cmd/compile: use new for loop semantics for Go 1.22+ compilations

This includes version-dependent support for GOEXPERIMENT and
-d=loopvar, -d=loopvarhash, to allow testing/porting of old code.

Includes tests of downgrade (1.22 -> 1.21) and upgrade (1.21 -> 1.22)
based on //go:build lines (while running a 1.22 build/compiler).

Change-Id: Idd3be61a2b46acec33c7e7edac0924158cc726b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/508819
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

7 files changed, 238 insertions, 11 deletions

diff --git a/src/cmd/compile/internal/loopvar/loopvar.go b/src/cmd/compile/internal/loopvar/loopvar.go
index 43f081c10a..1d8e42f5e5 100644
--- a/src/cmd/compile/internal/loopvar/loopvar.go
+++ b/src/cmd/compile/internal/loopvar/loopvar.go
@@ -605,7 +605,7 @@ func LogTransformations(transformed []VarAndLoop) {
 				// Intended to help with performance debugging, we record whole loop ranges
 				logopt.LogOptRange(pos, last, "loop-modified-"+loopKind, "loopvar", ir.FuncName(l.curfn))
 			}
-			if print && 3 <= base.Debug.LoopVar {
+			if print && 4 <= base.Debug.LoopVar {
 				// TODO decide if we want to keep this, or not.  It was helpful for validating logopt, otherwise, eh.
 				inner := base.Ctxt.InnermostPos(pos)
 				outer := base.Ctxt.OutermostPos(pos)
diff --git a/src/cmd/compile/internal/loopvar/loopvar_test.go b/src/cmd/compile/internal/loopvar/loopvar_test.go
index 03e6eec437..c8e11dbd07 100644
--- a/src/cmd/compile/internal/loopvar/loopvar_test.go
+++ b/src/cmd/compile/internal/loopvar/loopvar_test.go
@@ -51,7 +51,7 @@ var cases = []testcase{
 }
 
 // TestLoopVar checks that the GOEXPERIMENT and debug flags behave as expected.
-func TestLoopVar(t *testing.T) {
+func TestLoopVarGo1_21(t *testing.T) {
 	switch runtime.GOOS {
 	case "linux", "darwin":
 	default:
@@ -71,7 +71,7 @@ func TestLoopVar(t *testing.T) {
 	for i, tc := range cases {
 		for _, f := range tc.files {
 			source := f
-			cmd := testenv.Command(t, gocmd, "build", "-o", output, "-gcflags=-d=loopvar="+tc.lvFlag, source)
+			cmd := testenv.Command(t, gocmd, "build", "-o", output, "-gcflags=-lang=go1.21 -d=loopvar="+tc.lvFlag, source)
 			cmd.Env = append(cmd.Env, "GOEXPERIMENT=loopvar", "HOME="+tmpdir)
 			cmd.Dir = "testdata"
 			t.Logf("File %s loopvar=%s expect '%s' exit code %d", f, tc.lvFlag, tc.buildExpect, tc.expectRC)
@@ -103,7 +103,7 @@ func TestLoopVar(t *testing.T) {
 	}
 }
 
-func TestLoopVarInlines(t *testing.T) {
+func TestLoopVarInlinesGo1_21(t *testing.T) {
 	switch runtime.GOOS {
 	case "linux", "darwin":
 	default:
@@ -125,7 +125,7 @@ func TestLoopVarInlines(t *testing.T) {
 		// This disables the loopvar change, except for the specified package.
 		// The effect should follow the package, even though everything (except "c")
 		// is inlined.
-		cmd := testenv.Command(t, gocmd, "run", "-gcflags="+pkg+"=-d=loopvar=1", root)
+		cmd := testenv.Command(t, gocmd, "run", "-gcflags="+root+"/...=-lang=go1.21", "-gcflags="+pkg+"=-d=loopvar=1", root)
 		cmd.Env = append(cmd.Env, "GOEXPERIMENT=noloopvar", "HOME="+tmpdir)
 		cmd.Dir = filepath.Join("testdata", "inlines")
 
