cmd/compile: experimental loop iterator capture semantics change

Adds:
GOEXPERIMENT=loopvar (expected way of invoking)
-d=loopvar={-1,0,1,2,11,12} (for per-package control and/or logging)
-d=loopvarhash=... (for hash debugging)

loopvar=11,12 are for testing, benchmarking, and debugging.

If enabled,for loops of the form `for x,y := range thing`, if x and/or
y are addressed or captured by a closure, are transformed by renaming
x/y to a temporary and prepending an assignment to the body of the
loop x := tmp_x.  This changes the loop semantics by making each
iteration's instance of x be distinct from the others (currently they
are all aliased, and when this matters, it is almost always a bug).

3-range with captured iteration variables are also transformed,
though it is a more complex transformation.

"Optimized" to do a simpler transformation for
3-clause for where the increment is empty.

(Prior optimization of address-taking under Return disabled, because
it was incorrect; returns can have loops for children.  Restored in
a later CL.)

Includes support for -d=loopvarhash=<binary string> intended for use
with hash search and GOCOMPILEDEBUG=loopvarhash=<binary string>
(use `gossahash -e loopvarhash command-that-fails`).

Minor feature upgrades to hash-triggered features; clients can specify
that file-position hashes use only the most-inline position, and/or that
they use only the basenames of source files (not the full directory path).
Most-inlined is the right choice for debugging loop-iteration change
once the semantics are linked to the package across inlining; basename-only
makes it tractable to write tests (which, otherwise, depend on the full
pathname of the source file and thus vary).

Updates #57969.

Change-Id: I180a51a3f8d4173f6210c861f10de23de8a1b1db
Reviewed-on: https://go-review.googlesource.com/c/go/+/411904
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

30 files changed, 1462 insertions, 20 deletions

diff --git a/src/cmd/compile/internal/base/debug.go b/src/cmd/compile/internal/base/debug.go
index 04b8469eef..288c2d82ef 100644
--- a/src/cmd/compile/internal/base/debug.go
+++ b/src/cmd/compile/internal/base/debug.go
@@ -33,6 +33,8 @@ type DebugFlags struct {
 	InlStaticInit         int    `help:"allow static initialization of inlined calls" concurrent:"ok"`
 	InterfaceCycles       int    `help:"allow anonymous interface cycles"`
 	Libfuzzer             int    `help:"enable coverage instrumentation for libfuzzer"`
+	LoopVar               int    `help:"shared (0, default), 1 (private loop variables), 2, private + log"`
+	LoopVarHash           string `help:"for debugging changes in loop behavior. Overrides experiment and loopvar flag."`
 	LocationLists         int    `help:"print information about DWARF location list creation"`
 	Nil                   int    `help:"print information about nil checks"`
 	NoOpenDefer           int    `help:"disable open-coded defers" concurrent:"ok"`
diff --git a/src/cmd/compile/internal/base/flag.go b/src/cmd/compile/internal/base/flag.go
index a833364c66..4baff74917 100644
--- a/src/cmd/compile/internal/base/flag.go
+++ b/src/cmd/compile/internal/base/flag.go
@@ -186,7 +186,61 @@ func ParseFlags() {
 	}
 
