tests: autogenerated test to confirm attribute strings

Test helps verify our attribute types match the macros in textflag.h.
Discovered https://golang.org/issue/29487 in the process.

Updates #2

tests/textflag/make_attrtest.go

@@ -0,0 +1,88 @@
+// +build ignore
+
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"runtime"
+
+	"github.com/mmcloughlin/avo"
+)
+
+var (
+	output = flag.String("output", "", "path to output file (default stdout)")
+	seed   = flag.Int64("seed", 0, "random seed")
+	num    = flag.Int("num", 32, "number of attributes to generate")
+)
+
+func GenerateAttributes(n int) []avo.Attribute {
+	as := make([]avo.Attribute, 0, n)
+
+	// Include each bitlevel.
+	for i := 0; i < 16 && i < n; i++ {
+		a := avo.Attribute(1 << uint(i))
+		as = append(as, a)
+	}
+
+	// Add randomly generated attributes.
+	for len(as) < n {
+		a := avo.Attribute(rand.Uint32())
+		as = append(as, a)
+	}
+
+	return as
+}
+
+func PrintAttributesTest(w io.Writer, as []avo.Attribute) {
+	_, self, _, _ := runtime.Caller(0)
+	fmt.Fprintf(w, "// Code generated by %s. DO NOT EDIT.\n\n", filepath.Base(self))
+	fmt.Fprintf(w, "#include \"textflag.h\"\n\n")
+	fmt.Fprintf(w, "TEXT ·AttrTest(SB), $0-1\n")
+	fmt.Fprintf(w, "\tMOVB $0, ret+0(FP)\n")
+
+	for i, a := range as {
+		fmt.Fprintf(w, "\tMOVW $(%d), R8\n", a)
+		fmt.Fprintf(w, "\tMOVW $(%s), R9\n", a.Asm())
+		fmt.Fprintf(w, "\tCMPW R8, R9\n")
+
+		cont := fmt.Sprintf("cont%d", i)
+		fmt.Fprintf(w, "\tJE %s\n", cont)
+		fmt.Fprintf(w, "\tRET\n")
+
+		fmt.Fprintf(w, "%s:\n", cont)
+	}
+
+	fmt.Fprintf(w, "\tMOVB $1, ret+0(FP)\n")
+	fmt.Fprintf(w, "\tRET\n")
+}
+
+func main() {
+	flag.Parse()
+
+	w := os.Stdout
+	if *output != "" {
+		f, err := os.Create(*output)
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+		w = f
+	}
+
+	rand.Seed(*seed)
+	as := GenerateAttributes(*num)
+
+	buf := bytes.NewBuffer(nil)
+	PrintAttributesTest(buf, as)
+
+	if _, err := w.Write(buf.Bytes()); err != nil {
+		log.Fatal(err)
+	}
+}