gimple-fold: Fix fold_truth_andor_for_ifcombine [PR120074]HEADtrunkmaster

The following testcase ICEs because of a mismatch between wide_int
precision, in particular lr_and_mask has 32-bit precision while sign has
16-bit.

decode_field_reference ensures that {ll,lr,rl,rr}_and_mask has
{ll,lr,rl,rr}_bitsize precision, so the
        ll_and_mask |= sign;
and
        rl_and_mask |= sign;
and
        ll_and_mask &= sign;
and
        rl_and_mask &= sign;
cases should work right, sign has in those cases {ll,rl}_bitsize
precision.  The problem is that nothing until much later guarantees
that ll_bitsize == lr_bitsize or rl_bitsize == rr_bitsize.
In the testcase there is
((b ^ a) & 3) < 0
where a is 16-bit and b is 32-bit, so it is the lsignbit handling,
and because of the xor the xor operand is moved to the *r_and_mask, so
with ll_and_mask being 16-bit 3 and lr_and_mask being 32-bit 3.

Now, either b in the above case would be INTEGER_CST, in that case
if rr_arg was also INTEGER_CST we'd use the l_const && r_const case
and try to handle it, or we'd run into (though much later)
      if (ll_bitsize != lr_bitsize || rl_bitsize != rr_bitsize
...
        return 0;

One possibility is dealing with a different precision using wide_int::from.

Another option used in this patch as it is safest is
+         if (ll_bitsize != lr_bitsize)
+           return 0;
          if (!lr_and_mask.get_precision ())
            lr_and_mask = sign;
          else
            lr_and_mask &= sign;
and similarly in the other hunk, i.e. punt if there is a mismatch
early.

And yet another option would be to compute
the sign
      wide_int sign = wi::mask (ll_bitsize - 1, true, ll_bitsize);
      /* If ll_arg is zero-extended and we're testing the sign bit, we know
         what the result should be.  Shifting the sign bit out of sign will get
         us to mask the entire field out, yielding zero, i.e., the sign bit of
         the zero-extended value.  We know the masked value is being compared
         with zero, so the compare will get us the result we're looking
         for: TRUE if EQ_EXPR, FALSE if NE_EXPR.  */
      if (lsignbit > ll_bitsize && ll_unsignedp)
        sign <<= 1;
once again for the lr_and_mask and rr_and_mask cases using rl_bitsize.

As we just return 0; anyway unless l_const && r_const, if l_const & r_const
are false it doesn't really matter what is chosen, but for the const
cases it matters and I'm not sure what is right.  So the second option
might be safest.

2025-05-06  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/120074
	* gimple-fold.cc (fold_truth_andor_for_ifcombine): For
	lsignbit && l_xor case, punt if ll_bitsize != lr_bitsize.  Similarly
	for rsignbit && r_xor case, punt if rl_bitsize != rr_bitsize.
	Formatting fix.

	* gcc.dg/pr120074.c: New test.

0 files changed, 0 insertions, 0 deletions