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libmp: optimize case x/0xffffffff in mpdigdiv() (helps arm)

--- a/sys/src/libmp/port/mpdigdiv.c

+++ b/sys/src/libmp/port/mpdigdiv.c

@@ -21,6 +21,19 @@

 		return;

 	}

+	// very common case

+	if(~divisor == 0){

+		lo += hi;

+		if(lo < hi){

+			hi++;

+			lo++;

+		}

+		if(lo+1 == 0)

+			hi++;

+		*quotient = hi;

+		return;

+	}

+

 	// at this point we know that hi < divisor

 	// just shift and subtract till we're done

 	q = 0;