reassoc: Fix up inter-bb range optimization [PR104196]

reassoc: Fix up inter-bb range optimization [PR104196]

As mentioned in the PR, reassoc1 miscompiles following testcase.
We have:
  if (a.1_1 >= 0)
    goto <bb 5>; [59.00%]
  else
    goto <bb 4>; [41.00%]

  <bb 4> [local count: 440234144]:
  _3 = -2147483647 - a.1_1;
  _9 = a.1_1 != -2147479551;
  _4 = _3 == 1;
  _8 = _4 | _9;
  if (_8 != 0)
    goto <bb 5>; [34.51%]
  else
    goto <bb 3>; [65.49%]

and the inter-bb range test optimization treats it as:
  if ((a.1_1 >= 0)
      | (-2147483647 - a.1_1 == 1)
      | (a.1_1 != -2147479551))
    goto bb5;
  else
    goto bb3;
and recognizes that a.1_1 >= 0 is redundant with a.1_1 != -2147479551
and so will optimize it into:
  if (0
      | (-2147483647 - a.1_1 == 1)
      | (a.1_1 != -2147479551))
    goto bb5;
  else
    goto bb3;
When merging 2 comparisons, we use update_range_test which picks one
of the comparisons as the one holding the result (currently always
the RANGE one rather than all the OTHERRANGE* ones) and adjusts the
others to be true or false.
The problem with doing that is that means the
  _3 = -2147483647 - a.1_1;
stmt with undefined behavior on overflow used to be conditional before
but now is unconditional.  reassoc performs a no_side_effect_bb check
which among other checks punts on gimple_has_side_effects and
gimple_assign_rhs_could_trap_p stmts as well as ones that have uses of
their lhs outside of the same bb, but it doesn't punt for this potential
signed overflow case.

Now, for this testcase, it can be fixed in update_range_test by being
smarter and choosing the other comparison to modify.  This is achieved
by storing into oe->id index of the bb with GIMPLE_COND the
comparison feeds into not just for the cases where the comparison is
the GIMPLE_COND itself, but in all cases, and then finding oe->id that
isn't dominated by others.  If we find such, use that oe for the merge
test and if successful, swap range->idx and swap_with->idx.
So for the above case we optimize it into:
  if ((a.1_1 != -2147479551)
      | (-2147483647 - a.1_1 == 1)
      | 0)
    goto bb5;
  else
    goto bb3;
instead.

Unfortunately, this doesn't work in all cases,
optimize_range_tests_to_bit_test and
optimize_range_tests_cmp_bitwise optimizations use non-NULL seq
to update_range_test and they carefully choose a particular comparison
because the sequence then uses SSA_NAMEs that may be defined only in
their blocks.  For that case, the patch keeps using the chosen comparison
but if the merge is successful, rewrites stmts with signed overflow behavior
into defined overflow.
For this I ran into a problem, rewrite_to_defined_overflow returns a
sequence that includes the original stmt with modified arguments, this means
it needs to be gsi_remove first.  Unfortunately, gsi_remove regardless of
the remove_permanently argument creates debug temps for the lhs, which I
think is quite undesirable here.  So I've added an argument (default to
false) to rewrite_to_defined_overflow to do the modification in place
without the need to remove the stmt.

2022-01-27  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/104196
	* gimple-fold.h (rewrite_to_defined_overflow): Add IN_PLACE argument.
	* gimple-fold.cc (rewrite_to_defined_overflow): Likewise.  If true,
	return NULL and emit needed stmts before and after stmt.
	* tree-ssa-reassoc.cc (update_range_test): For inter-bb range opt
	pick as operand_entry that will hold the merged test the one feeding
	earliest condition, ensure that by swapping range->idx with some
	other range's idx if needed.  If seq is non-NULL, don't actually swap
	it but instead rewrite stmts with undefined overflow in between
	the two locations.
	(maybe_optimize_range_tests): Set ops[]->id to bb->index with the
	corresponding condition even if they have non-NULL ops[]->op.
	Formatting fix.

	* gcc.c-torture/execute/pr104196.c: New test.