 	if Debug.Gossahash != "" {
-		hashDebug = NewHashDebug("gosshash", Debug.Gossahash, nil)
+		hashDebug = NewHashDebug("gossahash", Debug.Gossahash, nil)
+	}
+
+	// Three inputs govern loop iteration variable rewriting, hash, experiment, flag.
+	// The loop variable rewriting is:
+	// IF non-empty hash, then hash determines behavior (function+line match) (*)
+	// ELSE IF experiment and flag==0, then experiment (set flag=1)
+	// ELSE flag (note that build sets flag per-package), with behaviors:
+	//  -1 => no change to behavior.
+	//   0 => no change to behavior (unless non-empty hash, see above)
+	//   1 => apply change to likely-iteration-variable-escaping loops
+	//   2 => apply change, log results
+	//   11 => apply change EVERYWHERE, do not log results (for debugging/benchmarking)
+	//   12 => apply change EVERYWHERE, log results (for debugging/benchmarking)
+	//
+	// The expected uses of the these inputs are, in believed most-likely to least likely:
+	//  GOEXPERIMENT=loopvar -- apply change to entire application
+	//  -gcflags=some_package=-d=loopvar=1 -- apply change to some_package (**)
+	//  -gcflags=some_package=-d=loopvar=2 -- apply change to some_package, log it
+	//  GOEXPERIMENT=loopvar -gcflags=some_package=-d=loopvar=-1 -- apply change to all but one package
+	//  GOCOMPILEDEBUG=loopvarhash=... -- search for failure cause
+	//
+	//  (*) For debugging purposes, providing loopvar flag >= 11 will expand the hash-eligible set of loops to all.
+	// (**) Currently this applies to all code in the compilation of some_package, including
+	//     inlines from other packages that may have been compiled w/o the change.
+
+	if Debug.LoopVarHash != "" {
+		// This first little bit controls the inputs for debug-hash-matching.
+		basenameOnly := false
+		mostInlineOnly := true
+		if strings.HasPrefix(Debug.LoopVarHash, "FS") {
+			// Magic handshake for testing, use file suffixes only when hashing on a position.
+			// i.e., rather than /tmp/asdfasdfasdf/go-test-whatever/foo_test.go,
+			// hash only on "foo_test.go", so that it will be the same hash across all runs.
+			Debug.LoopVarHash = Debug.LoopVarHash[2:]
+			basenameOnly = true
+		}
+		if strings.HasPrefix(Debug.LoopVarHash, "IL") {
+			// When hash-searching on a position that is an inline site, default is to use the
+			// most-inlined position only.  This makes the hash faster, plus there's no point
+			// reporting a problem with all the inlining; there's only one copy of the source.
+			// However, if for some reason you wanted it per-site, you can get this.  (The default
+			// hash-search behavior for compiler debugging is at an inline site.)
+			Debug.LoopVarHash = Debug.LoopVarHash[2:]
+			mostInlineOnly = false
+		}
+		// end of testing trickiness
+		LoopVarHash = NewHashDebug("loopvarhash", Debug.LoopVarHash, nil)
+		if Debug.LoopVar < 11 { // >= 11 means all loops are rewrite-eligible
+			Debug.LoopVar = 1 // 1 means those loops that syntactically escape their dcl vars are eligible.
+		}
+		LoopVarHash.SetInlineSuffixOnly(mostInlineOnly)
+		LoopVarHash.SetFileSuffixOnly(basenameOnly)
+	} else if buildcfg.Experiment.LoopVar && Debug.LoopVar == 0 {
+		Debug.LoopVar = 1
 	}
 
 	if Debug.Fmahash != "" {
diff --git a/src/cmd/compile/internal/base/hashdebug.go b/src/cmd/compile/internal/base/hashdebug.go
index 6c4821bbf6..a13c66646b 100644
--- a/src/cmd/compile/internal/base/hashdebug.go
+++ b/src/cmd/compile/internal/base/hashdebug.go
@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"sync"
@@ -34,16 +35,38 @@ type HashDebug struct {
 	name string     // base name of the flag/variable.
 	// what file (if any) receives the yes/no logging?
 	// default is os.Stdout
-	logfile  writeSyncer
-	posTmp   []src.Pos
-	bytesTmp bytes.Buffer
-	matches  []hashAndMask // A hash matches if one of these matches.
-	yes, no  bool
+	logfile          writeSyncer
+	posTmp           []src.Pos
+	bytesTmp         bytes.Buffer
+	matches          []hashAndMask // A hash matches if one of these matches.
+	yes, no          bool
+	fileSuffixOnly   bool // for Pos hashes, remove the directory prefix.
+	inlineSuffixOnly bool // for Pos hashes, remove all but the most inline position.
+}
+
+// SetFileSuffixOnly controls whether hashing and reporting use the entire
+// file path name, just the basename.  This makes hashing more consistent,
+// at the expense of being able to certainly locate the file.
+func (d *HashDebug) SetFileSuffixOnly(b bool) *HashDebug {
+	d.fileSuffixOnly = b
+	return d
+}
+
+// SetInlineSuffixOnly controls whether hashing and reporting use the entire
+// inline position, or just the most-inline suffix.  Compiler debugging tends
+// to want the whole inlining, debugging user problems (loopvarhash, e.g.)
+// typically does not need to see the entire inline tree, there is just one
+// copy of the source code.
+func (d *HashDebug) SetInlineSuffixOnly(b bool) *HashDebug {
+	d.inlineSuffixOnly = b
+	return d
 }
 
 // The default compiler-debugging HashDebug, for "-d=gossahash=..."
 var hashDebug *HashDebug
-var FmaHash *HashDebug
+
+var FmaHash *HashDebug     // for debugging fused-multiply-add floating point changes
+var LoopVarHash *HashDebug // for debugging shared/private loop variable changes
 
