cmd/compile: add framework for logging optimizer (non)actions to LSP

This is intended to allow IDEs to note where the optimizer
was not able to improve users' code.  There may be other
applications for this, for example in studying effectiveness
of optimizer changes more quickly than running benchmarks,
or in verifying that code changes did not accidentally disable
optimizations in performance-critical code.

Logging of nilcheck (bad) for amd64 is implemented as
proof-of-concept.  In general, the intent is that optimizations
that didn't happen are what will be logged, because that is
believed to be what IDE users want.

Added flag -json=version,dest

Check that version=0.  (Future compilers will support a
few recent versions, I hope that version is always <=3.)

Dest is expected to be one of:

/path (or \path in Windows)
  will create directory /path and fill it w/ json files
file://path
  will create directory path, intended either for
     I:\dont\know\enough\about\windows\paths
     trustme_I_know_what_I_am_doing_probably_testing

Not passing an absolute path name usually leads to
json splattered all over source directories,
or failure when those directories are not writeable.
If you want a foot-gun, you have to ask for it.

The JSON output is directed to subdirectories of dest,
where each subdirectory is net/url.PathEscape of the
package name, and each for each foo.go in the package,
net/url.PathEscape(foo).json is created.  The first line
of foo.json contains version and context information,
and subsequent lines contains LSP-conforming JSON
describing the missing optimizations.

Change-Id: Ib83176a53a8c177ee9081aefc5ae05604ccad8a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/204338
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/logopt/logopt_test.go

@@ -0,0 +1,126 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package logopt
+
+import (
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+const srcCode = `package x
+type pair struct {a,b int}
+func bar(y *pair) *int {
+	return &y.b
+}
+
+func foo(w, z *pair) *int {
+	if *bar(w) > 0 {
+		return bar(z)
+	}
+	return nil
+}
+`
+
+func want(t *testing.T, out string, desired string) {
+	if !strings.Contains(out, desired) {
+		t.Errorf("did not see phrase %s in \n%s", desired, out)
+	}
+}
+
+func TestLogOpt(t *testing.T) {
+	t.Parallel()
+
+	testenv.MustHaveGoBuild(t)
+
+	dir, err := ioutil.TempDir("", "TestLogOpt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	dir = fixSlash(dir) // Normalize the directory name as much as possible, for Windows testing
+	src := filepath.Join(dir, "file.go")
+	if err := ioutil.WriteFile(src, []byte(srcCode), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	outfile := filepath.Join(dir, "file.o")
+
+	t.Run("JSON_fails", func(t *testing.T) {
+		// Test malformed flag
+		out, err := testLogOpt(t, "-json=foo", src, outfile)
+		if err == nil {
+			t.Error("-json=foo succeeded unexpectedly")
+		}
+		want(t, out, "option should be")
+		want(t, out, "number")
+
+		// Test a version number that is currently unsupported (and should remain unsupported for a while)
+		out, err = testLogOpt(t, "-json=9,foo", src, outfile)
+		if err == nil {
+			t.Error("-json=0,foo succeeded unexpectedly")
+		}
+		want(t, out, "version must be")
+
+	})
+
+	// Some architectures don't fault on nil dereference, so nilchecks are eliminated differently.
+	if runtime.GOARCH != "amd64" {
+		return
+	}
+
+	t.Run("Success", func(t *testing.T) {
+		// This test is supposed to succeed
+
+		// replace d (dir)  with t ("tmpdir") and convert path separators to '/'
+		normalize := func(out []byte, d, t string) string {
+			s := string(out)
+			s = strings.ReplaceAll(s, d, t)
+			s = strings.ReplaceAll(s, string(os.PathSeparator), "/")
+			return s
+		}
+
+		// Note 'file://' is the I-Know-What-I-Am-Doing way of specifying a file, also to deal with corner cases for Windows.
+		_, err := testLogOptDir(t, dir, "-json=0,file://log/opt", src, outfile)
+		if err != nil {
+			t.Error("-json=0,file://log/opt should have succeeded")
+		}
+		logged, err := ioutil.ReadFile(filepath.Join(dir, "log", "opt", "x", "file.json"))
+		if err != nil {
+			t.Error("-json=0,file://log/opt missing expected log file")
+		}
+		// All this delicacy with uriIfy and filepath.Join is to get this test to work right on Windows.
+		slogged := normalize(logged, string(uriIfy(dir)), string(uriIfy("tmpdir")))
+		t.Logf("%s", slogged)
+		// below shows proper inlining and nilcheck
+		want(t, slogged, `{"range":{"start":{"line":9,"character":13},"end":{"line":9,"character":13}},"severity":3,"code":"nilcheck","source":"go compiler","message":"","relatedInformation":[{"location":{"uri":"file://tmpdir/file.go","range":{"start":{"line":4,"character":11},"end":{"line":4,"character":11}}},"message":"inlineLoc"}]}`)
+	})
+}
+
+func testLogOpt(t *testing.T, flag, src, outfile string) (string, error) {
+	run := []string{testenv.GoToolPath(t), "tool", "compile", flag, "-o", outfile, src}
+	t.Log(run)
+	cmd := exec.Command(run[0], run[1:]...)
+	out, err := cmd.CombinedOutput()
+	t.Logf("%s", out)
+	return string(out), err
+}
+
+func testLogOptDir(t *testing.T, dir, flag, src, outfile string) (string, error) {
+	// Notice the specified import path "x"
+	run := []string{testenv.GoToolPath(t), "tool", "compile", "-p", "x", flag, "-o", outfile, src}
+	t.Log(run)
+	cmd := exec.Command(run[0], run[1:]...)
+	cmd.Dir = dir
+	out, err := cmd.CombinedOutput()
+	t.Logf("%s", out)
+	return string(out), err
+}