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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-19 01:47:29 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-19 01:47:29 +0000 |
commit | 0ebf5bdf043a27fd3dfb7f92e0cb63d88954c44d (patch) | |
tree | a31f07c9bcca9d56ce61e9a1ffd30ef350d513aa /security/sandbox/chromium/base/bit_cast.h | |
parent | Initial commit. (diff) | |
download | firefox-esr-0ebf5bdf043a27fd3dfb7f92e0cb63d88954c44d.tar.xz firefox-esr-0ebf5bdf043a27fd3dfb7f92e0cb63d88954c44d.zip |
Adding upstream version 115.8.0esr.upstream/115.8.0esr
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'security/sandbox/chromium/base/bit_cast.h')
-rw-r--r-- | security/sandbox/chromium/base/bit_cast.h | 77 |
1 files changed, 77 insertions, 0 deletions
diff --git a/security/sandbox/chromium/base/bit_cast.h b/security/sandbox/chromium/base/bit_cast.h new file mode 100644 index 0000000000..90dd925e86 --- /dev/null +++ b/security/sandbox/chromium/base/bit_cast.h @@ -0,0 +1,77 @@ +// Copyright 2016 The Chromium Authors. All rights reserved. +// Use of this source code is governed by a BSD-style license that can be +// found in the LICENSE file. + +#ifndef BASE_BIT_CAST_H_ +#define BASE_BIT_CAST_H_ + +#include <string.h> +#include <type_traits> + +#include "base/compiler_specific.h" +#include "base/template_util.h" +#include "build/build_config.h" + +// 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 +// the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15. +// (This did not substantially change in C++11.) 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 this paragraph is to allow optimizing compilers to assume that +// expressions with different types refer to different memory. Compilers are +// known to take 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, compilers replace +// calls to memcpy() with inline object code when the size argument is a +// compile-time constant. 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. + +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"); + static_assert(base::is_trivially_copyable<Dest>::value, + "bit_cast requires the destination type to be copyable"); + static_assert(base::is_trivially_copyable<Source>::value, + "bit_cast requires the source type to be copyable"); + + Dest dest; + memcpy(&dest, &source, sizeof(dest)); + return dest; +} + +#endif // BASE_BIT_CAST_H_ |