a little optimization of portable mulUnit

include/mcl/vint.hpp

@@ -90,35 +90,19 @@ inline uint32_t mulUnit(uint32_t *pH, uint32_t x, uint32_t y)
 inline uint64_t mulUnit(uint64_t *pH, uint64_t x, uint64_t y)
 {
 #ifdef MCL_VINT_64BIT_PORTABLE
-	uint32_t a = uint32_t(x >> 32);
-	uint32_t b = uint32_t(x);
-	uint32_t c = uint32_t(y >> 32);
-	uint32_t d = uint32_t(y);
-
-	uint64_t ad = uint64_t(d) * a;
-	uint64_t bd = uint64_t(d) * b;
-	uint64_t L = uint32_t(bd);
-	ad += bd >> 32; // [ad:L]
-
-	uint64_t ac = uint64_t(c) * a;
-	uint64_t bc = uint64_t(c) * b;
-	uint64_t H = uint32_t(bc);
-	ac += bc >> 32; // [ac:H]
-	/*
-		  adL
-		 acH
-	*/
-	uint64_t t = (ac << 32) | H;
-	ac >>= 32;
-	H = t + ad;
-	if (H < t) {
-		ac++;
-	}
-	/*
-		ac:H:L
-	*/
+	const uint64_t mask = 0xffffffff;
+	uint64_t v = (x & mask) * (y & mask);
+	uint64_t L = uint32_t(v);
+	uint64_t H = v >> 32;
+	uint64_t ad = (x & mask) * uint32_t(y >> 32);
+	uint64_t bc = uint32_t(x >> 32) * (y & mask);
+	H += uint32_t(ad);
+	H += uint32_t(bc);
 	L |= H << 32;
-	H = (ac << 32) | uint32_t(H >> 32);
+	H >>= 32;
+	H += ad >> 32;
+	H += bc >> 32;
+	H += (x >> 32) * (y >> 32);
 	*pH = H;
 	return L;
 #elif defined(_WIN64) && !defined(__INTEL_COMPILER)