[gcc r12-3064] Improve return slot handling in ipa-modref

[Jan Hubicka <hubicka@gcc.gnu.org>]

https://gcc.gnu.org/g:59f38935d189027f172b4813a6056addaceb7cd3

commit r12-3064-g59f38935d189027f172b4813a6056addaceb7cd3
Author: Jan Hubicka <hubicka@ucw.cz>
Date:   Mon Aug 23 09:21:36 2021 +0200

    Improve return slot handling in ipa-modref
    
    gcc/ChangeLog:
    
            * ipa-modref.c (analyze_ssa_name_flags): Improve handling of return slot.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/tree-ssa/modref-1.C: New test.

Diff:
---
 gcc/ipa-modref.c                         | 23 ++++++++++-------------
 gcc/testsuite/g++.dg/tree-ssa/modref-1.C | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 13 deletions(-)

diff --git a/gcc/ipa-modref.c b/gcc/ipa-modref.c
index 549153865b8..cb0a314cbeb 100644
--- a/gcc/ipa-modref.c
+++ b/gcc/ipa-modref.c
@@ -1709,19 +1709,8 @@ analyze_ssa_name_flags (tree name, vec<modref_lattice> &lattice, int depth,
 	  if (gimple_call_fn (use_stmt) == name)
 	    lattice[index].merge (~EAF_NOCLOBBER);
 
-	  /* Return slot optimization 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.  */
-	  else if (callee && !ipa && recursive_call_p (current_function_decl,
+	  if (callee && !ipa && recursive_call_p (current_function_decl,
 						  callee))
 	    lattice[index].merge (0);
 	  else
@@ -1735,7 +1724,15 @@ analyze_ssa_name_flags (tree name, vec<modref_lattice> &lattice, int depth,
 	      /* Handle *name = func (...).  */
 	      if (gimple_call_lhs (call)
 		  && memory_access_to (gimple_call_lhs (call), name))
-		lattice[index].merge_direct_store ();
+		{
+		  lattice[index].merge_direct_store ();
+		  /* Return slot optimization passes address of
+		     LHS to callee via hidden parameter and this
+		     may make LHS to escape.  See PR 98499.  */
+		  if (gimple_call_return_slot_opt_p (call)
+		      && TREE_ADDRESSABLE (TREE_TYPE (gimple_call_lhs (call))))
+		    lattice[index].merge (EAF_NOREAD | EAF_DIRECT);
+		}
 
 	      /* We do not track accesses to the static chain (we could)
 		 so give up.  */
diff --git a/gcc/testsuite/g++.dg/tree-ssa/modref-1.C b/gcc/testsuite/g++.dg/tree-ssa/modref-1.C
new file mode 100644
index 00000000000..c742dfe8b33
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/modref-1.C
@@ -0,0 +1,32 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+struct S { int a; char b[20]; S(); S(S const&); };
+volatile int global;
+
+__attribute__ ((noinline,noclone))
+struct S noescape (int *b)
+{
+  struct S a;
+  a.a = b!=0;
+  global = 1;
+  return a;
+}
+
+void escape (struct S *p);
+
+__attribute__ ((noinline,noclone))
+int
+test(int *b)
+{
+  struct S s = noescape (b);
+  escape (&s);
+  return *b;
+}
+int test2()
+{
+  int b=1234;
+  test (&b);
+  return b;
+}
+// ipa-modref should analyze parameter B of test as noescape.
+// { dg-final { scan-tree-dump "return 1234" } }