From 33be07be9e46f15b9556521050356c47460651ee Mon Sep 17 00:00:00 2001
From: Jakub Jelinek <jakub@redhat.com>
Date: Fri, 4 Dec 2020 18:00:54 +0100
Subject: [PATCH] fold-const: Don't use build_constructor for non-aggregate
 types in native_encode_initializer [PR93121]

The following testcase is rejected, because when trying to encode a zeroing
CONSTRUCTOR, the code was using build_constructor to build initializers for
the elements but when recursing the function handles CONSTRUCTOR only for
aggregate types.

The following patch fixes that by using build_zero_cst instead for
non-aggregates.  Another option would be add handling CONSTRUCTOR for
non-aggregates in native_encode_initializer.  Or we can do both, I guess
the middle-end generally doesn't like CONSTRUCTORs for scalar variables, but
am not 100% sure if the FE doesn't produce those sometimes.

2020-12-04  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/93121
	* fold-const.c (native_encode_initializer): Use build_zero_cst
	instead of build_constructor.

	* g++.dg/cpp2a/bit-cast6.C: New test.
---
 gcc/fold-const.c                       | 10 ++++++----
 gcc/testsuite/g++.dg/cpp2a/bit-cast6.C | 31 +++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/bit-cast6.C

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index e77d74e..1241b13 100644
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -8104,11 +8104,12 @@ native_encode_initializer (tree init, unsigned char *ptr, int len,
 		{
 		  if (valueinit == -1)
 		    {
-		      tree zero = build_constructor (TREE_TYPE (type), NULL);
+		      tree zero = build_zero_cst (TREE_TYPE (type));
 		      r = native_encode_initializer (zero, ptr + curpos,
 						     fieldsize, 0,
 						     mask + curpos);
-		      ggc_free (zero);
+		      if (TREE_CODE (zero) == CONSTRUCTOR)
+			ggc_free (zero);
 		      if (!r)
 			return 0;
 		      valueinit = curpos;
@@ -8255,8 +8256,9 @@ native_encode_initializer (tree init, unsigned char *ptr, int len,
 		    {
 		      cnt--;
 		      field = fld;
-		      val = build_constructor (TREE_TYPE (fld), NULL);
-		      to_free = val;
+		      val = build_zero_cst (TREE_TYPE (fld));
+		      if (TREE_CODE (val) == CONSTRUCTOR)
+			to_free = val;
 		    }
 		}
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/bit-cast6.C b/gcc/testsuite/g++.dg/cpp2a/bit-cast6.C
new file mode 100644
index 0000000..4b70da1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/bit-cast6.C
@@ -0,0 +1,31 @@
+// PR libstd++/93121
+// { dg-do compile { target c++20 } }
+
+namespace std
+{
+enum class byte : unsigned char {};
+template <typename To, typename From>
+constexpr To
+bit_cast (const From &from)
+{
+  return __builtin_bit_cast (To, from);
+}
+}
+
+struct S { unsigned short s[2]; };
+constexpr std::byte from1[sizeof (S)]{};
+constexpr auto to1 = std::bit_cast<S>(from1);
+constexpr unsigned char from2[sizeof (S)]{};
+constexpr auto to2 = std::bit_cast<S>(from2);
+
+constexpr bool
+cmp (const S &s1, const S &s2)
+{
+  for (int i = 0; i < sizeof (s1.s) / sizeof (s1.s[0]); i++)
+    if (s1.s[i] != s2.s[i])
+      return false;
+  return true;
+}
+
+static_assert (cmp (to1, S{}));
+static_assert (cmp (to2, S{}));
-- 
2.7.4