@@ -166,6 +166,7 @@ func countMatches(s, re string) int {
 }
 
 func TestLoopVarHashes(t *testing.T) {
+	// This behavior does not depend on Go version (1.21 or greater)
 	switch runtime.GOOS {
 	case "linux", "darwin":
 	default:
@@ -187,7 +188,7 @@ func TestLoopVarHashes(t *testing.T) {
 		// This disables the loopvar change, except for the specified hash pattern.
 		// -trimpath is necessary so we get the same answer no matter where the
 		// Go repository is checked out. This is not normally a concern since people
-		// do not rely on the meaning of specific hashes.
+		// do not normally rely on the meaning of specific hashes.
 		cmd := testenv.Command(t, gocmd, "run", "-trimpath", root)
 		cmd.Env = append(cmd.Env, "GOCOMPILEDEBUG=loopvarhash="+hash, "HOME="+tmpdir)
 		cmd.Dir = filepath.Join("testdata", "inlines")
@@ -225,7 +226,8 @@ func TestLoopVarHashes(t *testing.T) {
 	}
 }
 
-func TestLoopVarOpt(t *testing.T) {
+// TestLoopVarVersionEnableFlag checks for loopvar transformation enabled by command line flag (1.22).
+func TestLoopVarVersionEnableFlag(t *testing.T) {
 	switch runtime.GOOS {
 	case "linux", "darwin":
 	default:
@@ -240,7 +242,8 @@ func TestLoopVarOpt(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 	gocmd := testenv.GoToolPath(t)
 
-	cmd := testenv.Command(t, gocmd, "run", "-gcflags=-d=loopvar=2", "opt.go")
+	// loopvar=3 logs info but does not change loopvarness
+	cmd := testenv.Command(t, gocmd, "run", "-gcflags=-lang=go1.22 -d=loopvar=3", "opt.go")
 	cmd.Dir = filepath.Join("testdata")
 
 	b, err := cmd.CombinedOutput()
@@ -260,5 +263,121 @@ func TestLoopVarOpt(t *testing.T) {
 	if err != nil {
 		t.Errorf("err=%v != nil", err)
 	}
+}
+
+// TestLoopVarVersionEnableGoBuild checks for loopvar transformation enabled by go:build version (1.22).
+func TestLoopVarVersionEnableGoBuild(t *testing.T) {
+	switch runtime.GOOS {
+	case "linux", "darwin":
+	default:
+		t.Skipf("Slow test, usually avoid it, os=%s not linux or darwin", runtime.GOOS)
+	}
+	switch runtime.GOARCH {
+	case "amd64", "arm64":
+	default:
+		t.Skipf("Slow test, usually avoid it, arch=%s not amd64 or arm64", runtime.GOARCH)
+	}
+
+	testenv.MustHaveGoBuild(t)
+	gocmd := testenv.GoToolPath(t)
+
+	// loopvar=3 logs info but does not change loopvarness
+	cmd := testenv.Command(t, gocmd, "run", "-gcflags=-lang=go1.21 -d=loopvar=3", "opt-122.go")
+	cmd.Dir = filepath.Join("testdata")
+
+	b, err := cmd.CombinedOutput()
+	m := string(b)
+
+	t.Logf(m)
+
+	yCount := strings.Count(m, "opt-122.go:18:6: loop variable private now per-iteration, heap-allocated (loop inlined into ./opt-122.go:32)")
+	nCount := strings.Count(m, "shared")
+
+	if yCount != 1 {
+		t.Errorf("yCount=%d != 1", yCount)
+	}
+	if nCount > 0 {
+		t.Errorf("nCount=%d > 0", nCount)
+	}
+	if err != nil {
+		t.Errorf("err=%v != nil", err)
+	}
+}
+
+// TestLoopVarVersionDisableFlag checks for loopvar transformation DISABLED by command line version (1.21).
+func TestLoopVarVersionDisableFlag(t *testing.T) {
+	switch runtime.GOOS {
+	case "linux", "darwin":
+	default:
+		t.Skipf("Slow test, usually avoid it, os=%s not linux or darwin", runtime.GOOS)
+	}
+	switch runtime.GOARCH {
+	case "amd64", "arm64":
+	default:
+		t.Skipf("Slow test, usually avoid it, arch=%s not amd64 or arm64", runtime.GOARCH)
+	}
 
