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Unified Diff: src/base/macros.h

Issue 553843002: Replace our home-grown BitCast with bit_cast from Chrome/Google3. (Closed) Base URL: https://v8.googlecode.com/svn/branches/bleeding_edge
Patch Set: Created 6 years, 3 months ago
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Index: src/base/macros.h
diff --git a/src/base/macros.h b/src/base/macros.h
index a951c2ba81f903468da7665245b76757cefe391a..7a3561878b6debcdfb248370d2ef48b3eef62938 100644
--- a/src/base/macros.h
+++ b/src/base/macros.h
@@ -5,6 +5,8 @@
#ifndef V8_BASE_MACROS_H_
#define V8_BASE_MACROS_H_
+#include <cstring>
+
#include "include/v8stdint.h"
#include "src/base/build_config.h"
#include "src/base/compiler-specific.h"
@@ -102,6 +104,141 @@ char (&ArraySizeHelper(const T (&array)[N]))[N];
#endif // V8_OS_NACL
+// The COMPILE_ASSERT macro can be used to verify that a compile time
+// expression is true. For example, you could use it to verify the
+// size of a static array:
+//
+// COMPILE_ASSERT(ARRAYSIZE_UNSAFE(content_type_names) == CONTENT_NUM_TYPES,
+// content_type_names_incorrect_size);
+//
+// or to make sure a struct is smaller than a certain size:
+//
+// COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large);
+//
+// The second argument to the macro is the name of the variable. If
+// the expression is false, most compilers will issue a warning/error
+// containing the name of the variable.
+#if V8_HAS_CXX11_STATIC_ASSERT
+
+// Under C++11, just use static_assert.
+#define COMPILE_ASSERT(expr, msg) static_assert(expr, #msg)
+
+#else
+
+template <bool>
+struct CompileAssert {};
+
+#define COMPILE_ASSERT(expr, msg) \
+ typedef CompileAssert<static_cast<bool>(expr)> \
+ msg[static_cast<bool>(expr) ? 1 : -1] ALLOW_UNUSED
+
+// Implementation details of COMPILE_ASSERT:
+//
+// - COMPILE_ASSERT works by defining an array type that has -1
+// elements (and thus is invalid) when the expression is false.
+//
+// - The simpler definition
+//
+// #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1]
+//
+// does not work, as gcc supports variable-length arrays whose sizes
+// are determined at run-time (this is gcc's extension and not part
+// of the C++ standard). As a result, gcc fails to reject the
+// following code with the simple definition:
+//
+// int foo;
+// COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is
+// // not a compile-time constant.
+//
+// - By using the type CompileAssert<(bool(expr))>, we ensures that
+// expr is a compile-time constant. (Template arguments must be
+// determined at compile-time.)
+//
+// - The outer parentheses in CompileAssert<(bool(expr))> are necessary
+// to work around a bug in gcc 3.4.4 and 4.0.1. If we had written
+//
+// CompileAssert<bool(expr)>
+//
+// instead, these compilers will refuse to compile
+//
+// COMPILE_ASSERT(5 > 0, some_message);
+//
+// (They seem to think the ">" in "5 > 0" marks the end of the
+// template argument list.)
+//
+// - The array size is (bool(expr) ? 1 : -1), instead of simply
+//
+// ((expr) ? 1 : -1).
+//
+// This is to avoid running into a bug in MS VC 7.1, which
+// causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1.
+
+#endif
+
+
+// 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>(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) {
+ COMPILE_ASSERT(sizeof(Dest) == sizeof(Source), VerifySizesAreEqual);
+
+ Dest dest;
+ memcpy(&dest, &source, sizeof(dest));
+ return dest;
+}
+
+
// A macro to disallow the evil copy constructor and operator= functions
// This should be used in the private: declarations for a class
#define DISALLOW_COPY_AND_ASSIGN(TypeName) \
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