 // DebugHashMatch reports whether debug variable Gossahash
 //
@@ -56,10 +79,10 @@ var FmaHash *HashDebug
 //  4. is a suffix of the sha1 hash of pkgAndName (returns true)
 //
 //  5. OR
-//     if the value is in the regular language "[01]+(;[01]+)+"
+//     if the value is in the regular language "[01]+(/[01]+)+"
 //     test the [01]+ substrings after in order returning true
 //     for the first one that suffix-matches. The substrings AFTER
-//     the first semicolon are numbered 0,1, etc and are named
+//     the first slash are numbered 0,1, etc and are named
 //     fmt.Sprintf("%s%d", varname, number)
 //     Clause 5 is not really intended for human use and only
 //     matters for failures that require multiple triggers.
@@ -235,6 +258,8 @@ func (d *HashDebug) DebugHashMatchParam(pkgAndName string, param uint64) bool {
 // package name and path. The output trigger string is prefixed with "POS=" so
 // that tools processing the output can reliably tell the difference. The mutex
 // locking is also more frequent and more granular.
+// Note that the default answer for no environment variable (d == nil)
+// is "yes", do the thing.
 func (d *HashDebug) DebugHashMatchPos(ctxt *obj.Link, pos src.XPos) bool {
 	if d == nil {
 		return true
@@ -242,6 +267,11 @@ func (d *HashDebug) DebugHashMatchPos(ctxt *obj.Link, pos src.XPos) bool {
 	if d.no {
 		return false
 	}
+	// Written this way to make inlining likely.
+	return d.debugHashMatchPos(ctxt, pos)
+}
+
+func (d *HashDebug) debugHashMatchPos(ctxt *obj.Link, pos src.XPos) bool {
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
@@ -278,9 +308,17 @@ func (d *HashDebug) bytesForPos(ctxt *obj.Link, pos src.XPos) []byte {
 	// Reverse posTmp to put outermost first.
 	b := &d.bytesTmp
 	b.Reset()
-	for i := len(d.posTmp) - 1; i >= 0; i-- {
+	start := len(d.posTmp) - 1
+	if d.inlineSuffixOnly {
+		start = 0
+	}
+	for i := start; i >= 0; i-- {
 		p := &d.posTmp[i]
-		fmt.Fprintf(b, "%s:%d:%d", p.Filename(), p.Line(), p.Col())
+		f := p.Filename()
+		if d.fileSuffixOnly {
+			f = filepath.Base(f)
+		}
+		fmt.Fprintf(b, "%s:%d:%d", f, p.Line(), p.Col())
 		if i != 0 {
 			b.WriteByte(';')
 		}
diff --git a/src/cmd/compile/internal/escape/stmt.go b/src/cmd/compile/internal/escape/stmt.go
index 1ce04d98f3..98cd2d53a6 100644
--- a/src/cmd/compile/internal/escape/stmt.go
+++ b/src/cmd/compile/internal/escape/stmt.go
@@ -64,7 +64,7 @@ func (e *escape) stmt(n ir.Node) {
 			}
 			e.loopDepth++
 		default:
-			base.Fatalf("label missing tag")
+			base.Fatalf("label %v missing tag", n.Label)
 		}
 		delete(e.labels, n.Label)
 
diff --git a/src/cmd/compile/internal/gc/main.go b/src/cmd/compile/internal/gc/main.go
index 3c2b9c48ec..fa9f429a1e 100644
--- a/src/cmd/compile/internal/gc/main.go
+++ b/src/cmd/compile/internal/gc/main.go
@@ -16,6 +16,7 @@ import (
 	"cmd/compile/internal/inline"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/logopt"
+	"cmd/compile/internal/loopvar"
 	"cmd/compile/internal/noder"
 	"cmd/compile/internal/pgo"
 	"cmd/compile/internal/pkginit"
@@ -265,10 +266,12 @@ func Main(archInit func(*ssagen.ArchInfo)) {
 	}
 	noder.MakeWrappers(typecheck.Target) // must happen after inlining
 
-	// Devirtualize.
+	// Devirtualize and get variable capture right in for loops
+	var transformed []*ir.Name
 	for _, n := range typecheck.Target.Decls {
 		if n.Op() == ir.ODCLFUNC {
 			devirtualize.Func(n.(*ir.Func))
+			transformed = append(transformed, loopvar.ForCapture(n.(*ir.Func))...)
 		}
 	}
 	ir.CurFunc = nil
@@ -293,6 +296,46 @@ func Main(archInit func(*ssagen.ArchInfo)) {
 	base.Timer.Start("fe", "escapes")
 	escape.Funcs(typecheck.Target.Decls)
 
