[Bug c/45637] New: Suspicious code for bit fields

["sebastian dot huber at embedded-brains dot de" <gcc-bugzilla at gcc dot gnu dot org>]

Please have a look at this test case:

#include <stdint.h>

struct type1 {
        union {
                uint8_t reg;
                struct {
                        uint8_t : 7;
                        uint8_t b : 1;
                } bit;

        } foo [1];
        uint8_t bar;
};

volatile struct type1 *var1;

void f1(int i)
{
        var1->foo [i].bit.b = 1;
        var1->foo [0].bit.b = 1;
}

struct type2 {
        union {
                uint8_t reg;
                struct {
                        uint8_t : 7;
                        uint8_t b : 1;
                } bit;

        } foo [1];
        uint16_t bar;
};

volatile struct type2 *var2;

void f2(int i)
{
        var2->foo [i].bit.b = 1;
        var2->foo [0].bit.b = 1;
}

struct type3 {
        union {
                uint8_t reg;
                struct {
                        uint8_t : 7;
                        uint8_t b : 1;
                } bit;

        } foo [1];
        uint32_t bar;
};

volatile struct type3 *var3;

void f3(int i)
{
        var3->foo [i].bit.b = 1;
        var3->foo [0].bit.b = 1;
}

And the produced assembler file generated with (powerpc-rtems4.11-gcc -O2 -c
-mcpu=8540 -meabi -msdata -fno-common -save-temps):

        .file   "bitfields.c"
        .gnu_attribute 4, 2
        .gnu_attribute 8, 1
        .gnu_attribute 12, 1
        .section        ".text"
        .align 2
        .globl f1
        .type   f1, @function
f1:
        lwz 9,var1@sda21(0)
        lbzx 0,9,3
        ori 0,0,1
        stbx 0,9,3
        lbz 0,0(9)
        ori 0,0,1
        stb 0,0(9)
        blr
        .size   f1, .-f1
        .align 2
        .globl f2
        .type   f2, @function
f2:
        lwz 9,var2@sda21(0)
        lbzx 0,9,3
        ori 0,0,1
        stbx 0,9,3
        lhz 0,0(9)
        ori 0,0,256
        sth 0,0(9)
        blr
        .size   f2, .-f2
        .align 2
        .globl f3
        .type   f3, @function
f3:
        lwz 9,var3@sda21(0)
        li 11,1
        lbzx 0,9,3
        ori 0,0,1
        stbx 0,9,3
        lwz 0,0(9)
        rlwimi 0,11,24,7,7
        stw 0,0(9)
        blr
        .size   f3, .-f3
        .globl var1
        .globl var2
        .globl var3
        .section        .sbss,"aw",@nobits
        .align 2
        .type   var1, @object
        .size   var1, 4
var1:
        .zero   4
        .type   var2, @object
        .size   var2, 4
var2:
        .zero   4
        .type   var3, @object
        .size   var3, 4
var3:
        .zero   4
        .ident  "GCC: (GNU) 4.5.1 20100731 (RTEMS
gcc-4.5.1-5.suse11.2/newlib-1.18.0-20.suse11.2)"

Here you can see, that the access to the 'foo' field depends on

1. index is constant or variable, and
2. the 'bar' field type.

I think that both dependencies are wrong.  In any case byte accesses should be
used.


-- 
           Summary: Suspicious code for bit fields
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sebastian dot huber at embedded-brains dot de
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: powerpc-rtems4.11


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45637