[Bug rtl-optimization/18577] New: [3.3 regression] variable use moved before initialization

[falk at debian dot org]

Test case:

static float tfcos12[3];
__attribute__((noinline)) double f(double x) { return x; }
int g;
int main(void) {
    int i, j;
    for (i = 0; i < 1; i++) 
        tfcos12[i] = 0.5;
    
    for (i = 0; i < 1; i++) {
        tfcos12[i] = 0.5 * f(i);
        for (j = 0; j < 12; j++)
            g++;
    }
    return 0;
}

% gcc -funroll-all-loops -O2 test.c && ./a.out
zsh: floating point exception (core dumped)  ./a.out

The reason is that the constant 0.5 is first used and then loaded:

        [...]
        cpys $f10,$f10,$f2                              # use $f10
        lds $f10,$LC0($1)               !gprellow       # before loading it
        [...]
        mult $f2,$f0,$f0                                # and this FPEs

This does not happen without -funroll-all-loops. Also not at -O, or with
3.4.

-- 
           Summary: [3.3 regression] variable use moved before
                    initialization
           Product: gcc
           Version: 3.3.5
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: critical
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: falk at debian dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: alphaev68-unknown-linux-gnu
  GCC host triplet: alphaev68-unknown-linux-gnu
GCC target triplet: alphaev68-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18577