yoga::bit_cast

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

java/jni/YGJNIVanilla.cpp

@@ -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,

yoga/bits/BitCast.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <cstring>
+#include <type_traits>
+
+namespace facebook::yoga {
+
+// Polyfill for std::bit_cast() from C++20, to allow safe type punning
+// https://en.cppreference.com/w/cpp/numeric/bit_cast
+template <class To, class From>
+std::enable_if_t<
+    sizeof(To) == sizeof(From) && std::is_trivially_copyable_v<From> &&
+        std::is_trivially_copyable_v<To> &&
+        std::is_trivially_constructible_v<To>,
+    To>
+bit_cast(const From& src) noexcept {
+  To dst;
+  std::memcpy(&dst, &src, sizeof(To));
+  return dst;
+}
+
+} // namespace facebook::yoga

yoga/style/CompactValue.h

@@ -14,19 +14,7 @@
 #include <yoga/YGMacros.h>
 #include <yoga/YGValue.h>
 
-#if defined(__has_include) && __has_include(<version>)
-// needed to be able to evaluate defined(__cpp_lib_bit_cast)
-#include <version>
-#else
-// needed to be able to evaluate defined(__cpp_lib_bit_cast)
-#include <ciso646>
-#endif
-
-#ifdef __cpp_lib_bit_cast
-#include <bit>
-#else
-#include <cstring>
-#endif
+#include <yoga/bits/BitCast.h>
 
 static_assert(
     std::numeric_limits<float>::is_iec559,
@@ -76,7 +64,7 @@ public:
     }
 
     uint32_t unitBit = Unit == YGUnitPercent ? PERCENT_BIT : 0;
-    auto data = asU32(value);
+    auto data = yoga::bit_cast<uint32_t>(value);
     data -= BIAS;
     data |= unitBit;
     return {data};
@@ -129,7 +117,7 @@ public:
         return YGValue{0.0f, YGUnitPercent};
     }
 
-    if (std::isnan(asFloat(repr_))) {
+    if (std::isnan(yoga::bit_cast<float>(repr_))) {
       return YGValueUndefined;
     }
 
@@ -138,13 +126,14 @@ public:
     data += BIAS;
 
     return YGValue{
-        asFloat(data), repr_ & 0x40000000 ? YGUnitPercent : YGUnitPoint};
+        yoga::bit_cast<float>(data),
+        repr_ & 0x40000000 ? YGUnitPercent : YGUnitPoint};
   }
 
   bool isUndefined() const noexcept {
     return (
         repr_ != AUTO_BITS && repr_ != ZERO_BITS_POINT &&
-        repr_ != ZERO_BITS_PERCENT && std::isnan(asFloat(repr_)));
+        repr_ != ZERO_BITS_PERCENT && std::isnan(yoga::bit_cast<float>(repr_)));
   }
 
   bool isAuto() const noexcept { return repr_ == AUTO_BITS; }
@@ -164,30 +153,6 @@ private:
   constexpr CompactValue(uint32_t data) noexcept : repr_(data) {}
 
   VISIBLE_FOR_TESTING uint32_t repr() { return repr_; }
-
-  static uint32_t asU32(float f) {
-#ifdef __cpp_lib_bit_cast
-    return std::bit_cast<uint32_t>(f);
-#else
-    uint32_t u;
-    static_assert(
-        sizeof(u) == sizeof(f), "uint32_t and float must have the same size");
-    std::memcpy(&u, &f, sizeof(f));
-    return u;
-#endif
-  }
-
-  static float asFloat(uint32_t u) {
-#ifdef __cpp_lib_bit_cast
-    return std::bit_cast<float>(u);
-#else
-    float f;
-    static_assert(
-        sizeof(f) == sizeof(u), "uint32_t and float must have the same size");
-    std::memcpy(&f, &u, sizeof(u));
-    return f;
-#endif
-  }
 };
 
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