[Bug tree-optimization/45704] New: [4.5 Regression] load byte instruction is used for volatile int

["anemo at mba dot ocn dot ne dot jp" <gcc-bugzilla at gcc dot gnu dot org>]

A cast to volatile int pointer is ignored on gcc 4.5 with this test case.

struct st {
        int ptr;
};

int foo(struct st *st)
{
        int v = *(volatile int *)&st->ptr;
        return v & 0xff;
}

mipsel-linux-gcc-4.5.1 -O2 foo.c -S output:
        lbu     $2,0($4)
        j       $31
        nop

It seems the cast are ignored and "load int and mask" was optimized to "load
byte".
gcc 4.4.4 works fine.
mipsel-linux-gcc-4.4.4 -O2 foo.c -S output:
        lw      $2,0($4)
        j       $31
        andi    $2,$2,0x00ff

git-bisect tell me 0d9f1189f3df5ce5c0efc3ecadc7c0a4f75b202d is the first bad
commit.
URL: http://gcc.gnu.org/viewcvs?view=revision&revision=156571
The commit is fix for Bug 42956.

The PR 42956 bugzilla shows same fix was applied to both 4.5.0 and 4.4.4,
but they behave differently on this test case.

Comparing patches for 4.4 branch and 4.5, I see the latter contains two more
changes for gimplify.c:
A STRIP_NOP line and lines starting with "/* *(foo *)&complexfoo => __real__
complexfoo */" comment.

I do not know gcc internal at all but it seems reverting this change fixes this
problem.

-  STRIP_USELESS_TYPE_CONVERSION (sub);
+  STRIP_NOPS (sub);

Hope this helps.


-- 
           Summary: [4.5 Regression] load byte instruction is used for
                    volatile int
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: anemo at mba dot ocn dot ne dot jp


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704