[PATCH] tree-optimization/98499 - fix modref analysis on RVO statements

[Sergei Trofimovich]

From: Sergei Trofimovich <siarheit@google.com>

Before the change RVO gimple statements were treated as local
stores by modres analysis. But in practice RVO escapes target.

2021-01-30  Sergei Trofimovich  <siarheit@google.com>

gcc/ChangeLog:

	PR tree-optimization/98499
	* ipa-modref.c: treat RVO conservatively and assume
	all possible side-effects.

gcc/testsuite/ChangeLog:

	PR tree-optimization/98499
	* g++.dg/pr98499.C: new test.
---
 gcc/ipa-modref.c               | 14 ++++++++++++--
 gcc/testsuite/g++.dg/pr98499.C | 31 +++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/pr98499.C

diff --git a/gcc/ipa-modref.c b/gcc/ipa-modref.c
index b362de77e74..7aaf53be8f4 100644
--- a/gcc/ipa-modref.c
+++ b/gcc/ipa-modref.c
@@ -1621,9 +1621,19 @@ analyze_ssa_name_flags (tree name, vec<modref_lattice> &lattice, int depth,
       else if (gcall *call = dyn_cast <gcall *> (use_stmt))
 	{
 	  tree callee = gimple_call_fndecl (call);
-
+	  /* Return slot optiomization would require bit of propagation;
+	     give up for now.  */
+	  if (gimple_call_return_slot_opt_p (call)
+	      && gimple_call_lhs (call) != NULL_TREE
+	      && TREE_ADDRESSABLE (TREE_TYPE (gimple_call_lhs (call))))
+	    {
+	      if (dump_file)
+		fprintf (dump_file, "%*s  Unhandled return slot opt\n",
+			 depth * 4, "");
+	      lattice[index].merge (0);
+	    }
 	  /* Recursion would require bit of propagation; give up for now.  */
-	  if (callee && !ipa && recursive_call_p (current_function_decl,
+	  else if (callee && !ipa && recursive_call_p (current_function_decl,
 						  callee))
 	    lattice[index].merge (0);
 	  else
diff --git a/gcc/testsuite/g++.dg/pr98499.C b/gcc/testsuite/g++.dg/pr98499.C
new file mode 100644
index 00000000000..ace088aeed9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr98499.C
@@ -0,0 +1,31 @@
+/* PR tree-optimization/98499.  */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+struct string {
+  // pointer to local store
+  char * _M_buf;
+  // local store
+  char _M_local_buf[16];
+
+  __attribute__((noinline)) string() : _M_buf(_M_local_buf) {}
+
+  ~string() {
+    if (_M_buf != _M_local_buf)
+      __builtin_trap();
+  }
+
+  string(const string &__str); // no copies
+};
+
+__attribute__((noinline)) static string dir_name() { return string(); }
+class Importer {
+  string base_path;
+
+public:
+  __attribute__((noinline)) Importer() : base_path (dir_name()) {}
+};
+
+int main() {
+  Importer imp;
+}
-- 
2.30.0