/*
* gcc-3.4 -O2 miscompiles this program on armv5b-unknown-linux:
* a negative value for y is passed to I_lshift(), but I_lshift()
* behaves as though y is non-negative (chooses the wrong branch
* of the second if-statement).
*
* The original code comes from the bignum support code in the
* runtime system for the Erlang functional language, where the
* bug caused bignum right shifts to become left shifts.
*
* The bug occurs with gcc-3.4.4 and gcc-3.4-20051122.
* Reducing optimisation level to -O1 masks the bug.
*
* The bug does not occur with gcc-3.3.6, which makes this a
* regression. It also does not occur with gcc-4.0.2.
*/
#include <stdio.h>
/* Shift a bignum left or right. */
unsigned int I_lshift(unsigned short *x, unsigned int xl, int y, unsigned short
*r)
{
if (xl == 1)
return 1;
{
int ay = (y < 0) ? -y : y;
int bw = ay / 16;
unsigned int rl;
if (y > 0) { /* shift left */
rl = xl + bw + 1;
return rl;
}
else { /* shift right */
rl = xl - bw;
r += (rl-1);
xl -= bw;
r -= xl;
rl--;
return rl;
}
}
}
unsigned short x[3] = { 0x0000, 0x0000, 0x0010 };
unsigned int xl = 3;
int y = -8;
unsigned short r[4]; /* should be { 0x0000, 0x1000 } after I_lshift() */
int main(void)
{
unsigned int res = I_lshift(x, xl, y, r);
if (res != 2) {
fprintf(stderr, "I_lshift() returned %u, gcc is buggy\n", res);
return 1;
}
return 0;
}
--
Summary: gcc-3.4 wrong code for conditionals on arm
Product: gcc
Version: 3.4.5
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: regression
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: mikpe at csd dot uu dot se
GCC build triplet: armv5b-unknown-linux
GCC host triplet: armv5b-unknown-linux
GCC target triplet: armv5b-unknown-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25133
