[PATCH] Fix devirt from dropping lhs with TREE_ADDRESSABLE type on noreturn calls (PR c++/71210)

[Jakub Jelinek]

Hi!

This is another case in the never ending story of dropping lhs of noreturn
calls when we shouldn't.

Though, in this case, while we can optimize a call to a direct call to
normal [[noreturn]] method, we can also optimize into __cxa_pure_virtual
or __builtin_unreachable.  And in those cases IMHO it is desirable to
not have the lhs, but we should also adjust gimple_call_set_fntype,
because we are now calling something different, we've just reused the
same call stmt for that.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2016-05-20  Jakub Jelinek  <ja...@redhat.com>

        PR c++/71210
        * gimple-fold.c (gimple_fold_call): Do not remove lhs of noreturn
        calls if the LHS is variable length or has addressable type.
        If targets[0]->decl is a noreturn call with void return type and
        zero arguments, adjust fntype and remove lhs in that case.

        * g++.dg/opt/pr71210-1.C: New test.
        * g++.dg/opt/pr71210-2.C: New test.

--- gcc/gimple-fold.c.jj        2016-05-03 14:12:19.000000000 +0200
+++ gcc/gimple-fold.c   2016-05-20 10:18:22.818728240 +0200
@@ -3039,10 +3039,25 @@ gimple_fold_call (gimple_stmt_iterator *
                }
              if (targets.length () == 1)
                {
-                 gimple_call_set_fndecl (stmt, targets[0]->decl);
+                 tree fndecl = targets[0]->decl;
+                 gimple_call_set_fndecl (stmt, fndecl);
                  changed = true;
+                 /* If changing the call to __cxa_pure_virtual
+                    or similar noreturn function, adjust gimple_call_fntype
+                    too.  */
+                 if ((gimple_call_flags (stmt) & ECF_NORETURN)
+                     && VOID_TYPE_P (TREE_TYPE (TREE_TYPE (fndecl)))
+                     && TYPE_ARG_TYPES (TREE_TYPE (fndecl))
+                     && (TREE_VALUE (TYPE_ARG_TYPES (TREE_TYPE (fndecl)))
+                         == void_type_node))
+                   gimple_call_set_fntype (stmt, TREE_TYPE (fndecl));
                  /* If the call becomes noreturn, remove the lhs.  */
-                 if (lhs && (gimple_call_flags (stmt) & ECF_NORETURN))
+                 if (lhs
+                     && (gimple_call_flags (stmt) & ECF_NORETURN)
+                     && (VOID_TYPE_P (TREE_TYPE (gimple_call_fntype (stmt)))
+                         || ((TREE_CODE (TYPE_SIZE_UNIT (TREE_TYPE (lhs)))
+                              == INTEGER_CST)
+                             && !TREE_ADDRESSABLE (TREE_TYPE (lhs)))))
                    {
                      if (TREE_CODE (lhs) == SSA_NAME)
                        {
--- gcc/testsuite/g++.dg/opt/pr71210-1.C.jj     2016-05-20 09:30:13.119724805 
+0200
+++ gcc/testsuite/g++.dg/opt/pr71210-1.C        2016-05-20 09:29:42.000000000 
+0200
@@ -0,0 +1,14 @@
+// PR c++/71210
+// { dg-do compile }
+// { dg-options "-O2" }
+
+#include <typeinfo>
+
+void f1 (const std::type_info&) __attribute__((noreturn));
+struct S1 { ~S1 (); };
+struct S2
+{
+  virtual S1 f2 () const { f1 (typeid (*this)); }
+  S1 f3 () const { return f2 (); }
+};
+void f4 () { S2 a; a.f3 (); }
--- gcc/testsuite/g++.dg/opt/pr71210-2.C.jj     2016-05-20 10:20:00.918402232 
+0200
+++ gcc/testsuite/g++.dg/opt/pr71210-2.C        2016-05-20 10:19:48.000000000 
+0200
@@ -0,0 +1,23 @@
+// PR c++/71210
+// { dg-do compile }
+// { dg-options "-O2" }
+
+struct C { int a; int b; C (); ~C (); };
+
+namespace
+{
+  struct A
+  {
+    A () {}
+    virtual C bar (int) = 0;
+    C baz (int x) { return bar (x); }
+  };
+}
+
+A *a;
+
+void
+foo ()
+{
+  C c = a->baz (0);
+}

        Jakub