[Bug c/45819] New: [4.5 Regression] unexpected unaligned access to volatile int

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

           Summary: [4.5 Regression] unexpected unaligned access to
                    volatile int
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: anemo@mba.ocn.ne.jp


An unexpected unaligned access instructions for volatile int are
generated on gcc 4.5 or 4.6 with this test case.

struct st {
    int ptr;
} __attribute__ ((packed));

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

This test case is derived from something like this in ARM linux kernel.

struct st {
    unsigned int ptr;
} __attribute__ ((packed));
unsigned int foo(struct st *st)
{
    return readl(&st->ptr);
}

mipsel-linux-gcc-4.5.1 -O2 foo.c -S output:
    lwl    $2,3($4)
    nop
    lwr    $2,0($4)
    j    $31
    nop
arm-linux-gnueabi-gcc-4.5.1 -O2 foo.c -S output:
    ldrb    r3, [r0, #0]    @ zero_extendqisi2
    ldrb    r1, [r0, #1]    @ zero_extendqisi2
    ldrb    r2, [r0, #2]    @ zero_extendqisi2
    orr    r3, r3, r1, asl #8
    ldrb    r0, [r0, #3]    @ zero_extendqisi2
    orr    r3, r3, r2, asl #16
    orr    r0, r3, r0, asl #24
    bx    lr

It seems the cast is ignored and unaligned access is assumed.
gcc 4.4.4 works fine.
mipsel-linux-gcc-4.4.4 -O2 foo.c -S output:
    lw    $2,0($4)
    j    $31
    nop
arm-linux-gnueabi-gcc-4.4.4 -O2 foo.c -S output:
    ldr    r0, [r0, #0]
    bx    lr

The similar problem was found as PR 45704.

git-bisect tell me same commit
0d9f1189f3df5ce5c0efc3ecadc7c0a4f75b202d is the first bad commit.

Like PR 45704, reverting this change fixes this problme too.

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

But the final fix for PR 45704 does not fix this problem.

mipsel-linux-gcc (4.6.0 20100927):
    lbu    $5,0($4)
    lbu    $6,1($4)
    lbu    $3,2($4)
    andi    $6,$6,0x00ff
    lbu    $2,3($4)
    andi    $5,$5,0x00ff
    sll    $6,$6,8
    andi    $3,$3,0x00ff
    or    $4,$6,$5
    sll    $3,$3,16
    or    $3,$3,$4
    sll    $2,$2,24
    j    $31
    or    $2,$2,$3
arm-linux-gnueabi-gcc (4.6.0 20100927):
    ldrb    r3, [r0, #0]    @ zero_extendqisi2
    ldrb    r1, [r0, #1]    @ zero_extendqisi2
    ldrb    r2, [r0, #2]    @ zero_extendqisi2
    orr    r3, r3, r1, asl #8
    ldrb    r0, [r0, #3]    @ zero_extendqisi2
    orr    r3, r3, r2, asl #16
    orr    r0, r3, r0, asl #24
    bx    lr