[gcc r12-8913] SRA: Limit replacement creation for accesses propagated from LHSs

[Martin Jambor <jamborm@gcc.gnu.org>]

https://gcc.gnu.org/g:985a8962712a56d1c9418cf05e1761f8bd20cf8f

commit r12-8913-g985a8962712a56d1c9418cf05e1761f8bd20cf8f
Author: Martin Jambor <mjambor@suse.cz>
Date:   Wed Nov 16 13:11:27 2022 +0100

    SRA: Limit replacement creation for accesses propagated from LHSs
    
    PR 107206 is fallout from the fix to PR 92706 where we started
    propagating accesses across assignments also from LHS to RHS of
    assignments so that we would not do harmful total scalarization of the
    aggregates on the RHS.
    
    But this can lead to new scalarization of these aggregates and in the
    testcase of PR 107206 these can appear in superfluous uses of
    un-initialized values and spurious warnings.
    
    Fixed by making sure the the accesses created by propagation in this
    direction are only used as a basis for replacements when the structure
    would be totally scalarized anyway.
    
    gcc/ChangeLog:
    
    2022-10-18  Martin Jambor  <mjambor@suse.cz>
    
            PR tree-optimization/107206
            * tree-sra.cc (struct access): New field grp_result_of_prop_from_lhs.
            (analyze_access_subtree): Do not create replacements for accesses with
            this flag when not toally scalarizing.
            (propagate_subaccesses_from_lhs): Set the new flag.
    
    gcc/testsuite/ChangeLog:
    
    2022-10-18  Martin Jambor  <mjambor@suse.cz>
    
            PR tree-optimization/107206
            * g++.dg/tree-ssa/pr107206.C: New test.
    
    (cherry picked from commit f6c168f8c06047bfaa3005e570126831b8855dcc)

Diff:
---
 gcc/testsuite/g++.dg/tree-ssa/pr107206.C | 27 +++++++++++++++++++++++++++
 gcc/tree-sra.cc                          |  7 +++++++
 2 files changed, 34 insertions(+)

diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr107206.C b/gcc/testsuite/g++.dg/tree-ssa/pr107206.C
new file mode 100644
index 00000000000..34810ad2b7b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr107206.C
@@ -0,0 +1,27 @@
+// { dg-do compile }
+// { dg-require-effective-target c++17 }
+// { dg-options "-O -Wuninitialized" }
+
+#include <optional>
+struct X {
+    X() = default;
+    X(X const& r) : i(r.i) {}
+    int i;
+};
+struct Y {
+    Y() : x() {}
+    X x;
+    std::optional<int> o;
+};
+struct Z {
+    Y y;
+    explicit Z(Y y) : y(y) {}
+};
+void f(Y const&);
+void test() {
+    Y const y;
+    Z z(y);
+    z.y.o = 1;
+    auto const w = z;
+    f(w.y);
+}
diff --git a/gcc/tree-sra.cc b/gcc/tree-sra.cc
index 099e8dbe873..95291e8df93 100644
--- a/gcc/tree-sra.cc
+++ b/gcc/tree-sra.cc
@@ -260,6 +260,9 @@ struct access
 
   /* Should TREE_NO_WARNING of a replacement be set?  */
   unsigned grp_no_warning : 1;
+
+  /* Result of propagation accross link from LHS to RHS.  */
+  unsigned grp_result_of_prop_from_lhs : 1;
 };
 
 typedef struct access *access_p;
@@ -2532,6 +2535,9 @@ analyze_access_subtree (struct access *root, struct access *parent,
   if (allow_replacements && expr_with_var_bounded_array_refs_p (root->expr))
     allow_replacements = false;
 
+  if (!totally && root->grp_result_of_prop_from_lhs)
+    allow_replacements = false;
+
   for (child = root->first_child; child; child = child->next_sibling)
     {
       hole |= covered_to < child->offset;
@@ -2959,6 +2965,7 @@ propagate_subaccesses_from_lhs (struct access *lacc, struct access *racc)
 	  struct access *new_acc
 	    = create_artificial_child_access (racc, lchild, norm_offset,
 					      true, false);
+	  new_acc->grp_result_of_prop_from_lhs = 1;
 	  propagate_subaccesses_from_lhs (lchild, new_acc);
 	}
       else