Improve handling of return slots in ipa-modref

[Jan Hubicka]

Hi,
while looking at Martin's patch I also noticed that return slots are
handled but overactively.  We only care if the SSA name we analyze is
base of return slot.

Bootstrapped/regtested x86_64-linux, comitted.

Honza

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 --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" } }