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.

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index 1241b1395d49a6a061e8af508f9ad64cda363373..81467f19fdb203231fe45ff89c2cd57a1c1bfd25 100644 (file)
--- 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 (file)
index 0000000..4a3c682
--- /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}, "");