Avoid aliasing in mspnr_operator

Author: solidpixel

Source/astcenccli_error_metrics.cpp

@@ -68,13 +68,15 @@ 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;
+	// Fast implementation of pow(2.0, fstop), assuming IEEE float layout
+	uint32_t uscale = 0x3f800000 + (fstop << 23);
 
-	return astc::clamp(val, 0.0f, 255.0f);
+	// Future: Can use std:bit_cast with C++20
+	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);
 }
 
 /**