+	if 2 <= base.Debug.LoopVar && base.Debug.LoopVar != 11 || logopt.Enabled() { // 11 is do them all, quietly, 12 includes debugging.
+		fileToPosBase := make(map[string]*src.PosBase) // used to remove inline context for innermost reporting.
+		for _, n := range transformed {
+			pos := n.Pos()
+			if logopt.Enabled() {
+				// For automated checking of coverage of this transformation, include this in the JSON information.
+				if n.Esc() == ir.EscHeap {
+					logopt.LogOpt(pos, "transform-escape", "loopvar", ir.FuncName(n.Curfn))
+				} else {
+					logopt.LogOpt(pos, "transform-noescape", "loopvar", ir.FuncName(n.Curfn))
+				}
+			}
+			inner := base.Ctxt.InnermostPos(pos)
+			outer := base.Ctxt.OutermostPos(pos)
+			if inner == outer {
+				if n.Esc() == ir.EscHeap {
+					base.WarnfAt(pos, "transformed loop variable %v escapes", n)
+				} else {
+					base.WarnfAt(pos, "transformed loop variable %v does not escape", n)
+				}
+			} else {
+				// Report the problem at the line where it actually occurred.
+				afn := inner.AbsFilename()
+				pb, ok := fileToPosBase[afn]
+				if !ok {
+					pb = src.NewFileBase(inner.Filename(), afn)
+					fileToPosBase[afn] = pb
+				}
+				inner.SetBase(pb) // rebasing w/o inline context makes it print correctly in WarnfAt; otherwise it prints as outer.
+				innerXPos := base.Ctxt.PosTable.XPos(inner)
+
+				if n.Esc() == ir.EscHeap {
+					base.WarnfAt(innerXPos, "transformed loop variable %v escapes (loop inlined into %s:%d)", n, outer.Filename(), outer.Line())
+				} else {
+					base.WarnfAt(innerXPos, "transformed loop variable %v does not escape (loop inlined into %s:%d)", n, outer.Filename(), outer.Line())
+				}
+			}
+		}
+	}
+
 	// Collect information for go:nowritebarrierrec
 	// checking. This must happen before transforming closures during Walk
 	// We'll do the final check after write barriers are
diff --git a/src/cmd/compile/internal/ir/stmt.go b/src/cmd/compile/internal/ir/stmt.go
index 9f2d04f450..dd3908e665 100644
--- a/src/cmd/compile/internal/ir/stmt.go
+++ b/src/cmd/compile/internal/ir/stmt.go
@@ -163,6 +163,15 @@ func NewBranchStmt(pos src.XPos, op Op, label *types.Sym) *BranchStmt {
 	return n
 }
 
+func (n *BranchStmt) SetOp(op Op) {
+	switch op {
+	default:
+		panic(n.no("SetOp " + op.String()))
+	case OBREAK, OCONTINUE, OFALL, OGOTO:
+		n.op = op
+	}
+}
+
 func (n *BranchStmt) Sym() *types.Sym { return n.Label }
 
