Add support for reading and writing values with runtime-endianness

[zturner]

Summary

llvm/Support/Endian.h contains a bunch of functions and classes for working with values whose endianness is known at compile time. But it does not contain anything for working with values where the endianness is only known at runtime.

This patch adds overloads of read and write that take the endianness not as a template parameter, but as a function argument. The versions of the function that use the template parameter are updated to call the version that takes the runtime argument. Since all these functions are inlined, the demotion from a compile-time value to a runtime value should not incur any additional overhead.

The long term motivation for this is to convert LLDB's endian-aware code to use LLVM instead of reinventing all of their own code for doing byte swaps, theyir own enumeration for representing byte order, etc. For now, this just adds the necessary machinery.

Since the template functions now call the runtime functions, no additional tests are necessary, as there are already tests written against the templated versions of the functions.

[Bigcheese]

This seems fine. Can you first verify that -O2 with gcc, clang, and msvc still get it right? I expect them to, just want to make sure.

[zturner]

Code looks fine on MSVC and Clang. I don't have the ability to test on GCC, but given that both MSVC and Clang get it right, I would be rather surprised if GCC does not. Seem reasonable?

llvm/include/llvm/Support/Endian.h

namespace detail {
   /// \brief ::value is either alignment, or alignof(T) if alignment is 0.
   template<class T, int alignment>
   struct PickAlignment {
     enum { value = alignment == 0 ? alignof(T) : alignment };
   };
 } // end namespace detail
 
 namespace endian {
+constexpr endianness system_endianness() {
+  return sys::IsBigEndianHost ? big : little;
+}
+
+template <typename value_type>
+inline value_type byte_swap(value_type value, endianness endian) {
+  if ((endian != native) && (endian != system_endianness()))
+    sys::swapByteOrder(value);
+  return value;
+}
+
 /// Swap the bytes of value to match the given endianness.
 template<typename value_type, endianness endian>
 inline value_type byte_swap(value_type value) {
-  if (endian != native && sys::IsBigEndianHost != (endian == big))
-    sys::swapByteOrder(value);
-  return value;
+  return byte_swap(value, endian);
 }
 
 /// Read a value of a particular endianness from memory.
-template<typename value_type,
-         endianness endian,
-         std::size_t alignment>
-inline value_type read(const void *memory) {
+template <typename value_type, std::size_t alignment>
+inline value_type read(const void *memory, endianness endian) {
   value_type ret;
 
   memcpy(&ret,
-         LLVM_ASSUME_ALIGNED(memory,
-           (detail::PickAlignment<value_type, alignment>::value)),
+         LLVM_ASSUME_ALIGNED(
+             memory, (detail::PickAlignment<value_type, alignment>::value)),
          sizeof(value_type));
-  return byte_swap<value_type, endian>(ret);
+  return byte_swap<value_type>(ret, endian);
+}
+
+template<typename value_type,
+         endianness endian,
+         std::size_t alignment>
+inline value_type read(const void *memory) {
+  return read<value_type, alignment>(memory, endian);
 }
 
 /// Read a value of a particular endianness from a buffer, and increment the
 /// buffer past that value.
+template <typename value_type, std::size_t alignment, typename CharT>
+inline value_type readNext(const CharT *&memory, endianness endian) {
+  value_type ret = read<value_type, alignment>(memory, endian);
+  memory += sizeof(value_type);
+  return ret;
+}
+
 template<typename value_type, endianness endian, std::size_t alignment,
          typename CharT>
 inline value_type readNext(const CharT *&memory) {
-  value_type ret = read<value_type, endian, alignment>(memory);
-  memory += sizeof(value_type);
-  return ret;
+  return readNext<value_type, alignment, CharT>(memory, endian);
 }
 
