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
