[PATCH] c++: Further fix for get_member_function_from_ptrfunc [PR117259]

[Jakub Jelinek]

Hi!

The following testcase shows that the previous get_member_function_from_ptrfunc
changes weren't sufficient and we still have cases where
-fsanitize=undefined with pointers to member functions can cause wrong code
being generated and related false positive warnings.

The problem is that save_expr doesn't always create SAVE_EXPR, it can skip
some invariant arithmetics and in the end it could be really large
expressions which would be evaluated several times (and what is worse, with
-fsanitize=undefined those expressions then can have SAVE_EXPRs added to
their subparts for -fsanitize=bounds or -fsanitize=null or
-fsanitize=alignment instrumentation).  Tried to just build1 a SAVE_EXPR
+ add TREE_SIDE_EFFECTS instead of save_expr, but that doesn't work either,
because cp_fold happily optimizes those SAVE_EXPRs away when it sees
SAVE_EXPR operand is tree_invariant_p.

So, the following patch instead of using save_expr or building SAVE_EXPR
manually builds a TARGET_EXPR.  Both types are pointers, so it doesn't need
to be destroyed in any way, but TARGET_EXPR is what doesn't get optimized
away immediately.

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

2024-10-22  Jakub Jelinek  <jakub@redhat.com>

	PR c++/117259
	* typeck.cc (get_member_function_from_ptrfunc): Use force_target_expr
	rather than save_expr for instance_ptr and function.  Don't call it
	for TREE_CONSTANT.

	* g++.dg/ubsan/pr117259.C: New test.

--- gcc/cp/typeck.cc.jj	2024-10-16 14:42:58.835725318 +0200
+++ gcc/cp/typeck.cc	2024-10-22 16:12:58.462731292 +0200
@@ -4193,24 +4193,27 @@ get_member_function_from_ptrfunc (tree *
       if (!nonvirtual && is_dummy_object (instance_ptr))
 	nonvirtual = true;
 
-      /* Use save_expr even when instance_ptr doesn't have side-effects,
-	 unless it is a simple decl (save_expr won't do anything on
-	 constants), so that we don't ubsan instrument the expression
-	 multiple times.  See PR116449.  */
+      /* Use force_target_expr even when instance_ptr doesn't have
+	 side-effects, unless it is a simple decl or constant, so
+	 that we don't ubsan instrument the expression multiple times.
+	 Don't use save_expr, as save_expr can avoid building a SAVE_EXPR
+	 and building a SAVE_EXPR manually can be optimized away during
+	 cp_fold.  See PR116449 and PR117259.  */
       if (TREE_SIDE_EFFECTS (instance_ptr)
-	  || (!nonvirtual && !DECL_P (instance_ptr)))
-	{
-	  instance_save_expr = save_expr (instance_ptr);
-	  if (instance_save_expr == instance_ptr)
-	    instance_save_expr = NULL_TREE;
-	  else
-	    instance_ptr = instance_save_expr;
-	}
+	  || (!nonvirtual
+	      && !DECL_P (instance_ptr)
+	      && !TREE_CONSTANT (instance_ptr)))
+	instance_ptr = instance_save_expr
+	  = force_target_expr (TREE_TYPE (instance_ptr), instance_ptr,
+			       complain);
 
       /* See above comment.  */
       if (TREE_SIDE_EFFECTS (function)
-	  || (!nonvirtual && !DECL_P (function)))
-	function = save_expr (function);
+	  || (!nonvirtual
+	      && !DECL_P (function)
+	      && !TREE_CONSTANT (function)))
+	function
+	  = force_target_expr (TREE_TYPE (function), function, complain);
 
       /* Start by extracting all the information from the PMF itself.  */
       e3 = pfn_from_ptrmemfunc (function);
--- gcc/testsuite/g++.dg/ubsan/pr117259.C.jj	2024-10-22 17:00:52.156114344 +0200
+++ gcc/testsuite/g++.dg/ubsan/pr117259.C	2024-10-22 17:05:20.470324367 +0200
@@ -0,0 +1,13 @@
+// PR c++/117259
+// { dg-do compile }
+// { dg-options "-Wuninitialized -fsanitize=undefined" }
+
+struct A { void foo () {} };
+struct B { void (A::*b) (); B (void (A::*x) ()) : b(x) {}; };
+const B c[1] = { &A::foo };
+
+void
+foo (A *x, int y)
+{
+  (x->*c[y].b) ();
+}

	Jakub