Merge branch 'main' into mem_ops

Reviewed-on: TCShenanigans/symphony_stdlib#16

src/imath.asm

@@ -6,20 +6,29 @@
 ; r1 - The lower 32 bits of the result
 ; Clobbers: r2, r3, r4, r5
 pub mul_low:
-	mov r3, 0 ; result
-	mov r4, 31 ; loop counter
-	
+	add r3, r2, r2 ; r3 = r2 * 2
+	mov r4, 0 ; r4 has the result
+
 	mul_low_loop:
-		asr r5, r2, 31
-		and r5, r5, r1
-		lsl r5, r5, r4
-		add r3, r3, r5
-		lsl r2, r2, 1
-		sub r4, r4, 1
-		cmp r4, 0
-		jge mul_low_loop
-	mov r1, r3
-	
+
+		and r5, r1, 1 ; a0
+		neg r5, r5 ; mask
+		and r5, r2, r5 ; a0 ? r2 : 0
+		add r4, r4, r5
+
+		and r5, r1, 2 ; a1 is now 0 or 2
+		lsr r5, r5, 1 ; normalize to 0/1
+		neg r5, r5 ; mask
+		and r5, r3, r5 ; a1 ? r2 * 2 : 0
+		add r4, r4, r5
+
+		lsl r2, r2, 2
+		lsl r3, r3, 2
+		lsr r1, r1, 2
+		cmp r1, zr
+		jne mul_low_loop
+
+	mov r1, r4
 	jmp r13
 
 ; Calculates the absolute value of the value provided in the r1 register