[Bug tree-optimization/29631] New: [4.1 regression] Incorrect zero extension of signed short IV

["falk at debian dot org" <gcc-bugzilla at gcc dot gnu dot org>]

This is extracted from gnushogi (http://bugs.debian.org/393075).

falk@juist:~% cat bug.c
const signed char nunmap[] = { 17, -1, 1 };

__attribute__((noinline))
void ff(int i) {
    asm volatile("");
}

__attribute__((noinline))
void f(short delta) {
    short p0 = 2, s;
    for (s = 0; s < 2; s++) {
        p0 += delta;
        ff(s);
        if (nunmap[p0] == 17)
            asm volatile("# nop");
    }
}

int main() {
    f(-1);
    return 0;
}

falk@juist:~% gcc-3.4 -O2 bug.c && ./a.out   
falk@juist:~% gcc-4.2 -O2 bug.c && ./a.out    
falk@juist:~% gcc-4.1 -O2 bug.c && ./a.out 
zsh: segmentation fault (core dumped)  ./a.out

Reason seems to be that delta is zero extended, not sign extended:

$f..ng:
        lda $30,-32($30)
        ldah $1,nunmap($29)             !gprelhigh
        lda $1,nunmap($1)               !gprellow
        stq $9,8($30)
        stq $10,16($30)
        mov $31,$9
        stq $11,24($30)
        lda $10,3($1)        # $10 = numap + 3
        zapnot $16,3,$11     # $11 = zero extend delta (=65535)
        stq $26,0($30)
        .prologue 1
$L4:
        addq $10,$11,$10     # $10 += $11
        addl $31,$9,$16
        bsr $26,ff              !samegp
        ldq_u $1,-1($10)     # access numap[65537] -> SEGV
        extqh $1,$10,$1
        sra $1,56,$1
        cmpeq $1,17,$1
        bne $1,$L12
$L5:
        lda $9,1($9)
        cmpeq $9,2,$1
        beq $1,$L4
        ldq $26,0($30)
        ldq $9,8($30)
        ldq $10,16($30)
        ldq $11,24($30)
        lda $30,32($30)
        ret $31,($26),1
        .align 4
$L12:
        .set    macro
        # nop
        .set    nomacro
        br $31,$L5


-- 
           Summary: [4.1 regression] Incorrect zero extension of signed
                    short IV
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: falk at debian dot org
 GCC build triplet: alpha-linux-gnu
  GCC host triplet: alpha-linux-gnu
GCC target triplet: alpha-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29631