cmd/compile: log rules to a file for rule coverage tool

When rules are generated with -log, log rule application to a file.

The file is opened in append mode so multiple calls to the compiler
union their logs.

Change-Id: Ib35c7c85bf58e5909ea9231043f8cbaa6bf278b7
Reviewed-on: https://go-review.googlesource.com/23406Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -150,9 +150,6 @@ func genRules(arch arch) {
 	fmt.Fprintln(w, "// generated with: cd gen; go run *.go")
 	fmt.Fprintln(w)
 	fmt.Fprintln(w, "package ssa")
-	if *genLog {
-		fmt.Fprintln(w, "import \"fmt\"")
-	}
 	fmt.Fprintln(w, "import \"math\"")
 	fmt.Fprintln(w, "var _ = math.MinInt8 // in case not otherwise used")
 
@@ -196,7 +193,7 @@ func genRules(arch arch) {
 
 			genResult(w, arch, result, rule.loc)
 			if *genLog {
-				fmt.Fprintf(w, "fmt.Println(\"rewrite %s\")\n", rule.loc)
+				fmt.Fprintf(w, "logRule(\"%s\")\n", rule.loc)
 			}
 			fmt.Fprintf(w, "return true\n")
 
@@ -300,7 +297,7 @@ func genRules(arch arch) {
 			}
 
 			if *genLog {
-				fmt.Fprintf(w, "fmt.Println(\"rewrite %s\")\n", rule.loc)
+				fmt.Fprintf(w, "logRule(\"%s\")\n", rule.loc)
 			}
 			fmt.Fprintf(w, "return true\n")
 

src/cmd/compile/internal/ssa/rewrite.go

@@ -7,6 +7,8 @@ package ssa
 import (
 	"fmt"
 	"math"
+	"os"
+	"path/filepath"
 )
 
 func applyRewrite(f *Func, rb func(*Block) bool, rv func(*Value, *Config) bool) {
@@ -357,3 +359,28 @@ func clobber(v *Value) bool {
 	// Note: leave v.Block intact.  The Block field is used after clobber.
 	return true
 }
+
+// logRule logs the use of the rule s. This will only be enabled if
+// rewrite rules were generated with the -log option, see gen/rulegen.go.
+func logRule(s string) {
+	if ruleFile == nil {
+		// Open a log file to write log to. We open in append
+		// mode because all.bash runs the compiler lots of times,
+		// and we want the concatenation of all of those logs.
+		// This means, of course, that users need to rm the old log
+		// to get fresh data.
+		// TODO: all.bash runs compilers in parallel. Need to synchronize logging somehow?
+		w, err := os.OpenFile(filepath.Join(os.Getenv("GOROOT"), "src", "rulelog"),
+			os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0666)
+		if err != nil {
+			panic(err)
+		}
+		ruleFile = w
+	}
+	_, err := fmt.Fprintf(ruleFile, "rewrite %s\n", s)
+	if err != nil {
+		panic(err)
+	}
+}
+
+var ruleFile *os.File