Backport: Avoid type-pun union when converting from RGBA colour to DWORD as this may be

undefined or implementation defined behaviour.
Drop some casts by hoisting out part of dwordMultiplied.

Backport of changeset 8289:a53064f376c5.

1 files changed, 9 insertions, 13 deletions

diff --git a/win32/PlatWin.cxx b/win32/PlatWin.cxx
index 629a49963..09cb2f517 100644
--- a/win32/PlatWin.cxx
+++ b/win32/PlatWin.cxx
@@ -743,23 +743,19 @@ void AllFour(DWORD *pixels, int width, int height, int x, int y, DWORD val) noex
 	pixels[(height-1-y)*width+width-1-x] = val;
 }
 
-DWORD dwordFromBGRA(byte b, byte g, byte r, byte a) noexcept {
-	union {
-		byte pixVal[4];
-		DWORD val;
-	} converter;
-	converter.pixVal[0] = b;
-	converter.pixVal[1] = g;
-	converter.pixVal[2] = r;
-	converter.pixVal[3] = a;
-	return converter.val;
+constexpr DWORD dwordFromBGRA(byte b, byte g, byte r, byte a) noexcept {
+	return (a << 24) | (r << 16) | (g << 8) | b;
+}
+
+constexpr byte AlphaScaled(unsigned char component, unsigned int alpha) noexcept {
+	return static_cast<byte>(component * alpha / 255);
 }
 
 DWORD dwordMultiplied(ColourDesired colour, unsigned int alpha) noexcept {
 	return dwordFromBGRA(
-		static_cast<byte>(colour.GetBlue() * alpha / 255),
-		static_cast<byte>(colour.GetGreen() * alpha / 255),
-		static_cast<byte>(colour.GetRed() * alpha / 255),
+		AlphaScaled(colour.GetBlue(), alpha),
+		AlphaScaled(colour.GetGreen(), alpha),
+		AlphaScaled(colour.GetRed(), alpha),
 		static_cast<byte>(alpha));
 }