From 4164e58bfd5d57e7399f91c7c27ae6ccebc7dcb0 Mon Sep 17 00:00:00 2001
From: Jakub Jelinek <jakub@redhat.com>
Date: Wed, 9 Dec 2020 09:34:51 +0100
Subject: [PATCH] fold-const: Fix up native_encode_initializer missing field
 handling [PR98193]

When native_encode_initializer is called with non-NULL mask (i.e. ATM
bit_cast only), it checks if the current index in the CONSTRUCTOR (if any)
is the next initializable FIELD_DECL, and if not, decrements cnt and
performs the iteration with that FIELD_DECL as field and val of zero
(so that it computes mask properly).  As the testcase shows, I forgot to
set pos to the byte position of the field though (like it is done
for e.g. index referenced FIELD_DECLs in the constructor.

2020-12-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98193
	* fold-const.c (native_encode_initializer): Set pos to field's
	byte position if iterating over a field with missing initializer.

	* g++.dg/cpp2a/bit-cast7.C: New test.
---
 gcc/fold-const.c                       |  1 +
 gcc/testsuite/g++.dg/cpp2a/bit-cast7.C | 39 ++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/bit-cast7.C

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index 1241b1395d4..81467f19fdb 100644
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -8256,6 +8256,7 @@ native_encode_initializer (tree init, unsigned char *ptr, int len,
 		    {
 		      cnt--;
 		      field = fld;
+		      pos = int_byte_position (field);
 		      val = build_zero_cst (TREE_TYPE (fld));
 		      if (TREE_CODE (val) == CONSTRUCTOR)
 			to_free = val;
diff --git a/gcc/testsuite/g++.dg/cpp2a/bit-cast7.C b/gcc/testsuite/g++.dg/cpp2a/bit-cast7.C
new file mode 100644
index 00000000000..4a3c6820070
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/bit-cast7.C
@@ -0,0 +1,39 @@
+// PR c++/98193
+// { dg-do compile { target c++20 } }
+
+template <typename To, typename From>
+constexpr To
+bit_cast (const From &from)
+{
+  return __builtin_bit_cast (To, from);
+}
+
+struct J
+{
+  long int a, b : 11, h;
+};
+
+struct K
+{
+  long int a, b : 11, c;
+  constexpr bool operator == (const K &x)
+  {
+    return a == x.a && b == x.b && c == x.c;
+  }
+};
+
+struct L
+{
+  long long int a, b : 11, h;
+};
+struct M
+{
+  long long int a, b : 11, c;
+  constexpr bool operator == (const M &x)
+  {
+    return a == x.a && b == x.b && c == x.c;
+  }
+};
+
+static_assert (bit_cast <K> (J{}) == K{}, "");
+static_assert (bit_cast <M> (L{0x0feedbacdeadbeefLL}) == M{0x0feedbacdeadbeefLL}, "");
-- 
2.30.2