cmd/compile: add pgohash for debugging/bisecting PGO optimizations

When a PGO build fails or produces incorrect program, it is often
unclear what the problem is. Add pgo hash so we can bisect to
individual optimization decisions, which often helps debugging.

Related to #58153.

Change-Id: I651ffd9c53bad60f2f28c8ec2a90a3f532982712
Reviewed-on: https://go-review.googlesource.com/c/go/+/528400
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/cmd/compile/internal/base/debug.go

@@ -55,6 +55,7 @@ type DebugFlags struct {
 	ABIWrap               int    `help:"print information about ABI wrapper generation"`
 	MayMoreStack          string `help:"call named function before all stack growth checks" concurrent:"ok"`
 	PGODebug              int    `help:"debug profile-guided optimizations"`
+	PGOHash               string `help:"hash value for debugging profile-guided optimizations" concurrent:"ok"`
 	PGOInline             int    `help:"enable profile-guided inlining" concurrent:"ok"`
 	PGOInlineCDFThreshold string `help:"cumulative threshold percentage for determining call sites as hot candidates for inlining" concurrent:"ok"`
 	PGOInlineBudget       int    `help:"inline budget for hot functions" concurrent:"ok"`

src/cmd/compile/internal/base/flag.go

@@ -255,6 +255,9 @@ func ParseFlags() {
 	if Debug.Fmahash != "" {
 		FmaHash = NewHashDebug("fmahash", Debug.Fmahash, nil)
 	}
+	if Debug.PGOHash != "" {
+		PGOHash = NewHashDebug("pgohash", Debug.PGOHash, nil)
+	}
 
 	if Flag.MSan && !platform.MSanSupported(buildcfg.GOOS, buildcfg.GOARCH) {
 		log.Fatalf("%s/%s does not support -msan", buildcfg.GOOS, buildcfg.GOARCH)

src/cmd/compile/internal/base/hashdebug.go

@@ -55,6 +55,7 @@ var hashDebug *HashDebug
 
 var FmaHash *HashDebug     // for debugging fused-multiply-add floating point changes
 var LoopVarHash *HashDebug // for debugging shared/private loop variable changes
+var PGOHash *HashDebug     // for debugging PGO optimization decisions
 
 // DebugHashMatchPkgFunc reports whether debug variable Gossahash
 //
@@ -274,8 +275,36 @@ func (d *HashDebug) MatchPos(pos src.XPos, desc func() string) bool {
 }
 
 func (d *HashDebug) matchPos(ctxt *obj.Link, pos src.XPos, note func() string) bool {
+	return d.matchPosWithInfo(ctxt, pos, nil, note)
+}
+
+func (d *HashDebug) matchPosWithInfo(ctxt *obj.Link, pos src.XPos, info any, note func() string) bool {
 	hash := d.hashPos(ctxt, pos)
-	return d.matchAndLog(hash, func() string { return d.fmtPos(ctxt, pos) }, note)
+	if info != nil {
+		hash = bisect.Hash(hash, info)
+	}
+	return d.matchAndLog(hash,
+		func() string {
+			r := d.fmtPos(ctxt, pos)
+			if info != nil {
+				r += fmt.Sprintf(" (%v)", info)
+			}
+			return r
+		},
+		note)
+}
+
+// MatchPosWithInfo is similar to MatchPos, but with additional information
+// that is included for hash computation, so it can distinguish multiple
+// matches on the same source location.
+// Note that the default answer for no environment variable (d == nil)
+// is "yes", do the thing.
+func (d *HashDebug) MatchPosWithInfo(pos src.XPos, info any, desc func() string) bool {
+	if d == nil {
+		return true
+	}
+	// Written this way to make inlining likely.
+	return d.matchPosWithInfo(Ctxt, pos, info, desc)
 }
 
 // matchAndLog is the core matcher. It reports whether the hash matches the pattern.

src/cmd/compile/internal/devirtualize/pgo.go

@@ -155,6 +155,11 @@ func ProfileGuided(fn *ir.Func, p *pgo.Profile) {
 			return n
 		}
 
+		if !base.PGOHash.MatchPosWithInfo(n.Pos(), "devirt", nil) {
+			// De-selected by PGO Hash.
+			return n
+		}
+
 		if stat != nil {
 			stat.Devirtualized = ir.LinkFuncName(callee)
 			stat.DevirtualizedWeight = weight

src/cmd/compile/internal/inline/inl.go

@@ -1044,6 +1044,11 @@ func inlineCostOK(n *ir.CallExpr, caller, callee *ir.Func, bigCaller bool) (bool
 		return false, inlineHotMaxBudget
 	}
 
+	if !base.PGOHash.MatchPosWithInfo(n.Pos(), "inline", nil) {
+		// De-selected by PGO Hash.
+		return false, maxCost
+	}
+
 	if base.Debug.PGODebug > 0 {
 		fmt.Printf("hot-budget check allows inlining for call %s (cost %d) at %v in function %s\n", ir.PkgFuncName(callee), callee.Inl.Cost, ir.Line(n), ir.PkgFuncName(caller))
 	}

src/cmd/compile/internal/test/pgo_inl_test.go

@@ -6,6 +6,7 @@ package test
 
 import (
 	"bufio"
+	"bytes"
 	"fmt"
 	"internal/profile"
 	"internal/testenv"
@@ -17,11 +18,7 @@ import (
 	"testing"
 )
 
