[gcc r13-6422] c++: more mce_false folding from cp_fully_fold_init [PR108243]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:5425159d176a7a92afc932cbb22d8822667099c4

commit r13-6422-g5425159d176a7a92afc932cbb22d8822667099c4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Mar 2 14:04:50 2023 -0500

    c++: more mce_false folding from cp_fully_fold_init [PR108243]
    
    We should also fold the overall initializer passed to cp_fully_fold_init
    with mce_false, which allows folding of the copy-initialization of
    'a1' in the below testcase (the initializer here is an AGGR_INIT_EXPR).
    
    Unfortunately this doesn't help with direct- or default-initialization
    because we don't call cp_fully_fold_init in that case, and even if we
    did the initializer in that case is expressed as a bare CALL_EXPR
    instead of an AGGR_INIT_EXPR, which cp_fully_fold_init can't really
    fold.
    
            PR c++/108243
            PR c++/97553
    
    gcc/cp/ChangeLog:
    
            * cp-gimplify.cc (cp_fully_fold): Add an internal overload that
            additionally takes and propagate an mce_value parameter, and
            define the existing public overload in terms of it.
            (cp_fully_fold_init): Pass mce_false to cp_fully_fold.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/opt/is_constant_evaluated3.C: New test.

Diff:
---
 gcc/cp/cp-gimplify.cc                             | 19 ++++++++++++++-----
 gcc/testsuite/g++.dg/opt/is_constant_evaluated3.C | 23 +++++++++++++++++++++++
 2 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index 921b0e4b120..b995b4f81a5 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -2449,8 +2449,8 @@ cp_fold_rvalue (tree x)
 
 /* Perform folding on expression X.  */
 
-tree
-cp_fully_fold (tree x)
+static tree
+cp_fully_fold (tree x, mce_value manifestly_const_eval)
 {
   if (processing_template_decl)
     return x;
@@ -2458,7 +2458,7 @@ cp_fully_fold (tree x)
      have to call both.  */
   if (cxx_dialect >= cxx11)
     {
-      x = maybe_constant_value (x);
+      x = maybe_constant_value (x, /*decl=*/NULL_TREE, manifestly_const_eval);
       /* Sometimes we are given a CONSTRUCTOR but the call above wraps it into
 	 a TARGET_EXPR; undo that here.  */
       if (TREE_CODE (x) == TARGET_EXPR)
@@ -2468,7 +2468,16 @@ cp_fully_fold (tree x)
 	       && TREE_TYPE (TREE_OPERAND (x, 0)) == TREE_TYPE (x))
 	x = TREE_OPERAND (x, 0);
     }
-  return cp_fold_rvalue (x);
+  fold_flags_t flags = ff_none;
+  if (manifestly_const_eval == mce_false)
+    flags |= ff_mce_false;
+  return cp_fold_rvalue (x, flags);
+}
+
+tree
+cp_fully_fold (tree x)
+{
+  return cp_fully_fold (x, mce_unknown);
 }
 
 /* Likewise, but also fold recursively, which cp_fully_fold doesn't perform
@@ -2479,7 +2488,7 @@ cp_fully_fold_init (tree x)
 {
   if (processing_template_decl)
     return x;
-  x = cp_fully_fold (x);
+  x = cp_fully_fold (x, mce_false);
   cp_fold_data data (ff_mce_false);
   cp_walk_tree (&x, cp_fold_r, &data, NULL);
   return x;
diff --git a/gcc/testsuite/g++.dg/opt/is_constant_evaluated3.C b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3.C
new file mode 100644
index 00000000000..0a1e46e5638
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/is_constant_evaluated3.C
@@ -0,0 +1,23 @@
+// PR c++/108243
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-O -fdump-tree-original" }
+
+struct A {
+  constexpr A(int n) : n(n), m(__builtin_is_constant_evaluated()) { }
+  constexpr A() : A(42) { }
+  int n, m;
+};
+
+int main() {
+  A a1 = {42};
+  A a2{42};
+  A a3(42);
+  A a4;
+  A a5{};
+}
+
+// { dg-final { scan-tree-dump "a1 = {\\.n=42, \\.m=0}" "original" } }
+// { dg-final { scan-tree-dump "a2 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }
+// { dg-final { scan-tree-dump "a3 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }
+// { dg-final { scan-tree-dump "a4 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }
+// { dg-final { scan-tree-dump "a5 = {\\.n=42, \\.m=0}" "original" { xfail *-*-* } } }