Rewrite CompactValue to avoid undefined behavior from the use of a union for type-punning #1154

2022-07-18T16:32:59-07:00

htpiv commented

2022-07-18 16:32:59 -07:00

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C++ does not, pedantically, allow the use of unions for type-punning in the way that C does. Most compilers, in practice, do support it; however, recent versions of MSVC appear to have a bug that cause bad code to be generated due to this U.B. (see: https://developercommunity.visualstudio.com/t/Bad-code-generated-for-std::isnan-compil/10082631). This led to a series of issues in the react-native-windows project, see:

In C++20, the <bit> header and bit_cast function provide a pleasant API for type-punning. Since C++20 is not universally available, if the feature-test macro for bit_cast is not defined, memcpy is used instead.

C++ does not, pedantically, allow the use of unions for type-punning in the way that C does. Most compilers, in practice, do support it; however, recent versions of MSVC appear to have a bug that cause bad code to be generated due to this U.B. (see: https://developercommunity.visualstudio.com/t/Bad-code-generated-for-std::isnan-compil/10082631). This led to a series of issues in the react-native-windows project, see: * https://github.com/microsoft/react-native-windows/issues/4122 * https://github.com/microsoft/react-native-windows/issues/8675 In C++20, the `<bit>` header and `bit_cast` function provide a pleasant API for type-punning. Since C++20 is not universally available, if the feature-test macro for `bit_cast` is not defined, memcpy is used instead.

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2022-07-18 16:33:04 -07:00

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2022-07-19 11:38:33 -07:00

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2022-07-19 11:54:16 -07:00

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facebook-github-bot commented

2022-07-22 11:57:41 -07:00

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@rozele has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

@rozele has imported this pull request. If you are a Meta employee, you can view this diff [on Phabricator](https://www.internalfb.com/diff/D38082048).

rozele (Migrated from github.com) reviewed 2022-07-25 10:02:53 -07:00

yoga/CompactValue.h

						
				@@ -12,0 +13,4 @@

				#include <bit>

				#else

				#include <cstring>

				#endif

rozele (Migrated from github.com) commented

2022-07-25 10:01:53 -07:00

Looks like internal builds are failing because std::memcpy requires #include <cstring>

Looks like internal builds are failing because std::memcpy requires `#include <cstring>`

yoga/CompactValue.h

rozele (Migrated from github.com) commented

2022-07-25 10:02:43 -07:00

Should this be:

    return std::bit_cast<float>(u);

Should this be: ``` return std::bit_cast<float>(u); ```

Pull request closed

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Reference: DaddyFrosty/yoga#1154