[gcc r11-8203] c++: constexpr and volatile member function [PR80456]

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:3682052e4ccf0a29d1f61df1c8e31f8190eafafe

commit r11-8203-g3682052e4ccf0a29d1f61df1c8e31f8190eafafe
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Apr 15 15:13:18 2021 -0400

    c++: constexpr and volatile member function [PR80456]
    
    When calling a static member function we still need to evaluate an explicit
    object argument.  But we don't want to force a load of the entire object
    if the argument is volatile, so we take its address.  If as a result it no
    longer has any side-effects, we don't need to evaluate it after all.
    
    gcc/cp/ChangeLog:
    
            PR c++/80456
            * call.c (build_new_method_call_1): Check again for side-effects
            with a volatile object.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/80456
            * g++.dg/cpp0x/constexpr-volatile3.C: New test.

Diff:
---
 gcc/cp/call.c                                    |  3 ++-
 gcc/testsuite/g++.dg/cpp0x/constexpr-volatile3.C | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index c9a8c0d305f..678e120a165 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -10793,7 +10793,8 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
 		  tree a = instance;
 		  if (TREE_THIS_VOLATILE (a))
 		    a = build_this (a);
-		  call = build2 (COMPOUND_EXPR, TREE_TYPE (call), a, call);
+		  if (TREE_SIDE_EFFECTS (a))
+		    call = build2 (COMPOUND_EXPR, TREE_TYPE (call), a, call);
 		}
 	      else if (call != error_mark_node
 		       && DECL_DESTRUCTOR_P (cand->fn)
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-volatile3.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-volatile3.C
new file mode 100644
index 00000000000..5c1e865e0ac
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-volatile3.C
@@ -0,0 +1,15 @@
+// PR c++/80456
+// { dg-do compile { target c++11 } }
+
+struct A {
+  static constexpr bool test() noexcept { return true; }
+
+  void f() volatile {
+    constexpr bool b = test();
+  }
+};
+
+void g() {
+  A a;
+  a.f();
+}