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

["Paulo.Matos at csr dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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