[Bug tree-optimization/102134] [12 Regression] wrong code at -O1

[jakub at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102134

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #3)
> This is triggering the:
>           else if (!wi::neg_p (r1val | r1mask, sgn))
>             {
>               /* Logical right shift, or zero sign bit.  */
>               widest_int arg = r1val | r1mask;
>               int lzcount = wi::clz (arg);
>               lzcount -= wi::get_precision (arg) - width;
>               widest_int tmp = wi::mask <widest_int> (width, false);
>               tmp = wi::lrshift (tmp, lzcount);
>               tmp = wi::lrshift (tmp, wi::bit_and_not (r2val, r2mask));
>               *mask = wi::ext (tmp, width, sgn);
>               *val = 0;
>             }
> sgn is UNSIGNED, so wi::neg_p is false even when r1val | r1mask is negative,
> so
> wi::clz (arg) is 0, but wi::get_precision (arg) is 576 and width is 64, so
> we end up with lzcount of -512 and I bet the code doesn't expect lzcount to
> be negative.

So perhaps either guard the subtraction on
  if (lzcount)
    lzcount -= wi::get_precision (arg) - width;
- when lzcount is 0, the sign bit is set and we shouldn't shift it right,
or do
  int lzcount;
  if (wi::neg_p (arg))
    lzcount = 0;
  else
    lzcount = wi::clz (arg) - (wi::get_precision (arg) - width);