 // A CaseClause is a case statement in a switch or select: case List: Body.
diff --git a/src/cmd/compile/internal/loopvar/loopvar.go b/src/cmd/compile/internal/loopvar/loopvar.go
new file mode 100644
index 0000000000..bc288657ab
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/loopvar.go
@@ -0,0 +1,449 @@
+// 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.
+
+// Package loopvar applies the proper variable capture, according
+// to experiment, flags, language version, etc.
+package loopvar
+
+import (
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
+	"fmt"
+)
+
+// ForCapture transforms for and range loops that declare variables that might be
+// captured by a closure or escaped to the heap, using a syntactic check that
+// conservatively overestimates the loops where capture occurs, but still avoids
+// transforming the (large) majority of loops. It returns the list of names
+// subject to this change, that may (once transformed) be heap allocated in the
+// process. (This allows checking after escape analysis to call out any such
+// variables, in case it causes allocation/performance problems).
+
+// For this code, the meaningful debug and hash flag settings
+//
+// base.Debug.LoopVar <= 0 => do not transform
+//
+// base.LoopVarHash != nil => use hash setting to govern transformation.
+// note that LoopVarHash != nil sets base.Debug.LoopVar to 1 (unless it is >= 11, for testing/debugging).
+//
+// base.Debug.LoopVar == 11 => transform ALL loops ignoring syntactic/potential escape. Do not log, can be in addition to GOEXPERIMENT.
+//
+// The effect of GOEXPERIMENT=loopvar is to change the default value (0) of base.Debug.LoopVar to 1 for all packages.
+
+func ForCapture(fn *ir.Func) []*ir.Name {
+	if base.Debug.LoopVar <= 0 { // code in base:flags.go ensures >= 1 if loopvarhash != ""
+		// TODO remove this when the transformation is made sensitive to inlining; this is least-risk for 1.21
+		return nil
+	}
+
+	// if a loop variable is transformed it is appended to this slice for later logging
+	var transformed []*ir.Name
+
+	forCapture := func() {
+		seq := 1
+
+		dclFixups := make(map[*ir.Name]ir.Stmt)
+
+		// possibly leaked includes names of declared loop variables that may be leaked;
+		// the mapped value is true if the name is *syntactically* leaked, and those loops
+		// will be transformed.
+		possiblyLeaked := make(map[*ir.Name]bool)
+
+		// noteMayLeak is called for candidate variables in for range/3-clause, and
+		// adds them (mapped to false) to possiblyLeaked.
+		noteMayLeak := func(x ir.Node) {
+			if n, ok := x.(*ir.Name); ok {
+				if n.Type().Kind() == types.TBLANK {
+					return
+				}
+				// default is false (leak candidate, not yet known to leak), but flag can make all variables "leak"
+				possiblyLeaked[n] = base.Debug.LoopVar >= 11
+			}
+		}
+
+		// maybeReplaceVar unshares an iteration variable for a range loop,
+		// if that variable was actually (syntactically) leaked,
+		// subject to hash-variable debugging.
+		maybeReplaceVar := func(k ir.Node, x *ir.RangeStmt) ir.Node {
+			if n, ok := k.(*ir.Name); ok && possiblyLeaked[n] {
+				if base.LoopVarHash.DebugHashMatchPos(base.Ctxt, n.Pos()) {
+					// Rename the loop key, prefix body with assignment from loop key
+					transformed = append(transformed, n)
+					tk := typecheck.Temp(n.Type())
+					tk.SetTypecheck(1)
+					as := ir.NewAssignStmt(x.Pos(), n, tk)
+					as.Def = true
+					as.SetTypecheck(1)
+					x.Body.Prepend(as)
+					dclFixups[n] = as
+					return tk
+				}
+			}
+			return k
+		}
+
+		// scanChildrenThenTransform processes node x to:
+		//  1. if x is a for/range, note declared iteration variables possiblyLeaked (PL)
+		//  2. search all of x's children for syntactically escaping references to v in PL,
+		//     meaning either address-of-v or v-captured-by-a-closure
+		//  3. for all v in PL that had a syntactically escaping reference, transform the declaration
+		//     and (in case of 3-clause loop) the loop to the unshared loop semantics.
+		//  This is all much simpler for range loops; 3-clause loops can have an arbitrary number
