[gcc r12-8837] tree-optimization/106922 - extend same-val clobber FRE

[gcc r12-8837] tree-optimization/106922 - extend same-val clobber FRE

[Richard Biener]

https://gcc.gnu.org/g:b9f58edfc2ccb0fb3840751a2fb4268ce5dd9b3d

commit r12-8837-gb9f58edfc2ccb0fb3840751a2fb4268ce5dd9b3d
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Sep 23 14:28:52 2022 +0200

    tree-optimization/106922 - extend same-val clobber FRE
    
    The following extends the skipping of same valued stores to
    handle an arbitrary number of them as long as they are from the
    same value (which we now record).  That's an obvious extension
    which allows to optimize the m_engaged member of std::optional
    more reliably.
    
            PR tree-optimization/106922
            * tree-ssa-sccvn.cc (vn_reference_lookup_3): Allow
            an arbitrary number of same valued skipped stores.
    
            * g++.dg/torture/pr106922.C: New testcase.
    
    (cherry picked from commit af611afe5fcc908a6678b5b205fb5af7d64fbcb2)

Diff:
---
 gcc/testsuite/g++.dg/torture/pr106922.C | 48 +++++++++++++++++++++++++++++++++
 gcc/tree-ssa-sccvn.cc                   | 10 ++++---
 2 files changed, 55 insertions(+), 3 deletions(-)

diff --git a/gcc/testsuite/g++.dg/torture/pr106922.C b/gcc/testsuite/g++.dg/torture/pr106922.C
new file mode 100644
index 00000000000..046fc6cce76
--- /dev/null
+++ b/gcc/testsuite/g++.dg/torture/pr106922.C
@@ -0,0 +1,48 @@
+// { dg-do compile }
+// { dg-require-effective-target c++17 }
+// { dg-additional-options "-Wall" }
+// -O1 doesn't iterate VN and thus has bogus uninit diagnostics
+// { dg-skip-if "" { *-*-* } { "-O1" } { "" } }
+
+#include <vector>
+
+#include <optional>
+template <class T>
+using Optional = std::optional<T>;
+
+#include <sstream>
+
+struct MyOptionalStructWithInt {
+    int myint; /* works without this */
+    Optional<std::vector<std::string>> myoptional;
+};
+
+struct MyOptionalsStruct {
+    MyOptionalStructWithInt external1;
+    MyOptionalStructWithInt external2;
+};
+
+struct MyStruct { };
+std::ostream &operator << (std::ostream &os, const MyStruct &myStruct);
+
+std::vector<MyStruct> getMyStructs();
+
+void test()
+{
+    MyOptionalsStruct externals;
+    MyOptionalStructWithInt internal1;
+    MyOptionalStructWithInt internal2;
+
+    std::vector<MyStruct> myStructs;
+    myStructs = getMyStructs();
+
+    for (const auto& myStruct : myStructs)
+    {
+        std::stringstream address_stream;
+        address_stream << myStruct;
+        internal1.myint = internal2.myint = 0;
+        externals.external1 = internal1;
+        externals.external2 = internal2;
+        externals.external2 = internal2;
+    }
+}
diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
index cfacf609f43..854c0c25bc7 100644
--- a/gcc/tree-ssa-sccvn.cc
+++ b/gcc/tree-ssa-sccvn.cc
@@ -2671,7 +2671,6 @@ vn_reference_lookup_3 (ao_ref *ref, tree vuse, void *data_,
       if (is_gimple_reg_type (TREE_TYPE (lhs))
 	  && types_compatible_p (TREE_TYPE (lhs), vr->type)
 	  && (ref->ref || data->orig_ref.ref)
-	  && !data->same_val
 	  && !data->mask
 	  && data->partial_defs.is_empty ()
 	  && multiple_p (get_object_alignment
@@ -2684,8 +2683,13 @@ vn_reference_lookup_3 (ao_ref *ref, tree vuse, void *data_,
 	     a different loop iteration but only to loop invariants.  Use
 	     CONSTANT_CLASS_P (unvalueized!) as conservative approximation.
 	     The one-hop lookup below doesn't have this issue since there's
-	     a virtual PHI before we ever reach a backedge to cross.  */
-	  if (CONSTANT_CLASS_P (rhs))
+	     a virtual PHI before we ever reach a backedge to cross.
+	     We can skip multiple defs as long as they are from the same
+	     value though.  */
+	  if (data->same_val
+	      && !operand_equal_p (data->same_val, rhs))
+	    ;
+	  else if (CONSTANT_CLASS_P (rhs))
 	    {
 	      if (dump_file && (dump_flags & TDF_DETAILS))
 		{