[Bug c/34552] New: optimization of bit-shifts leads to strange results

["blake dot tregre at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

The example below copies a float from one location to the next.  When compiling
with -O1 or less using 4.1.2, z equals *baz, and the program displays "ok, no
bug."  When compiling with -O2 or greater, z doesn't equal *baz, and the
program prints "whoah: 23423.234375 should equal
8523244312281203485966336.000000".  Also of note, when one prints the value of
q before defining *baz using -O2, the value of *baz changes to 0.00000.  This
problem did not occur using gcc 4.0.1 with -O2.  


#include <stdio.h>
#include <stdint.h>

int main(int argc, char **argv) {

  float z = 23423.23424;
  uint32_t* foo = (uint32_t*)&z;
  uint8_t zbuf[4] = {0,0,0,0};

  zbuf[3] = ((*foo) & 0x000000ffL);
  zbuf[2] = ((*foo) & 0x0000ff00L) >> 8;
  zbuf[1] = ((*foo) & 0x00ff0000L) >> 16;
  zbuf[0] = ((*foo) & 0xff000000L) >> 24;

  uint32_t q =
    (((uint32_t)zbuf[0] << 24) |
    ((uint32_t)zbuf[1] << 16)  |
    ((uint32_t)zbuf[2] << 8)   |
    ((uint32_t)zbuf[3]));

  float *baz = (float*)&q;

  if (z == *baz)
    fprintf (stderr, "okay, no bug\n");
  else
    fprintf (stderr, "whoah: %f should equal %f\n", z, *baz);

        return 0;
}


-- 
           Summary: optimization of bit-shifts leads to strange results
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: blake dot tregre at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34552