+		//  of iteration variables and the transformation is more involved, range loops have at most 2.
+		var scanChildrenThenTransform func(x ir.Node) bool
+		scanChildrenThenTransform = func(n ir.Node) bool {
+			switch x := n.(type) {
+			case *ir.ClosureExpr:
+				for _, cv := range x.Func.ClosureVars {
+					v := cv.Canonical()
+					if _, ok := possiblyLeaked[v]; ok {
+						possiblyLeaked[v] = true
+					}
+				}
+
+			case *ir.AddrExpr:
+				// Explicitly note address-taken so that return-statements can be excluded
+				y := ir.OuterValue(x.X)
+				if y.Op() != ir.ONAME {
+					break
+				}
+				z, ok := y.(*ir.Name)
+				if !ok {
+					break
+				}
+				switch z.Class {
+				case ir.PAUTO, ir.PPARAM, ir.PPARAMOUT, ir.PAUTOHEAP:
+					if _, ok := possiblyLeaked[z]; ok {
+						possiblyLeaked[z] = true
+					}
+				}
+
+			case *ir.RangeStmt:
+				if !x.Def {
+					break
+				}
+				noteMayLeak(x.Key)
+				noteMayLeak(x.Value)
+				ir.DoChildren(n, scanChildrenThenTransform)
+				x.Key = maybeReplaceVar(x.Key, x)
+				x.Value = maybeReplaceVar(x.Value, x)
+				return false
+
+			case *ir.ForStmt:
+				forAllDefInInit(x, noteMayLeak)
+				ir.DoChildren(n, scanChildrenThenTransform)
+				var leaked []*ir.Name
+				// Collect the leaking variables for the much-more-complex transformation.
+				forAllDefInInit(x, func(z ir.Node) {
+					if n, ok := z.(*ir.Name); ok && possiblyLeaked[n] {
+						// Hash on n.Pos() for most precise failure location.
+						if base.LoopVarHash.DebugHashMatchPos(base.Ctxt, n.Pos()) {
+							leaked = append(leaked, n)
+						}
+					}
+				})
+
+				if len(leaked) > 0 {
+					// need to transform the for loop just so.
+
+					/* Contrived example, w/ numbered comments from the transformation:
+									BEFORE:
+										var escape []*int
+										for z := 0; z < n; z++ {
+											if reason() {
+												escape = append(escape, &z)
+												continue
+											}
+											z = z + z
+											stuff
+										}
+									AFTER:
+										for z', tmp_first := 0, true; ; { // (4)
+											                              // (5) body' follows:
+											z := z'                       // (1)
+											if tmp_first {tmp_first = false} else {z++} // (6)
+											if ! (z < n) { break }        // (7)
+											                              // (3, 8) body_continue
+											if reason() {
+					                            escape = append(escape, &z)
+												goto next                 // rewritten continue
+											}
+											z = z + z
+											stuff
+										next:                             // (9)
+											z' = z                       // (2)
+										}
+
+										In the case that the loop contains no increment (z++),
+										there is no need for step 6,
+										and thus no need to test, update, or declare tmp_first (part of step 4).
+										Similarly if the loop contains no exit test (z < n),
+										then there is no need for step 7.
+					*/
+
+					// Expressed in terms of the input ForStmt
+					//
+					// 	type ForStmt struct {
+					// 	init     Nodes
+					// 	Label    *types.Sym
+					// 	Cond     Node  // empty if OFORUNTIL
+					// 	Post     Node
+					// 	Body     Nodes
+					// 	HasBreak bool
+					// }
+
+					// OFOR: init; loop: if !Cond {break}; Body; Post; goto loop
+
+					// (1) prebody = {z := z' for z in leaked}
+					// (2) postbody = {z' = z for z in leaked}
+					// (3) body_continue = {body : s/continue/goto next}
+					// (4) init' = (init : s/z/z' for z in leaked) + tmp_first := true
+					// (5) body' = prebody +        // appears out of order below
+					// (6)         if tmp_first {tmp_first = false} else {Post} +
+					// (7)         if !cond {break} +
+					// (8)         body_continue (3) +
+					// (9)         next: postbody (2)
+					// (10) cond' = {}
+					// (11) post' = {}
+
+					// minor optimizations:
+					//   if Post is empty, tmp_first and step 6 can be skipped.
+					//   if Cond is empty, that code can also be skipped.
+
+					var preBody, postBody ir.Nodes
+
+					// Given original iteration variable z, what is the corresponding z'
