fix(asm): resolve negative numeric PC-relative jumps

Assisted-by: GLM 5.3 Flash

asm/assemble.go

@@ -543,11 +543,27 @@ func pcJumpOffset(op *ast.Operand) (int, bool) {
 // pcJumpTarget resolves a numeric jump at statement index j: N counts the
 // instruction statements after the jump itself (N = 0 is the jump's own
 // address, the classic park loop), and the target is the start of the Nth
-// one.  It reports false when the count runs past the end of the function.
+// one.  A negative N counts the same way backwards, before the jump: the
+// exit loops write JMP -3(PC) to land three instructions earlier.  Labels
+// count not, in either direction.  It reports false when the count runs
+// past the end of the function, or before its first instruction.
 func pcJumpTarget(t *ast.Text, j, n int, pcs []int) (int, bool) {
 	if n == 0 {
 		return pcs[j], true
 	}
+	if n < 0 {
+		seen := 0
+		for k := j - 1; k >= 0; k-- {
+			if _, ok := t.Body[k].(*ast.Instr); !ok {
+				continue
+			}
+			seen--
+			if seen == n {
+				return pcs[k], true
+			}
+		}
+		return 0, false
+	}
 	seen := 0
 	for k := j + 1; k < len(t.Body); k++ {
 		if _, ok := t.Body[k].(*ast.Instr); !ok {