tree-optimization/103037 - PRE simplifying valueized expressions

tree-optimization/103037 - PRE simplifying valueized expressions

This fixes a long-standing issue in PRE where we track valueized
expressions in our expression sets that we use for PHI translation,
code insertion but also feed into match-and-simplify via
vn_nary_simplify.  But that's not what is expected from vn_nary_simplify
or match-and-simplify which assume we are simplifying with operands
available at the point of the expression so they can use contextual
information on the SSA names like ranges.  While the VN side was
updated to ensure this with the rewrite to RPO VN, thereby removing
all workarounds that nullified such contextual info on all SSA names,
the PRE side still suffers from this.

The following patch tries to apply minimal surgery at this point
and makes PRE track un-valueized expressions in the expression sets
but only for the NARY kind (both NAME and CONSTANT do not suffer
from this issue), leaving the REFERENCE kind alone.  The REFERENCE
kind is important when trying to remove the workarounds still in
place in compute_avail for code hoisting, but that's a separate issue
and we have a working workaround in place.

Doing this comes at the cost of duplicating the VN IL on the PRE side
for NARY and eventually some extra overhead for translated expressions
that is difficult to assess.

2022-02-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103037
	* tree-ssa-sccvn.h (alloc_vn_nary_op_noinit): Declare.
	(vn_nary_length_from_stmt): Likewise.
	(init_vn_nary_op_from_stmt): Likewise.
	(vn_nary_op_compute_hash): Likewise.
	* tree-ssa-sccvn.cc (alloc_vn_nary_op_noinit): Export.
	(vn_nary_length_from_stmt): Likewise.
	(init_vn_nary_op_from_stmt): Likewise.
	(vn_nary_op_compute_hash): Likewise.
	* tree-ssa-pre.cc (pre_expr_obstack): New obstack.
	(get_or_alloc_expr_for_nary): Pass in the value-id to use,
	(re-)compute the hash value and if the expression is not
	found allocate it from pre_expr_obstack.
	(phi_translate_1): Do not insert the NARY found in the
	VN tables but build a PRE expression from the valueized
	NARY with the value-id we eventually found.
	(find_or_generate_expression): Assert we have an entry
	for constant values.
	(compute_avail): Insert not valueized expressions into
	EXP_GEN using the value-id from the VN tables.
	(init_pre): Allocate pre_expr_obstack.
	(fini_pre): Free pre_expr_obstack.

	* gcc.dg/torture/pr103037.c: New testcase.