From: Jason Merrill <jason@redhat.com>
Date: Wed, 5 Oct 2022 15:50:59 +0000 (-0400)
Subject: gimplify: prevent some C++ temporary elision
X-Git-Tag: basepoints/gcc-14~4045
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d3e5465757c599ea64f611290b7793d3141a6b7c;p=thirdparty%2Fgcc.git

gimplify: prevent some C++ temporary elision

In this testcase, we were optimizing away the temporary for f(), but
C++17 and above are clear that there is a temporary, and because its
destructor has visible side-effects we can't optimize it away under the
as-if rule.  So disable this optimization for TREE_ADDRESSABLE type.

I moved the declaration of volatile_p after the call to
gimple_fold_indirect_ref_rhs to minimize indentation changes; I don't see
any way the value of that flag could be affected by the call.

gcc/ChangeLog:

	* gimplify.cc (gimplify_modify_expr_rhs): Don't optimize
	x = *(A*)&<expr> to x = <expr> for a TREE_ADDRESSABLE type.

gcc/testsuite/ChangeLog:

	* g++.dg/init/elide9.C: New test.
---

diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index 95e16f600b5d..d4209ea74a3d 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -5620,7 +5620,7 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p,
 	    }
 	  break;
 	case INDIRECT_REF:
-	  {
+	  if (!TREE_ADDRESSABLE (TREE_TYPE (*from_p)))
 	    /* If we have code like
 
 	     *(const A*)(A*)&x
@@ -5629,11 +5629,13 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p,
 	     of "A"), treat the entire expression as identical to "x".
 	     This kind of code arises in C++ when an object is bound
 	     to a const reference, and if "x" is a TARGET_EXPR we want
-	     to take advantage of the optimization below.  */
-	    bool volatile_p = TREE_THIS_VOLATILE (*from_p);
-	    tree t = gimple_fold_indirect_ref_rhs (TREE_OPERAND (*from_p, 0));
-	    if (t)
+	     to take advantage of the optimization below.  But not if
+	     the type is TREE_ADDRESSABLE; then C++17 says that the
+	     TARGET_EXPR needs to be a temporary.  */
+	    if (tree t
+		= gimple_fold_indirect_ref_rhs (TREE_OPERAND (*from_p, 0)))
 	      {
+		bool volatile_p = TREE_THIS_VOLATILE (*from_p);
 		if (TREE_THIS_VOLATILE (t) != volatile_p)
 		  {
 		    if (DECL_P (t))
@@ -5646,8 +5648,7 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p,
 		ret = GS_OK;
 		changed = true;
 	      }
-	    break;
-	  }
+	  break;
 
 	case TARGET_EXPR:
 	  {
diff --git a/gcc/testsuite/g++.dg/init/elide9.C b/gcc/testsuite/g++.dg/init/elide9.C
new file mode 100644
index 000000000000..810d60a94a44
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/elide9.C
@@ -0,0 +1,25 @@
+// The static_cast should prevent temporary elision.
+// { dg-do run { target c++11 } }
+
+int d;
+struct A
+{
+  int i;
+  A() { }
+  ~A() { ++d; }
+};
+
+A f() { return A(); }
+
+struct B
+{
+  A a;
+  B(): a(static_cast<A&&>(f())) {}
+};
+
+int main()
+{
+  { B b; }
+  if (d != 2)
+    return -1;
+}