[Bug regression/32687] New: Invalid code generation for reading signed negative bitfield value (g++ optimization)

[siarhei dot siamashka at gmail dot com]

Reading signed bitfield value when it needs to be extended to larger type (for
example assigning 24-bit value to int) results in zero extending instead of
sign extending when compiled with g++ using optimizations (-O1 or higher).
Compiling the same code with gcc or disabling optimizations makes the problem
disappear.

The following code reproduces the problem:

#include <stdio.h>

struct TEST_STRUCT
{
    int f_8  : 8;
    int f_24 : 24;
};

int main ()
{
    struct TEST_STRUCT x;
    int a = -123;
    x.f_24 = a;

    printf("a=%d (%08X)\n", (int)a, (int)a);
    printf("x.f_24=%d (%08X)\n", (int)x.f_24, (int)x.f_24);

    if ((int)x.f_24 != (int)a)
        printf("test failed\n");
    else
        printf("test ok\n");
    return 0;
}

////////////////////////////////

Expected correct result:
a=-123 (FFFFFF85)
x.f_24=-123 (FFFFFF85)
test ok

Faulty result:
a=-123 (FFFFFF85)
x.f_24=16777093 (00FFFF85)
test failed

It is a regression as gcc 3.4.6 did not have this bug. Also this problem may be
related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32346 and
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30332


-- 
           Summary: Invalid code generation for reading signed negative
                    bitfield value (g++ optimization)
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: regression
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: siarhei dot siamashka at gmail dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32687