu dot org  2006-08-23 01:40 -------
Subject: Re:  [4.2 Regression] wrong code with
 may_alias and structs

pinskia at gcc dot gnu dot org wrote:
> ------- Comment #3 from pinskia at gcc dot gnu dot org  2006-08-22 21:50 -------
> Created an attachment (id=12116)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=12116&action=view)
>  --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=12116&action=view)
> aliasing dump
> 
> aliasing dump for the newest testcase.
> 
> 

My guess is now that access_can_touch_variable is returning false for
the p->x store when it asks about a, so it uses the bare NMT.

This is a hard nut to crack, my *guess* is that if you just check the
alias set of the type of the access, we are going to end up claiming any
access to a structure containing a char variable is aliased.

Try it though :)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2006-08-23 08:06 -------
(In reply to comment #4)
> This is a hard nut to crack, my *guess* is that if you just check the
> alias set of the type of the access, we are going to end up claiming any
> access to a structure containing a char variable is aliased.
You are correct as we regress with the following testcase:
struct a
{
  char a1;
};

int *aa;

void g(int *a)
{
  aa = a;
  *a = 2;
}

int t(int i, struct a *b)
{
  g(&i);
  b->a1 = 1;
  i = 2;
  if (b->a1 != 1)
    link_failure ();
}
int main(void)
{
  struct a b;
  t(1, &b);
  return 0;
}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2006-08-23 08:11 -------
This patch fixes the problem without causing the missed optimization to
happen:Index: tree-ssa-operands.c
===================================================================
--- tree-ssa-operands.c (revision 116342)
+++ tree-ssa-operands.c (working copy)
@@ -1150,7 +1150,8 @@ access_can_touch_variable (tree ref, tre
               || TREE_CODE (TREE_TYPE (base)) != UNION_TYPE)
           && !AGGREGATE_TYPE_P (TREE_TYPE (alias))
           && TREE_CODE (TREE_TYPE (alias)) != COMPLEX_TYPE
-          && !POINTER_TYPE_P (TREE_TYPE (alias)))
+          && !POINTER_TYPE_P (TREE_TYPE (alias))
+          && get_alias_set (base))
     {
 #ifdef ACCESS_DEBUGGING
       fprintf (stderr, "Access to ");


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2006-08-24 15:16 -------
Mine, all mine.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |pinskia at gcc dot gnu dot
                   |dot org                     |org
                URL|                            |http://gcc.gnu.org/ml/gcc-
                   |                            |patches/2006-
                   |                            |08/msg00878.html
             Status|UNCONFIRMED                 |ASSIGNED
     Ever Confirmed|0                           |1
           Keywords|                            |patch
   Last reconfirmed|0000-00-00 00:00:00         |2006-08-24 15:16:34
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[patchapp at dberlin dot org]

------- Comment #8 from patchapp at dberlin dot org  2006-08-24 15:18 -------
Subject: Bug number PR 28807

A patch for this bug has been added to the patch tracker.
The mailing list url for the patch is
http://gcc.gnu.org/ml/gcc-patches/2006-08/msg00878.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2006-08-25 07:14 -------
Fixed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807

[Bug tree-optimization/28807] [4.2 Regression] wrong code with may_alias and structs

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2006-08-25 07:14 -------
Fixed.


------- Comment #10 from pinskia at gcc dot gnu dot org  2006-08-25 07:14 -------
Subject: Bug 28807

Author: pinskia
Date: Fri Aug 25 07:13:48 2006
New Revision: 116393

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=116393
Log:
2006-08-24  Andrew Pinski  <pinskia@physics.uc.edu>

        PR tree-opt/28807
        * tree-ssa-operands.c (access_can_touch_variable): Don't say
        the access through a base which has an alias set of 0 cannot
        touch the variable.

2006-08-24  Andrew Pinski  <pinskia@physics.uc.edu>

        PR tree-opt/28807
        * gcc.c-torture/execute/mayalias-2.c: New test.
        * gcc.dg/tree-ssa/alias-13.c: New test.



Added:
    trunk/gcc/testsuite/gcc.c-torture/execute/mayalias-2.c
    trunk/gcc/testsuite/gcc.dg/tree-ssa/alias-13.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/tree-ssa-operands.c


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28807