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yoga::bit_cast

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Summary:
X-link: https://github.com/facebook/react-native/pull/39358

This adds a function polyfilling C++ 20's `std::bit_cast`, using `memcpy()` to be safe with strict aliasing rules.

This replaces the conditional code in CompactValue for type punning, an unsafe place in YGJNI where we do it unsafely, and is used in ValuePool. The polyfill can be switched to `std::bit_cast` whenever we adopt C++ 20.

Note that this doesn't actually call into `memcpy()`, as verified by Godbolt. Compilers are aware of the memcpy type punning pattern and optimize it, but it's ugly and confusing to folks who haven't seen it before.

Reviewed By: javache

Differential Revision: D49082997

fbshipit-source-id: b848775a68286bdb11b2a3a95bef8069364ac9b5
This commit is contained in:
Nick Gerleman
2023-09-08 13:03:48 -07:00
committed by Facebook GitHub Bot
parent f8e2bc0875
commit 26d2a2682f
3 changed files with 39 additions and 45 deletions
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8
java/jni/YGJNIVanilla.cpp
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@@ -16,6 +16,7 @@
#include "YogaJniException.h"
#include <yoga/Yoga-internal.h>
#include <yoga/bits/BitCast.h>
// TODO: Reconcile missing layoutContext functionality from callbacks in the C
// API and use that
@@ -677,11 +678,10 @@ static YGSize YGJNIMeasureFunc(
uint32_t wBits = 0xFFFFFFFF & (measureResult >> 32);
uint32_t hBits = 0xFFFFFFFF & measureResult;
// TODO: this is unsafe under strict aliasing and should use bit_cast
const float* measuredWidth = reinterpret_cast<float*>(&wBits);
const float* measuredHeight = reinterpret_cast<float*>(&hBits);
const float measuredWidth = yoga::bit_cast<float>(wBits);
const float measuredHeight = yoga::bit_cast<float>(hBits);
return YGSize{*measuredWidth, *measuredHeight};
return YGSize{measuredWidth, measuredHeight};
} else {
return YGSize{
widthMode == YGMeasureModeUndefined ? 0 : width,