-// testPGOIntendedInlining tests that specific functions are inlined.
-func testPGOIntendedInlining(t *testing.T, dir string) {
-	testenv.MustHaveGoRun(t)
-	t.Parallel()
-
+func buildPGOInliningTest(t *testing.T, dir string, flags ...string) []byte {
 	const pkg = "example.com/pgo/inline"
 
 	// Add a go.mod so we have a consistent symbol names in this temp dir.
@@ -32,6 +29,25 @@ go 1.19
 		t.Fatalf("error writing go.mod: %v", err)
 	}
 
+	exe := filepath.Join(dir, "test.exe")
+	args := []string{"test", "-c", "-o", exe}
+	args = append(args, flags...)
+	cmd := testenv.CleanCmdEnv(testenv.Command(t, testenv.GoToolPath(t), args...))
+	cmd.Dir = dir
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("build failed: %v, output:\n%s", err, out)
+	}
+	return out
+}
+
+// testPGOIntendedInlining tests that specific functions are inlined.
+func testPGOIntendedInlining(t *testing.T, dir string) {
+	testenv.MustHaveGoRun(t)
+	t.Parallel()
+
+	const pkg = "example.com/pgo/inline"
+
 	want := []string{
 		"(*BS).NS",
 	}
@@ -71,25 +87,9 @@ go 1.19
 	// TODO: maybe adjust the test to work with default threshold.
 	pprof := filepath.Join(dir, "inline_hot.pprof")
 	gcflag := fmt.Sprintf("-gcflags=-m -m -pgoprofile=%s -d=pgoinlinebudget=160,pgoinlinecdfthreshold=90", pprof)
-	out := filepath.Join(dir, "test.exe")
-	cmd := testenv.CleanCmdEnv(testenv.Command(t, testenv.GoToolPath(t), "test", "-c", "-o", out, gcflag, "."))
-	cmd.Dir = dir
-
-	pr, pw, err := os.Pipe()
-	if err != nil {
-		t.Fatalf("error creating pipe: %v", err)
-	}
-	defer pr.Close()
-	cmd.Stdout = pw
-	cmd.Stderr = pw
-
-	err = cmd.Start()
-	pw.Close()
-	if err != nil {
-		t.Fatalf("error starting go test: %v", err)
-	}
+	out := buildPGOInliningTest(t, dir, gcflag)
 
-	scanner := bufio.NewScanner(pr)
+	scanner := bufio.NewScanner(bytes.NewReader(out))
 	curPkg := ""
 	canInline := regexp.MustCompile(`: can inline ([^ ]*)`)
 	haveInlined := regexp.MustCompile(`: inlining call to ([^ ]*)`)
@@ -128,11 +128,8 @@ go 1.19
 			continue
 		}
 	}
-	if err := cmd.Wait(); err != nil {
-		t.Fatalf("error running go test: %v", err)
-	}
 	if err := scanner.Err(); err != nil {
-		t.Fatalf("error reading go test output: %v", err)
+		t.Fatalf("error reading output: %v", err)
 	}
 	for fullName, reason := range notInlinedReason {
 		t.Errorf("%s was not inlined: %s", fullName, reason)
@@ -297,3 +294,48 @@ func copyFile(dst, src string) error {
 	_, err = io.Copy(d, s)
 	return err
 }
+
+// TestPGOHash tests that PGO optimization decisions can be selected by pgohash.
+func TestPGOHash(t *testing.T) {
+	testenv.MustHaveGoRun(t)
+	t.Parallel()
+
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("error getting wd: %v", err)
+	}
+	srcDir := filepath.Join(wd, "testdata/pgo/inline")
+
+	// Copy the module to a scratch location so we can add a go.mod.
+	dir := t.TempDir()
+
+	for _, file := range []string{"inline_hot.go", "inline_hot_test.go", "inline_hot.pprof"} {
+		if err := copyFile(filepath.Join(dir, file), filepath.Join(srcDir, file)); err != nil {
+			t.Fatalf("error copying %s: %v", file, err)
+		}
+	}
+
+	pprof := filepath.Join(dir, "inline_hot.pprof")
+	gcflag0 := fmt.Sprintf("-gcflags=-pgoprofile=%s -d=pgoinlinebudget=160,pgoinlinecdfthreshold=90,pgodebug=1,", pprof)
+
+	// Check that a hash match allows PGO inlining.
+	const srcPos = "example.com/pgo/inline/inline_hot.go:81:19"
+	const hashMatch = "pgohash triggered " + srcPos + " (inline)"
+	pgoDebugRE := regexp.MustCompile(`hot-budget check allows inlining for call .* at ` + strings.ReplaceAll(srcPos, ".", "\\."))
+	hash := "v1" // 1 matches srcPos, v for verbose (print source location)
+	gcflag := gcflag0 + ",pgohash=" + hash
+	// build with -trimpath so the source location (thus the hash)
+	// does not depend on the temporary directory path.
+	out := buildPGOInliningTest(t, dir, gcflag, "-trimpath")
+	if !bytes.Contains(out, []byte(hashMatch)) || !pgoDebugRE.Match(out) {
+		t.Errorf("output does not contain expected source line, out:\n%s", out)
+	}
+
+	// Check that a hash mismatch turns off PGO inlining.
+	hash = "v0" // 0 should not match srcPos
+	gcflag = gcflag0 + ",pgohash=" + hash
+	out = buildPGOInliningTest(t, dir, gcflag, "-trimpath")
+	if bytes.Contains(out, []byte(hashMatch)) || pgoDebugRE.Match(out) {
+		t.Errorf("output contains unexpected source line, out:\n%s", out)
+	}
+}