tree-optimization/105562 - avoid uninit diagnostic with better FRE

We can avoid some uninit diagnostics by making FRE disambiguate
against CLOBBERs since any aliasing there would invoke undefined
behavior for a read we are looking up.

2022-05-12  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105562
	* tree-ssa-sccvn.cc (vn_reference_lookup_3): Disambiguate
	against all CLOBBER defs if there's not an obvious must-alias
	and we are not doing redundant store elimination.
	(vn_walk_cb_data::redundant_store_removal_p): New field.
	(vn_reference_lookup_pieces): Initialize it.
	(vn_reference_lookup): Add argument to specify if we are
	doing redundant store removal.
	(eliminate_dom_walker::eliminate_stmt): Specify we do.
	* tree-ssa-sccvn.h (vn_reference_lookup): Adjust.

	* g++.dg/warn/uninit-pr105562.C: New testcase.

3 files changed, 40 insertions, 9 deletions

diff --git a/gcc/testsuite/g++.dg/warn/uninit-pr105562.C b/gcc/testsuite/g++.dg/warn/uninit-pr105562.C
new file mode 100644
index 0000000..ec3a550
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/uninit-pr105562.C
@@ -0,0 +1,10 @@
+// { dg-require-effective-target c++11 }
+// { dg-options "-O -Wall -fno-strict-aliasing" }
+
+#include <regex>
+
+int main()
+{
+  std::regex a(".");
+  std::regex b(std::move(a));
+}
diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
index 3732d06..f1bc372 100644
--- a/gcc/tree-ssa-sccvn.cc
+++ b/gcc/tree-ssa-sccvn.cc
@@ -1799,11 +1799,13 @@ struct pd_data
 struct vn_walk_cb_data
 {
   vn_walk_cb_data (vn_reference_t vr_, tree orig_ref_, tree *last_vuse_ptr_,
-		   vn_lookup_kind vn_walk_kind_, bool tbaa_p_, tree mask_)
+		   vn_lookup_kind vn_walk_kind_, bool tbaa_p_, tree mask_,
+		   bool redundant_store_removal_p_)
     : vr (vr_), last_vuse_ptr (last_vuse_ptr_), last_vuse (NULL_TREE),
       mask (mask_), masked_result (NULL_TREE), vn_walk_kind (vn_walk_kind_),
-      tbaa_p (tbaa_p_), saved_operands (vNULL), first_set (-2),
-      first_base_set (-2), known_ranges (NULL)
+      tbaa_p (tbaa_p_), redundant_store_removal_p (redundant_store_removal_p_),
+      saved_operands (vNULL), first_set (-2), first_base_set (-2),
+      known_ranges (NULL)
   {
     if (!last_vuse_ptr)
       last_vuse_ptr = &last_vuse;
@@ -1862,6 +1864,7 @@ struct vn_walk_cb_data
   tree masked_result;
   vn_lookup_kind vn_walk_kind;
   bool tbaa_p;
+  bool redundant_store_removal_p;
   vec<vn_reference_op_s> saved_operands;
 
   /* The VDEFs of partial defs we come along.  */
@@ -2620,6 +2623,19 @@ vn_reference_lookup_3 (ao_ref *ref, tree vuse, void *data_,
 	  return NULL;
 	}
 
+      /* When the def is a CLOBBER we can optimistically disambiguate
+	 against it since any overlap it would be undefined behavior.
+	 Avoid this for obvious must aliases to save compile-time though.
+	 We also may not do this when the query is used for redundant
+	 store removal.  */
+      if (!data->redundant_store_removal_p
+	  && gimple_clobber_p (def_stmt)
+	  && !operand_equal_p (ao_ref_base (&lhs_ref), base, OEP_ADDRESS_OF))
+	{
+	  *disambiguate_only = TR_DISAMBIGUATE;
+	  return NULL;
+	}
+
       /* Besides valueizing the LHS we can also use access-path based
          disambiguation on the original non-valueized ref.  */
       if (!ref->ref
@@ -3604,7 +3620,8 @@ vn_reference_lookup_pieces (tree vuse, alias_set_type set,
     {
       ao_ref r;
       unsigned limit = param_sccvn_max_alias_queries_per_access;
-      vn_walk_cb_data data (&vr1, NULL_TREE, NULL, kind, true, NULL_TREE);
+      vn_walk_cb_data data (&vr1, NULL_TREE, NULL, kind, true, NULL_TREE,
+			    false);
       vec<vn_reference_op_s> ops_for_ref;
       if (!valueized_p)
 	ops_for_ref = vr1.operands;
@@ -3649,12 +3666,14 @@ vn_reference_lookup_pieces (tree vuse, alias_set_type set,
    MASK is either NULL_TREE, or can be an INTEGER_CST if the result of the
    load is bitwise anded with MASK and so we are only interested in a subset
    of the bits and can ignore if the other bits are uninitialized or
-   not initialized with constants.  */
+   not initialized with constants.  When doing redundant store removal
+   the caller has to set REDUNDANT_STORE_REMOVAL_P.  */
 
 tree
 vn_reference_lookup (tree op, tree vuse, vn_lookup_kind kind,
 		     vn_reference_t *vnresult, bool tbaa_p,
-		     tree *last_vuse_ptr, tree mask)
+		     tree *last_vuse_ptr, tree mask,
+		     bool redundant_store_removal_p)
 {
   vec<vn_reference_op_s> operands;
   struct vn_reference_s vr1;
@@ -3732,7 +3751,8 @@ vn_reference_lookup (tree op, tree vuse, vn_lookup_kind kind,
 					     vr1.type, ops_for_ref))
 	ao_ref_init (&r, op);
       vn_walk_cb_data data (&vr1, r.ref ? NULL_TREE : op,
-			    last_vuse_ptr, kind, tbaa_p, mask);
+			    last_vuse_ptr, kind, tbaa_p, mask,
+			    redundant_store_removal_p);
 
       wvnresult
 	= ((vn_reference_t)
@@ -6592,7 +6612,8 @@ eliminate_dom_walker::eliminate_stmt (basic_block b, gimple_stmt_iterator *gsi)
       tree val = NULL_TREE;
       if (lookup_lhs)
 	val = vn_reference_lookup (lookup_lhs, gimple_vuse (stmt),
-				   VN_WALKREWRITE, &vnresult, false);
+				   VN_WALKREWRITE, &vnresult, false,
+				   NULL, NULL_TREE, true);
       if (TREE_CODE (rhs) == SSA_NAME)
 	rhs = VN_INFO (rhs)->valnum;
       if (val
diff --git a/gcc/tree-ssa-sccvn.h b/gcc/tree-ssa-sccvn.h
index c4e3410..a1b1e6b 100644
--- a/gcc/tree-ssa-sccvn.h
+++ b/gcc/tree-ssa-sccvn.h
@@ -265,7 +265,7 @@ tree vn_reference_lookup_pieces (tree, alias_set_type, alias_set_type, tree,
 				 vec<vn_reference_op_s> ,
 				 vn_reference_t *, vn_lookup_kind);
 tree vn_reference_lookup (tree, tree, vn_lookup_kind, vn_reference_t *, bool,
-			  tree * = NULL, tree = NULL_TREE);
+			  tree * = NULL, tree = NULL_TREE, bool = false);
 void vn_reference_lookup_call (gcall *, vn_reference_t *, vn_reference_t);
 vn_reference_t vn_reference_insert_pieces (tree, alias_set_type, alias_set_type,
 					   tree, vec<vn_reference_op_s>,