cmd/compile: add jump table codegen test

Change-Id: Ic67f676f5ebe146166a0d3c1d78a802881320e49
Reviewed-on: https://go-review.googlesource.com/c/go/+/400375
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>

test/codegen/switch.go

@@ -20,3 +20,53 @@ func f(x string) int {
 		return -3
 	}
 }
+
+// use jump tables for 8+ int cases
+func square(x int) int {
+	// amd64:`JMP\s\(.*\)\(.*\)$`
+	switch x {
+	case 1:
+		return 1
+	case 2:
+		return 4
+	case 3:
+		return 9
+	case 4:
+		return 16
+	case 5:
+		return 25
+	case 6:
+		return 36
+	case 7:
+		return 49
+	case 8:
+		return 64
+	default:
+		return x * x
+	}
+}
+
+// use jump tables for 8+ string lengths
+func length(x string) int {
+	// amd64:`JMP\s\(.*\)\(.*\)$`
+	switch x {
+	case "a":
+		return 1
+	case "bb":
+		return 2
+	case "ccc":
+		return 3
+	case "dddd":
+		return 4
+	case "eeeee":
+		return 5
+	case "ffffff":
+		return 6
+	case "ggggggg":
+		return 7
+	case "hhhhhhhh":
+		return 8
+	default:
+		return len(x)
+	}
+}