+					// that carries the value from iteration to iteration?
+					zPrimeForZ := make(map[*ir.Name]*ir.Name)
+
+					// (1,2) initialize preBody and postBody
+					for _, z := range leaked {
+						transformed = append(transformed, z)
+
+						tz := typecheck.Temp(z.Type())
+						tz.SetTypecheck(1)
+						zPrimeForZ[z] = tz
+
+						as := ir.NewAssignStmt(x.Pos(), z, tz)
+						as.Def = true
+						as.SetTypecheck(1)
+						preBody.Append(as)
+						dclFixups[z] = as
+
+						as = ir.NewAssignStmt(x.Pos(), tz, z)
+						as.SetTypecheck(1)
+						postBody.Append(as)
+
+					}
+
+					// (3) rewrite continues in body -- rewrite is inplace, so works for top level visit, too.
+					label := typecheck.Lookup(fmt.Sprintf(".3clNext_%d", seq))
+					seq++
+					labelStmt := ir.NewLabelStmt(x.Pos(), label)
+					labelStmt.SetTypecheck(1)
+
+					loopLabel := x.Label
+					loopDepth := 0
+					var editContinues func(x ir.Node) bool
+					editContinues = func(x ir.Node) bool {
+
+						switch c := x.(type) {
+						case *ir.BranchStmt:
+							// If this is a continue targeting the loop currently being rewritten, transform it to an appropriate GOTO
+							if c.Op() == ir.OCONTINUE && (loopDepth == 0 && c.Label == nil || loopLabel != nil && c.Label == loopLabel) {
+								c.Label = label
+								c.SetOp(ir.OGOTO)
+							}
+						case *ir.RangeStmt, *ir.ForStmt:
+							loopDepth++
+							ir.DoChildren(x, editContinues)
+							loopDepth--
+							return false
+						}
+						ir.DoChildren(x, editContinues)
+						return false
+					}
+					for _, y := range x.Body {
+						editContinues(y)
+					}
+					bodyContinue := x.Body
+
+					// (4) rewrite init
+					forAllDefInInitUpdate(x, func(z ir.Node, pz *ir.Node) {
+						// note tempFor[n] can be nil if hash searching.
+						if n, ok := z.(*ir.Name); ok && possiblyLeaked[n] && zPrimeForZ[n] != nil {
+							*pz = zPrimeForZ[n]
+						}
+					})
+
+					postNotNil := x.Post != nil
+					var tmpFirstDcl *ir.AssignStmt
+					if postNotNil {
+						// body' = prebody +
+						// (6)     if tmp_first {tmp_first = false} else {Post} +
+						//         if !cond {break} + ...
+						tmpFirst := typecheck.Temp(types.Types[types.TBOOL])
+
+						// tmpFirstAssign assigns val to tmpFirst
+						tmpFirstAssign := func(val bool) *ir.AssignStmt {
+							s := ir.NewAssignStmt(x.Pos(), tmpFirst, typecheck.OrigBool(tmpFirst, val))
+							s.SetTypecheck(1)
+							return s
+						}
+
+						tmpFirstDcl = tmpFirstAssign(true)
+						tmpFirstDcl.Def = true // also declares tmpFirst
+						tmpFirstSetFalse := tmpFirstAssign(false)
+						ifTmpFirst := ir.NewIfStmt(x.Pos(), tmpFirst, ir.Nodes{tmpFirstSetFalse}, ir.Nodes{x.Post})
+						ifTmpFirst.SetTypecheck(1)
+						preBody.Append(ifTmpFirst)
+					}
+
+					// body' = prebody +
+					//         if tmp_first {tmp_first = false} else {Post} +
+					// (7)     if !cond {break} + ...
+					if x.Cond != nil {
+						notCond := ir.NewUnaryExpr(x.Cond.Pos(), ir.ONOT, x.Cond)
+						notCond.SetType(x.Cond.Type())
+						notCond.SetTypecheck(1)
+						newBreak := ir.NewBranchStmt(x.Pos(), ir.OBREAK, nil)
+						newBreak.SetTypecheck(1)
+						ifNotCond := ir.NewIfStmt(x.Pos(), notCond, ir.Nodes{newBreak}, nil)
+						ifNotCond.SetTypecheck(1)
+						preBody.Append(ifNotCond)
+					}
+
+					if postNotNil {
+						x.PtrInit().Append(tmpFirstDcl)
+					}
+
+					// (8)
+					preBody.Append(bodyContinue...)
+					// (9)
+					preBody.Append(labelStmt)
+					preBody.Append(postBody...)
+
+					// (5) body' = prebody + ...
+					x.Body = preBody
+
+					// (10) cond' = {}
+					x.Cond = nil
+
+					// (11) post' = {}
+					x.Post = nil
+				}
+
+				return false
+			}
+
+			ir.DoChildren(n, scanChildrenThenTransform)
+
+			return false
+		}
+		scanChildrenThenTransform(fn)
+		if len(transformed) > 0 {
+			// editNodes scans a slice C of ir.Node, looking for declarations that
+			// appear in dclFixups.  Any declaration D whose "fixup" is an assignmnt
+			// statement A is removed from the C and relocated to the Init
+			// of A.  editNodes returns the modified slice of ir.Node.
