6 files changed, 113 insertions, 28 deletions
diff --git a/src/cmd/compile/internal/base/debug.go b/src/cmd/compile/internal/base/debug.go
index 244ba1bc9d..390ddf3176 100644
--- a/src/cmd/compile/internal/base/debug.go
+++ b/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"`
diff --git a/src/cmd/compile/internal/base/flag.go b/src/cmd/compile/internal/base/flag.go
index 0e44deae71..36340cb70b 100644
--- a/src/cmd/compile/internal/base/flag.go
+++ b/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)
diff --git a/src/cmd/compile/internal/base/hashdebug.go b/src/cmd/compile/internal/base/hashdebug.go
index 167b0df4f0..de7f01f09e 100644
--- a/src/cmd/compile/internal/base/hashdebug.go
+++ b/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.
diff --git a/src/cmd/compile/internal/devirtualize/pgo.go b/src/cmd/compile/internal/devirtualize/pgo.go
index b51028701e..a04ff16d60 100644
--- a/src/cmd/compile/internal/devirtualize/pgo.go
+++ b/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
diff --git a/src/cmd/compile/internal/inline/inl.go b/src/cmd/compile/internal/inline/inl.go
index f1dce85afb..cd5adc1421 100644
--- a/src/cmd/compile/internal/inline/inl.go
+++ b/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))
 	}
diff --git a/src/cmd/compile/internal/test/pgo_inl_test.go b/src/cmd/compile/internal/test/pgo_inl_test.go
index 4d6b5a134a..7aabf8b010 100644
--- a/src/cmd/compile/internal/test/pgo_inl_test.go
+++ b/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)
+	}
+}