Implement std_multiply using shift + add

asm/stdlib.asm

@@ -90,20 +90,22 @@
 
 
 ; FASTCALL uint std_multiply(uint A, uint B). Return value on RRET
-; Uses simple addition
-; TODO: use add + shift
+; Uses shift + add multiplication
 .constant std_mul @std_multiply
 .constant std_multiply @std_multiply
 std_multiply:
-    PUSH BSM_COUNTER_0      ; Save old value of counter
-    MOV BSM_COUNTER_0, P0   ; Move A to counter
-    MOV RRET, 0
-    JUMP @.loop             ; Skip first iteration
+    MOV S0, P0                      ; S0 = A
+    MOV S1, P1                      ; S1 = B
+    MOV RRET, 0                     ; result = 0
     .multiply:
-        ADD RRET, RRET, P1  ; Add B to itself A times
-    .loop: LOOP @.multiply  ; Check if 0, if not, decrease A and go back. Use COUNTER[0]
-    POP BSM_COUNTER_0       ; Restore old value of counter
-    RET                     ; Return, no need to get rid of stack as fastcall passed them on registers
+    BEQ S0, 0, 0xFFFFFFFF, @.return ; continue if S0 != 0
+        BNE S0, 1, 0x1, @.skipBit   ; If the lowest order bit is set in A?
+            ADD RRET, RRET, S1      ; result = result + b
+        .skipBit:
+        SHR S0, S0, 1               ; a = a >> 1, no sign copy
+        ADD S1, S1, S1              ; b = b + b
+        JUMP @.multiply
+    .return: RET                    ; Return, no need to get rid of stack as fastcall passed them on registers
 
 
 ; FASTCALL uint std_divide(uint dividend, uint divisor). Return quotient on RRET, remainder on RRET_X