[Bug regression/50472] New: Volatile qualification in data is not enough to avoid optimization over pointer to data

[Bug regression/50472] New: Volatile qualification in data is not enough to avoid optimization over pointer to data

[Paulo.Matos at csr dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

             Bug #: 50472
           Summary: Volatile qualification in data is not enough to avoid
                    optimization over pointer to data
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: regression
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: Paulo.Matos@csr.com


Given the following code:
static const unsigned int foo = 1;
unsigned int test( void )
{
  const volatile unsigned int *bar = &foo;
  return ( *bar );
}

GCC46 is generating:
$ local/gcc-4.6.1/bin/gcc -Os -S vol.c
$ cat vol.s
...
test:
.LFB0:
        .cfi_startproc
        movl    $1, %eax
        ret
        .cfi_endproc
...


This is optimizing away bar even though the data bar points to is being
qualified as volatile. The way to ensure this doesn't happen requires
qualifying the pointer as a volatile too:
static const unsigned int foo = 1;
unsigned int test( void )
{
  const volatile unsigned int * volatile bar = &foo;
  return ( *bar );
}

$ local/gcc-4.6.1/bin/gcc -Os -S vol.c
$ cat vol.s
...
test:
.LFB0:
        .cfi_startproc
        movq    $foo, -8(%rsp)
        movq    -8(%rsp), %rax
        movl    (%rax), %eax
        ret
        .cfi_endproc
...


This is a regression to what GCC45 used to do. Qualification of the data as
volatile used to be enough to block the optimization.

Posting this to the mailing list revealed Ian agrees this is a regression:
http://gcc.gnu.org/ml/gcc/2011-09/msg00197.html

[Bug regression/50472] Volatile qualification in data is not enough to avoid optimization over pointer to data

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

--- Comment #1 from Mikael Pettersson <mikpe at it dot uu.se> 2011-09-21 08:29:07 UTC ---
Duplicate of PR50078?

[Bug regression/50472] Volatile qualification in data is not enough to avoid optimization over pointer to data

[Paulo.Matos at csr dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

--- Comment #2 from Paulo J. Matos <Paulo.Matos at csr dot com> 2011-09-23 13:18:10 UTC ---
(In reply to comment #1)
> Duplicate of PR50078?

Might be but I can't confirm. Surely we will know once Mat fixes PR50078.

[Bug regression/50472] Volatile qualification in data is not enough to avoid optimization over pointer to data

[Paulo.Matos at csr dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

Paulo J. Matos <Paulo.Matos at csr dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Paulo.Matos at csr dot com

--- Comment #3 from Paulo J. Matos <Paulo.Matos at csr dot com> 2011-09-23 13:20:57 UTC ---
Richard raised concerns about this PR being an old bug and already fixed.
Initially I only confirmed it in 4.6.1 but I can reproduce it in git head
36181f98f (that's from 30mins ago).

[Bug regression/50472] Volatile qualification in data is not enough to avoid optimization over pointer to data

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2011-09-25
         AssignedTo|unassigned at gcc dot       |rguenth at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-09-25 11:52:49 UTC ---
CCP optimizes the load from the static const foo via fold_stmt.  I will have a
look.

Folding statement: D.2758_2 ={v} *bar_1;
Folded into: D.2758_2 = 1;

[Bug regression/50472] Volatile qualification in data is not enough to avoid optimization over pointer to data

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-09-26 12:59:02 UTC ---
Author: rguenth
Date: Mon Sep 26 12:58:35 2011
New Revision: 179196

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=179196
Log:
2011-09-26  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/50472
    * gimple-fold.c (fold_const_aggregate_ref_1): Do not fold
    volatile references.

    * gcc.dg/torture/pr50472.c: New testcase.

Added:
    trunk/gcc/testsuite/gcc.dg/torture/pr50472.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/gimple-fold.c
    trunk/gcc/testsuite/ChangeLog

[Bug regression/50472] Volatile qualification in data is not enough to avoid optimization over pointer to data

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

--- Comment #6 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-09-26 14:09:04 UTC ---
Author: rguenth
Date: Mon Sep 26 14:08:53 2011
New Revision: 179200

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=179200
Log:
2011-09-26  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/50472
    * tree-ssa-ccp.c (fold_const_aggregate_ref): Do not fold
    volatile references.

    * gcc.dg/torture/pr50472.c: New testcase.

Added:
    branches/gcc-4_6-branch/gcc/testsuite/gcc.dg/torture/pr50472.c
Modified:
    branches/gcc-4_6-branch/gcc/ChangeLog
    branches/gcc-4_6-branch/gcc/testsuite/ChangeLog
    branches/gcc-4_6-branch/gcc/tree-ssa-ccp.c

[Bug tree-optimization/50472] Volatile qualification in data is not enough to avoid optimization over pointer to data

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
          Component|regression                  |tree-optimization
         Resolution|                            |FIXED
   Target Milestone|---                         |4.6.2

--- Comment #7 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-09-26 14:09:49 UTC ---
Fixed for 4.6.2.

[Bug tree-optimization/50472] Volatile qualification in data is not enough to avoid optimization over pointer to data

[amker at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50472

--- Comment #8 from amker at gcc dot gnu.org 2012-06-12 02:33:29 UTC ---
Author: amker
Date: Tue Jun 12 02:33:23 2012
New Revision: 188414

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=188414
Log:
    Backport r179200 from GCC-4.6 branch
    2011-09-26  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/50472
    * tree-ssa-ccp.c (fold_const_aggregate_ref): Do not fold
    volatile references.

    PR tree-optimization/50472
    * gcc.dg/torture/pr50472.c: New testcase.

Added:
    branches/ARM/embedded-4_6-branch/gcc/testsuite/gcc.dg/torture/pr50472.c
Modified:
    branches/ARM/embedded-4_6-branch/gcc/ChangeLog.arm
    branches/ARM/embedded-4_6-branch/gcc/testsuite/ChangeLog.arm
    branches/ARM/embedded-4_6-branch/gcc/tree-ssa-ccp.c