 /// Write a value to memory with a particular endianness.
+template <typename value_type, std::size_t alignment>
+inline void write(void *memory, value_type value, endianness endian) {
+  value = byte_swap<value_type>(value, endian);
+  memcpy(LLVM_ASSUME_ALIGNED(
+             memory, (detail::PickAlignment<value_type, alignment>::value)),
+         &value, sizeof(value_type));
+}
+
 template<typename value_type,
          endianness endian,
          std::size_t alignment>
 inline void write(void *memory, value_type value) {
-  value = byte_swap<value_type, endian>(value);
-  memcpy(LLVM_ASSUME_ALIGNED(memory,
-           (detail::PickAlignment<value_type, alignment>::value)),
-         &value,
-         sizeof(value_type));
+  write<value_type, alignment>(memory, value, endian);
 }
 
 template <typename value_type>
 using make_unsigned_t = typename std::make_unsigned<value_type>::type;
 
 /// Read a value of a particular endianness from memory, for a location
 /// that starts at the given bit offset within the first byte.
 template <typename value_type, endianness endian, std::size_t alignment>

 typedef detail::packed_endian_specific_integral
                    <int16_t, native, unaligned> unaligned_int16_t;
 typedef detail::packed_endian_specific_integral
                    <int32_t, native, unaligned> unaligned_int32_t;
 typedef detail::packed_endian_specific_integral
                    <int64_t, native, unaligned> unaligned_int64_t;
 
 namespace endian {
+template <typename T> inline T read(const void *P, endianness E) {
+  return read<T, unaligned>(P, E);
+}
+
 template <typename T, endianness E> inline T read(const void *P) {
   return *(const detail::packed_endian_specific_integral<T, E, unaligned> *)P;
 }
 
+inline uint16_t read16(const void *P, endianness E) {
+  return read<uint16_t>(P, E);
+}
+inline uint32_t read32(const void *P, endianness E) {
+  return read<uint32_t>(P, E);
+}
+inline uint64_t read64(const void *P, endianness E) {
+  return read<uint64_t>(P, E);
+}
+
 template <endianness E> inline uint16_t read16(const void *P) {
   return read<uint16_t, E>(P);
 }
 template <endianness E> inline uint32_t read32(const void *P) {
   return read<uint32_t, E>(P);
 }
 template <endianness E> inline uint64_t read64(const void *P) {
   return read<uint64_t, E>(P);
 }
 
 inline uint16_t read16le(const void *P) { return read16<little>(P); }
 inline uint32_t read32le(const void *P) { return read32<little>(P); }
 inline uint64_t read64le(const void *P) { return read64<little>(P); }
 inline uint16_t read16be(const void *P) { return read16<big>(P); }
 inline uint32_t read32be(const void *P) { return read32<big>(P); }
 inline uint64_t read64be(const void *P) { return read64<big>(P); }
 
+template <typename T> inline void write(void *P, T V, endianness E) {
+  write<T, unaligned>(P, V, E);
+}
+
 template <typename T, endianness E> inline void write(void *P, T V) {
   *(detail::packed_endian_specific_integral<T, E, unaligned> *)P = V;
 }
 
+inline void write16(void *P, uint16_t V, endianness E) {
+  write<uint16_t>(P, V, E);
+}
+inline void write32(void *P, uint32_t V, endianness E) {
+  write<uint32_t>(P, V, E);
+}
+inline void write64(void *P, uint64_t V, endianness E) {
+  write<uint64_t>(P, V, E);
+}
+
 template <endianness E> inline void write16(void *P, uint16_t V) {
   write<uint16_t, E>(P, V);
 }
 template <endianness E> inline void write32(void *P, uint32_t V) {
   write<uint32_t, E>(P, V);
 }
 template <endianness E> inline void write64(void *P, uint64_t V) {
   write<uint64_t, E>(P, V);

llvm/include/llvm/Support/Host.h

 #endif
 
 #include <string>
 
 namespace llvm {
 namespace sys {
 
 #if defined(BYTE_ORDER) && defined(BIG_ENDIAN) && BYTE_ORDER == BIG_ENDIAN
-  static const bool IsBigEndianHost = true;
+constexpr bool IsBigEndianHost = true;
 #else
-  static const bool IsBigEndianHost = false;
+constexpr bool IsBigEndianHost = false;
 #endif
 
   static const bool IsLittleEndianHost = !IsBigEndianHost;
 
   /// getDefaultTargetTriple() - Return the default target triple the compiler
   /// has been configured to produce code for.
   ///
   /// The target triple is a string in the format of: