Replace type-punning unions by std::bit_cast clone

The type-punning unions are not allowed by the C++ standard. bit_cast is more succinct anyway.
DaemonEngine · May 27, 2024 · 2ae2181 · 2ae2181
1 parent bfa7b90
commit 2ae2181
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Showing 4 changed files with 19 additions and 39 deletions.
diff --git a/src/common/Endian.h b/src/common/Endian.h
@@ -166,15 +166,9 @@ inline uint64_t USwap64(uint64_t x)
 
 inline float SwapFloat(float x)
 {
-	union
-	{
-		float        f;
-		int          i;
-		unsigned int ui;
-	} fi;
-	fi.f = x;
-	fi.i = Swap32(fi.i);
-	return fi.f;
+	int32_t bits = Util::bit_cast<int32_t>(x);
+	bits = Swap32(bits);
+	return Util::bit_cast<float>(bits);
 }
 
 #ifdef Q3_LITTLE_ENDIAN

diff --git a/src/common/Util.h b/src/common/Util.h
@@ -44,6 +44,17 @@ char     *PRINTF_LIKE(1) va( const char *format, ... );
 
 namespace Util {
 
+// C++20 std::bit_cast
+template<typename ToT, typename FromT>
+ToT bit_cast(FromT from)
+{
+	static_assert(sizeof(ToT) == sizeof(FromT), "bit_cast: sizes must be equal");
+
+	ToT to;
+	memcpy(&to, &from, sizeof(to));
+	return to;
+}
+
 // Binary search function which returns an iterator to the result or end if not found
 template<typename Iter, typename T>
 Iter binary_find(Iter begin, Iter end, const T& value)

diff --git a/src/engine/qcommon/q_shared.h b/src/engine/qcommon/q_shared.h
@@ -144,13 +144,6 @@ void ignore_result(T) {}
 	using uint = unsigned int;
 	enum class qtrinary {qno, qyes, qmaybe};
 
-	union floatint_t
-	{
-		float f;
-		int i;
-		uint ui;
-	};
-
 //=============================================================
 
 #include "common/Platform.h"
@@ -351,22 +344,6 @@ extern const quat_t   quatIdentity;
 
 #define Q_ftol(x) ((long)(x))
 
-	inline unsigned int Q_floatBitsToUint( float number )
-	{
-		floatint_t t;
-
-		t.f = number;
-		return t.ui;
-	}
-
-	inline float Q_uintBitsToFloat( unsigned int number )
-	{
-		floatint_t t;
-
-		t.ui = number;
-		return t.f;
-	}
-
 	// Overall relative error bound (ignoring unknown powerpc case): 5 * 10^-6
 	// https://en.wikipedia.org/wiki/Fast_inverse_square_root#/media/File:2nd-iter.png
 	inline float Q_rsqrt( float number )
@@ -379,7 +356,7 @@ extern const quat_t   quatIdentity;
 		// SSE rsqrt relative error bound: 3.7 * 10^-4
 		_mm_store_ss( &y, _mm_rsqrt_ss( _mm_load_ss( &number ) ) );
 #else
-		y = Q_uintBitsToFloat( 0x5f3759df - (Q_floatBitsToUint( number ) >> 1) );
+		y = Util::bit_cast<float>( 0x5f3759df - ( Util::bit_cast<uint32_t>( number ) >> 1 ) );
 		y *= ( 1.5f - ( x * y * y ) ); // initial iteration
 		// relative error bound after the initial iteration: 1.8 * 10^-3
 #endif

diff --git a/src/engine/renderer/tr_local.h b/src/engine/renderer/tr_local.h
@@ -125,11 +125,10 @@ static inline void snorm16ToFloat( const i16vec4_t in, vec4_t out )
 
 static inline f16_t floatToHalf( float in ) {
 	static float scale = powf(2.0f, 15 - 127);
-	floatint_t fi;
 
-	fi.f = in * scale;
+	uint32_t ui = Util::bit_cast<uint32_t>( in * scale );
 
-	return { uint16_t(((fi.ui & 0x80000000) >> 16) | ((fi.ui & 0x0fffe000) >> 13)) };
+	return { uint16_t(((ui & 0x80000000) >> 16) | ((ui & 0x0fffe000) >> 13)) };
 }
 static inline void floatToHalf( const vec4_t in, f16vec4_t out )
 {
@@ -140,10 +139,9 @@ static inline void floatToHalf( const vec4_t in, f16vec4_t out )
 }
 static inline float halfToFloat( f16_t in ) {
 	static float scale = powf(2.0f, 127 - 15);
-	floatint_t fi;
 
-	fi.ui = (((unsigned int)in.bits & 0x8000) << 16) | (((unsigned int)in.bits & 0x7fff) << 13);
-	return fi.f * scale;
+	uint32_t ui = (((unsigned int)in.bits & 0x8000) << 16) | (((unsigned int)in.bits & 0x7fff) << 13);
+	return Util::bit_cast<float>(ui) * scale;
 }
 static inline void halfToFloat( const f16vec4_t in, vec4_t out )
 {