Allow shifting macro arguments by a negative amount

Fixes #733

src/asm/macro.c

@@ -168,12 +168,12 @@ void macro_ShiftCurrentArgs(int32_t count)
 {
 	if (!macroArgs) {
 		error("Cannot shift macro arguments outside of a macro\n");
-	} else if (count < 0) {
-		error("Cannot shift arguments by negative amount %" PRId32 "\n", count);
 	} else if (macroArgs->shift < macroArgs->nbArgs) {
 		macroArgs->shift += count;
 		if (macroArgs->shift > macroArgs->nbArgs)
 			macroArgs->shift = macroArgs->nbArgs;
+		else if (macroArgs->shift < 0)
+			macroArgs->shift = 0;
 	}
 }
 

src/asm/rgbasm.5

@@ -1487,6 +1487,7 @@ This is the only way of accessing the value of arguments from 10 to 256.
 .Pp
 .Ic SHIFT
 can optionally be given an integer parameter, and will apply the above shifting that number of times.
+A negative parameter will shift the arguments in reverse.
 .Ss Printing things during assembly
 The
 .Ic PRINT

test/asm/shift-negative.asm

@@ -1,4 +1,16 @@
+reverse: MACRO
+	for i, _NARG
+i = _NARG - i - 1
+		shift i
+		println \1
+		shift -i
+	endr
+ENDM
+
+	reverse $1, $2, $3
+
 m: MACRO
+	shift 2
 	shift -3
 ENDM
 	m

test/asm/shift-negative.err

@@ -1,3 +0,0 @@
-ERROR: shift-negative.asm(4) -> shift-negative.asm::m(2):
-    Cannot shift arguments by negative amount -3
-error: Assembly aborted (1 errors)!

test/asm/shift-negative.out

@@ -0,0 +1,3 @@
+$3
+$2
+$1