+	testenv.MustHaveGoBuild(t)
+	gocmd := testenv.GoToolPath(t)
+
+	// loopvar=3 logs info but does not change loopvarness
+	cmd := testenv.Command(t, gocmd, "run", "-gcflags=-lang=go1.21 -d=loopvar=3", "opt.go")
+	cmd.Dir = filepath.Join("testdata")
+
+	b, err := cmd.CombinedOutput()
+	m := string(b)
+
+	t.Logf(m) // expect error
+
+	yCount := strings.Count(m, "opt.go:16:6: loop variable private now per-iteration, heap-allocated (loop inlined into ./opt.go:30)")
+	nCount := strings.Count(m, "shared")
+
+	if yCount != 0 {
+		t.Errorf("yCount=%d != 0", yCount)
+	}
+	if nCount > 0 {
+		t.Errorf("nCount=%d > 0", nCount)
+	}
+	if err == nil { // expect error
+		t.Errorf("err=%v == nil", err)
+	}
+}
+
+// TestLoopVarVersionDisableGoBuild checks for loopvar transformation DISABLED by go:build version (1.21).
+func TestLoopVarVersionDisableGoBuild(t *testing.T) {
+	switch runtime.GOOS {
+	case "linux", "darwin":
+	default:
+		t.Skipf("Slow test, usually avoid it, os=%s not linux or darwin", runtime.GOOS)
+	}
+	switch runtime.GOARCH {
+	case "amd64", "arm64":
+	default:
+		t.Skipf("Slow test, usually avoid it, arch=%s not amd64 or arm64", runtime.GOARCH)
+	}
+
+	testenv.MustHaveGoBuild(t)
+	gocmd := testenv.GoToolPath(t)
+
+	// loopvar=3 logs info but does not change loopvarness
+	cmd := testenv.Command(t, gocmd, "run", "-gcflags=-lang=go1.22 -d=loopvar=3", "opt-121.go")
+	cmd.Dir = filepath.Join("testdata")
+
+	b, err := cmd.CombinedOutput()
+	m := string(b)
+
+	t.Logf(m) // expect error
+
+	yCount := strings.Count(m, "opt-121.go:18:6: loop variable private now per-iteration, heap-allocated (loop inlined into ./opt-121.go:32)")
+	nCount := strings.Count(m, "shared")
+
+	if yCount != 0 {
+		t.Errorf("yCount=%d != 0", yCount)
+	}
+	if nCount > 0 {
+		t.Errorf("nCount=%d > 0", nCount)
+	}
+	if err == nil { // expect error
+		t.Errorf("err=%v == nil", err)
+	}
 }
diff --git a/src/cmd/compile/internal/loopvar/testdata/opt-121.go b/src/cmd/compile/internal/loopvar/testdata/opt-121.go
new file mode 100644
index 0000000000..131033b13c
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/opt-121.go
@@ -0,0 +1,44 @@
+// Copyright 2023 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.
+
+//go:build go1.21
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var is []func() int
+
+func inline(j, k int) []*int {
+	var a []*int
+	for private := j; private < k; private++ {
+		a = append(a, &private)
+	}
+	return a
+
+}
+
+//go:noinline
+func notinline(j, k int) ([]*int, *int) {
+	for shared := j; shared < k; shared++ {
+		if shared == k/2 {
+			// want the call inlined, want "private" in that inline to be transformed,
+			// (believe it ends up on init node of the return).
+			// but do not want "shared" transformed,
+			return inline(j, k), &shared
+		}
+	}
+	return nil, &j
+}
+
+func main() {
+	a, p := notinline(2, 9)
+	fmt.Printf("a[0]=%d,*p=%d\n", *a[0], *p)
+	if *a[0] != 2 {
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/opt-122.go b/src/cmd/compile/internal/loopvar/testdata/opt-122.go
new file mode 100644
index 0000000000..0ed6feee04
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/opt-122.go
@@ -0,0 +1,44 @@
+// Copyright 2023 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.
+
+//go:build go1.22
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var is []func() int
+
+func inline(j, k int) []*int {
+	var a []*int
+	for private := j; private < k; private++ {
+		a = append(a, &private)
+	}
+	return a
+
+}
+
+//go:noinline
+func notinline(j, k int) ([]*int, *int) {
+	for shared := j; shared < k; shared++ {
+		if shared == k/2 {
+			// want the call inlined, want "private" in that inline to be transformed,
+			// (believe it ends up on init node of the return).
+			// but do not want "shared" transformed,
+			return inline(j, k), &shared
+		}
+	}
+	return nil, &j
+}
+
+func main() {
+	a, p := notinline(2, 9)
+	fmt.Printf("a[0]=%d,*p=%d\n", *a[0], *p)
+	if *a[0] != 2 {
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/compile/internal/noder/irgen.go b/src/cmd/compile/internal/noder/irgen.go
index df5de63620..6019c5986c 100644
--- a/src/cmd/compile/internal/noder/irgen.go
+++ b/src/cmd/compile/internal/noder/irgen.go
@@ -64,6 +64,7 @@ func checkFiles(m posMap, noders []*noder) (*types2.Package, *types2.Info) {
 		Implicits:          make(map[syntax.Node]types2.Object),
 		Scopes:             make(map[syntax.Node]*types2.Scope),
 		Instances:          make(map[*syntax.Name]types2.Instance),
+		FileVersions:       make(map[*syntax.PosBase]types2.Version),
 		// expand as needed
 	}
 
diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index 610d02c07c..6dec060c8c 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -1897,10 +1897,10 @@ func (r *reader) forStmt(label *types.Sym) ir.Node {
 	cond := r.optExpr()
 	post := r.stmt()
 	body := r.blockStmt()
-	dv := r.Bool()
+	perLoopVars := r.Bool()
 	r.closeAnotherScope()
 
-	stmt := ir.NewForStmt(pos, init, cond, post, body, dv)
+	stmt := ir.NewForStmt(pos, init, cond, post, body, perLoopVars)
 	stmt.Label = label
 	return stmt
 }
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index afe452bc9c..1d8c0bf933 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -1456,10 +1456,29 @@ func (w *writer) forStmt(stmt *syntax.ForStmt) {
 	}
 
 	w.blockStmt(stmt.Body)
-	w.Bool(base.Debug.LoopVar > 0)
+	w.Bool(w.distinctVars(stmt))
 	w.closeAnotherScope()
 }
 
+func (w *writer) distinctVars(stmt *syntax.ForStmt) bool {
+	lv := base.Debug.LoopVar
+	v := w.p.info.FileVersions[stmt.Pos().Base()]
+	is122 := v.Major == 0 && v.Minor == 0 || v.Major == 1 && v.Minor >= 22
+
+	// Turning off loopvar for 1.22 is only possible with loopvarhash=qn
+	//
+	// Debug.LoopVar values to be preserved for 1.21 compatibility are 1 and 2,
+	// which are also set (=1) by GOEXPERIMENT=loopvar.  The knobs for turning on
+	// the new, unshared, loopvar behavior apply to versions less than 1.21 because
+	// (1) 1.21 also did that and (2) this is believed to be the likely use case;
+	// anyone checking to see if it affects their code will just run the GOEXPERIMENT
+	// but will not also update all their go.mod files to 1.21.
+	//
+	// -gcflags=-d=loopvar=3 enables logging for 1.22 but does not turn loopvar on for <= 1.21.
+
+	return is122 || lv > 0 && lv != 3
+}
+
 // rangeTypes returns the types of values produced by ranging over
 // expr.
 func (pw *pkgWriter) rangeTypes(expr syntax.Expr) (key, value types2.Type) {