[PATCH 3/3] struct packed: Add fallback byte array implementation

Pedro Alves
2022-07-21T15:21:58Z

Attribute gcc_struct is not implemented in Clang targeting Windows, so
add a fallback standard-conforming implementation based on arrays.

I ran the testsuite on x86_64 GNU/Linux with this implementation
forced, and saw no regressions.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29373

Change-Id: I023315ee03622c59c397bf4affc0b68179c32374
---
gdbsupport/packed.h | 51 ++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 48 insertions(+), 3 deletions(-)

diff --git a/gdbsupport/packed.h b/gdbsupport/packed.h
index d721b02c056..dbece2a29aa 100644
--- a/gdbsupport/packed.h
+++ b/gdbsupport/packed.h
@@ -28,9 +28,27 @@
bit-fields (and ENUM_BITFIELD), when the fields must have separate
memory locations to avoid data races. */

-/* We need gcc_struct on Windows GCC, as otherwise the size of e.g.,
- "packed<int, 1>" will be larger than what we want. */
-#if defined _WIN32
+/* There are two implementations here -- one standard compliant, using
+ a byte array for internal representation, and another that relies
+ on bitfields and attribute packed (and attribute gcc_struct on
+ Windows). The latter is preferable, as it is more convenient when
+ debugging GDB -- printing a struct packed variable prints its field
+ using its natural type, which is particularly useful if the type is
+ an enum -- but may not work on all compilers. */
+
+/* Clang targeting Windows does not support attribute gcc_struct, so
+ we use the alternative byte array implemention there. */
+#if defined _WIN32 && defined __clang__
+# define PACKED_USE_ARRAY 1
+#else
+# define PACKED_USE_ARRAY 0
+#endif
+
+/* For the preferred implementation, we need gcc_struct on Windows, as
+ otherwise the size of e.g., "packed<int, 1>" will be larger than
+ what we want. Clang targeting Windows does not support attribute
+ gcc_struct. */
+#if !PACKED_USE_ARRAY && defined _WIN32 && !defined __clang__
# define ATTRIBUTE_GCC_STRUCT __attribute__((__gcc_struct__))
#else
# define ATTRIBUTE_GCC_STRUCT
@@ -44,7 +62,18 @@ struct ATTRIBUTE_GCC_STRUCT packed

packed (T val)
{
+ gdb_static_assert (sizeof (ULONGEST) >= sizeof (T));
+
+#if PACKED_USE_ARRAY
+ ULONGEST tmp = val;
+ for (int i = (Bytes - 1); i >= 0; --i)
+ {
+ m_bytes[i] = (gdb_byte) tmp;
+ tmp >>= HOST_CHAR_BIT;
+ }
+#else
m_val = val;
+#endif

/* Ensure size and aligment are what we expect. */
gdb_static_assert (sizeof (packed) == Bytes);
@@ -62,11 +91,27 @@ struct ATTRIBUTE_GCC_STRUCT packed

operator T () const noexcept
{
+#if PACKED_USE_ARRAY
+ ULONGEST tmp = 0;
+ for (int i = 0;;)
+ {
+ tmp |= m_bytes[i];
+ if (++i == Bytes)
+ break;
+ tmp <<= HOST_CHAR_BIT;
+ }
+ return (T) tmp;
+#else
return m_val;
+#endif
}

private:
+#if PACKED_USE_ARRAY
+ gdb_byte m_bytes[Bytes];
+#else
T m_val : (Bytes * HOST_CHAR_BIT) ATTRIBUTE_PACKED;
+#endif
};

/* Add some comparisons between std::atomic<packed<T>> and packed<T>
--
2.36.0