[llvm-commits] [PATCH] [Support][Endian] Add support for specifying the alignment and native unaligned types.

[Richard Smith]

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Comment at: include/llvm/Support/Endian.h:25
@@ -26,1 +24,3 @@
+enum endianness {big, little, native};
+enum {aligned = -1, unaligned = 1};
 
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In C++11 [dcl.align]p2, the value 0 to alignas means "use the native alignment". It would seem reasonable to replicate that here. It would also be nice to document that these values are actually used as alignments, not just as opaque enumerators.

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Comment at: include/llvm/Support/Endian.h:34-68
@@ -37,12 +33,37 @@
 
-// Provides unaligned loads and stores.
-#pragma pack(push)
-#pragma pack(1)
-template<typename value_type>
-struct alignment_access_helper<value_type, unaligned>
-{
-  value_type val;
+// We can't use AlignedCharArrayImpl here because it is larger than
+// the size of the alignment. It also doesn't provide an aligned Storage member.
+#if defined(_MSC_VER)
+# define LLVM_ALIGNED_DEF(a) template<> struct __declspec(align(a)) aligned<a>{}
+  template<int alignment> struct aligned;
+  LLVM_ALIGNED_DEF(1);
+  LLVM_ALIGNED_DEF(2);
+  LLVM_ALIGNED_DEF(4);
+  LLVM_ALIGNED_DEF(8);
+  LLVM_ALIGNED_DEF(16);
+  LLVM_ALIGNED_DEF(32);
+  LLVM_ALIGNED_DEF(64);
+# undef LLVM_ALIGNED_DEF
+
+template<typename value_type, int alignment>
+struct AlignmentHelper
+  : detail::aligned<detail::align<value_type, alignment>::value> {
+  uint8_t Storage[sizeof(value_type)];
 };
-#pragma pack(pop)
 
+#elif __has_feature(cxx_alignas)
+template<typename value_type, int alignment>
+struct AlignmentHelper {
+  uint8_t alignas(detail::align<value_type, alignment>::value)
+    Storage[sizeof(value_type)];
+};
+#elif defined(__GNUC__) || defined(__IBM_ATTRIBUTES)
+template<typename value_type, int alignment>
+struct AlignmentHelper {
+  uint8_t __attribute__((aligned(detail::align<value_type, alignment>::value)))
+    Storage[sizeof(value_type)];
+};
+#else
+# error "No supported alignment directive."
+#endif
 } // end namespace detail
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I don't like duplicating this. Is there some way we can extend or reimplement AlignedCharArrayImpl so that we can use the same code for both cases? Since you're using a memcpy anyway, is this actually necessasry?


http://llvm-reviews.chandlerc.com/D200