Move bit_cast from base/macros.h to its own header

base/macros.h

Index: base/macros.h
diff --git a/base/macros.h b/base/macros.h
index fa12dd2e6bca88aa06a2e425dc3a66ab83e48207..46ee1dadb47472edc0b2f228c4b390c89f766854 100644
--- a/base/macros.h
+++ b/base/macros.h
@@ -11,7 +11,6 @@
 #define BASE_MACROS_H_
 
 #include <stddef.h>  // For size_t.
-#include <string.h>  // For memcpy.
 
 // Put this in the declarations for a class to be uncopyable.
 #define DISALLOW_COPY(TypeName) \
@@ -49,70 +48,6 @@
 template <typename T, size_t N> char (&ArraySizeHelper(T (&array)[N]))[N];
 #define arraysize(array) (sizeof(ArraySizeHelper(array)))
 
-// bit_cast<Dest,Source> is a template function that implements the
-// equivalent of "*reinterpret_cast<Dest*>(&source)".  We need this in
-// very low-level functions like the protobuf library and fast math
-// support.
-//
-//   float f = 3.14159265358979;
-//   int i = bit_cast<int32_t>(f);
-//   // i = 0x40490fdb
-//
-// The classical address-casting method is:
-//
-//   // WRONG
-//   float f = 3.14159265358979;            // WRONG
-//   int i = * reinterpret_cast<int*>(&f);  // WRONG
-//
-// The address-casting method actually produces undefined behavior
-// according to ISO C++ specification section 3.10 -15 -.  Roughly, this
-// section says: if an object in memory has one type, and a program
-// accesses it with a different type, then the result is undefined
-// behavior for most values of "different type".
-//
-// This is true for any cast syntax, either *(int*)&f or
-// *reinterpret_cast<int*>(&f).  And it is particularly true for
-// conversions between integral lvalues and floating-point lvalues.
-//
-// The purpose of 3.10 -15- is to allow optimizing compilers to assume
-// that expressions with different types refer to different memory.  gcc
-// 4.0.1 has an optimizer that takes advantage of this.  So a
-// non-conforming program quietly produces wildly incorrect output.
-//
-// The problem is not the use of reinterpret_cast.  The problem is type
-// punning: holding an object in memory of one type and reading its bits
-// back using a different type.
-//
-// The C++ standard is more subtle and complex than this, but that
-// is the basic idea.
-//
-// Anyways ...
-//
-// bit_cast<> calls memcpy() which is blessed by the standard,
-// especially by the example in section 3.9 .  Also, of course,
-// bit_cast<> wraps up the nasty logic in one place.
-//
-// Fortunately memcpy() is very fast.  In optimized mode, with a
-// constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline
-// code with the minimal amount of data movement.  On a 32-bit system,
-// memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8)
-// compiles to two loads and two stores.
-//
-// I tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc 7.1.
-//
-// WARNING: if Dest or Source is a non-POD type, the result of the memcpy
-// is likely to surprise you.
-
-template <class Dest, class Source>
-inline Dest bit_cast(const Source& source) {
-  static_assert(sizeof(Dest) == sizeof(Source),
-                "bit_cast requires source and destination to be the same size");
-
-  Dest dest;
-  memcpy(&dest, &source, sizeof(dest));
-  return dest;
-}
-
 // Used to explicitly mark the return value of a function as unused. If you are
 // really sure you don't want to do anything with the return value of a function
 // that has been marked WARN_UNUSED_RESULT, wrap it with this. Example: