[Bug regression/25133] New: gcc-3.4 wrong code for conditionals on arm

[mikpe at csd dot uu dot se]

/*
 * 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