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
