Avoid type aliasing and UB in mspnr_operator (#578)

Author: solidpixel

Source/astcenc_color_unquantize.cpp

@@ -53,7 +53,7 @@ static ASTCENC_SIMD_INLINE vint4 uncontract_color(
  */
 static ASTCENC_SIMD_INLINE int32_t safe_signed_lsh(int32_t val, int shift)
 {
-	// Future: When we support C++20 can swap memcpy for std::bitcast
+	// Future: Can use std:bit_cast with C++20
 	uint32_t uval;
 	std::memcpy(&uval, &val, sizeof(uint32_t));
 	uval <<= shift;

Source/astcenccli_error_metrics.cpp

@@ -68,13 +68,19 @@ static float mpsnr_operator(
 	float val,
 	int fstop
 ) {
-	if32 p;
-	p.u = 0x3f800000 + (fstop << 23);  // 0x3f800000 is 1.0f
-	val *= p.f;
-	val = powf(val, (1.0f / 2.2f));
-	val *= 255.0f;
+	// Future: Can use std:bit_cast with C++20
 
-	return astc::clamp(val, 0.0f, 255.0f);
+	// Fast implementation of pow(2.0, fstop), assuming IEEE float layout
+	// Memcpy to uint avoids ubsan complaints shift of negative int
+	unsigned int fstopu;
+	std::memcpy(&fstopu, &fstop, sizeof(int));
+	uint32_t uscale = 0x3f800000 + (fstopu << 23);
+
+	float scale;
+	std::memcpy(&scale, &uscale, sizeof(float));
+
+	val = powf(val * scale, (1.0f / 2.2f));
+	return astc::clamp(val * 255.0f, 0.0f, 255.0f);
 }
 
 /**