[Bug middle-end/55555] [4.8 Regression] miscompilation at -O2 (number_of_iterations)

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55555

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> 2012-12-14 13:49:02 UTC ---
(In reply to comment #7)
> I've tried to rewrite this as C, but managed to turn it into something that is
> miscompiled at a different spot.  The fortran testcase starts having
> __builtin_unreachable () in it in the *.cunroll pass, this one already in
> *.cunrolli pass.  Still, I believe it doesn't do any out of bounds access
> anywhere.  -O2 on x86_64-linux.
> 
> double s[4] = { 1.0, 2.0, 3.0, 4.0 }, pol_x[2] = { 5.0, 6.0 };
> 
> __attribute__((noinline)) int
> foo (void)
> {
>   double coef_x[8] = { 0, 0, 0, 0, 0, 0, 0, 0 };
>   int lxp = 0;
>   if (lxp <= 1)
>     do
>       {
>     double t = pol_x[lxp];
>     long S;
>     long l = lxp * 4L - 1;
>     for (S = 1; S <= 4; S++)
>       coef_x[S + l] = coef_x[S + l] + s[S - 1] * t;
>       }
>     while (lxp++ != 1);
>   asm volatile ("" : : "r" (coef_x) : "memory");
>   for (lxp = 0; lxp < 8; lxp++)
>     if (coef_x[lxp] != ((lxp & 3) + 1) * (5.0 + (lxp >= 4)))
>       __builtin_abort ();
>   return 1;
> }
> 
> int
> main ()
> {
>   asm volatile ("" : : : "memory");
>   if (!foo ())
>     __builtin_abort ();
>   return 0;
> }
> 
> Works with r193067, fails with r193100, haven't tried to bisect exactly, but
> would guess this is r193098 again.  For the outer loop it prints:
> 
> Analyzing # of iterations of loop 1
>   exit condition [0, + , 1](no_overflow) != 1
>   bounds on difference of bases: 1 ... 1
>   result:
>     # of iterations 1, bounded by 1
> Loop 1 iterates 1 times.
> Loop 1 iterates at most 1 times.
> 
> but that is wrong, the outer loop iterates exactly 2 times.

That's latch block executions, so one latch block execution is correct.