+			editNodes := func(c ir.Nodes) ir.Nodes {
+				j := 0
+				for _, n := range c {
+					if d, ok := n.(*ir.Decl); ok {
+						if s := dclFixups[d.X]; s != nil {
+							switch a := s.(type) {
+							case *ir.AssignStmt:
+								a.PtrInit().Prepend(d)
+								delete(dclFixups, d.X) // can't be sure of visit order, wouldn't want to visit twice.
+							default:
+								base.Fatalf("not implemented yet for node type %v", s.Op())
+							}
+							continue // do not copy this node, and do not increment j
+						}
+					}
+					c[j] = n
+					j++
+				}
+				for k := j; k < len(c); k++ {
+					c[k] = nil
+				}
+				return c[:j]
+			}
+			// fixup all tagged declarations in all the statements lists in fn.
+			rewriteNodes(fn, editNodes)
+		}
+	}
+	ir.WithFunc(fn, forCapture)
+	return transformed
+}
+
+// forAllDefInInitUpdate applies "do" to all the defining assignemnts in the Init clause of a ForStmt.
+// This abstracts away some of the boilerplate from the already complex and verbose for-3-clause case.
+func forAllDefInInitUpdate(x *ir.ForStmt, do func(z ir.Node, update *ir.Node)) {
+	for _, s := range x.Init() {
+		switch y := s.(type) {
+		case *ir.AssignListStmt:
+			if !y.Def {
+				continue
+			}
+			for i, z := range y.Lhs {
+				do(z, &y.Lhs[i])
+			}
+		case *ir.AssignStmt:
+			if !y.Def {
+				continue
+			}
+			do(y.X, &y.X)
+		}
+	}
+}
+
+// forAllDefInInit is forAllDefInInitUpdate without the update option.
+func forAllDefInInit(x *ir.ForStmt, do func(z ir.Node)) {
+	forAllDefInInitUpdate(x, func(z ir.Node, _ *ir.Node) { do(z) })
+}
+
+// rewriteNodes applies editNodes to all statement lists in fn.
+func rewriteNodes(fn *ir.Func, editNodes func(c ir.Nodes) ir.Nodes) {
+	var forNodes func(x ir.Node) bool
+	forNodes = func(n ir.Node) bool {
+		if stmt, ok := n.(ir.InitNode); ok {
+			// process init list
+			stmt.SetInit(editNodes(stmt.Init()))
+		}
+		switch x := n.(type) {
+		case *ir.Func:
+			x.Body = editNodes(x.Body)
+			x.Enter = editNodes(x.Enter)
+			x.Exit = editNodes(x.Exit)
+		case *ir.InlinedCallExpr:
+			x.Body = editNodes(x.Body)
+
+		case *ir.CaseClause:
+			x.Body = editNodes(x.Body)
+		case *ir.CommClause:
+			x.Body = editNodes(x.Body)
+
+		case *ir.BlockStmt:
+			x.List = editNodes(x.List)
+
+		case *ir.ForStmt:
+			x.Body = editNodes(x.Body)
+		case *ir.RangeStmt:
+			x.Body = editNodes(x.Body)
+		case *ir.IfStmt:
+			x.Body = editNodes(x.Body)
+			x.Else = editNodes(x.Else)
+		case *ir.SelectStmt:
+			x.Compiled = editNodes(x.Compiled)
+		case *ir.SwitchStmt:
+			x.Compiled = editNodes(x.Compiled)
+		}
+		ir.DoChildren(n, forNodes)
+		return false
+	}
+	forNodes(fn)
+}
diff --git a/src/cmd/compile/internal/loopvar/loopvar_test.go b/src/cmd/compile/internal/loopvar/loopvar_test.go
new file mode 100644
index 0000000000..6ff4fc9e22
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/loopvar_test.go
@@ -0,0 +1,152 @@
+// 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.
+
+package loopvar_test
+
+import (
+	"internal/testenv"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+type testcase struct {
+	lvFlag      string // ==-2, -1, 0, 1, 2
+	buildExpect string // message, if any
+	expectRC    int
+	files       []string
+}
+
+var for_files = []string{
+	"for_esc_address.go",             // address of variable
+	"for_esc_closure.go",             // closure of variable
+	"for_esc_minimal_closure.go",     // simple closure of variable
+	"for_esc_method.go",              // method value of variable
+	"for_complicated_esc_address.go", // modifies loop index in body
+}
+
+var range_files = []string{
+	"range_esc_address.go",         // address of variable
+	"range_esc_closure.go",         // closure of variable
+	"range_esc_minimal_closure.go", // simple closure of variable
+	"range_esc_method.go",          // method value of variable
+}
+
+var cases = []testcase{
+	{"-1", "", 11, for_files[:1]},
+	{"0", "", 0, for_files[:1]},
+	{"1", "", 0, for_files[:1]},
+	{"2", "transformed loop variable i ", 0, for_files},
+
+	{"-1", "", 11, range_files[:1]},
+	{"0", "", 0, range_files[:1]},
+	{"1", "", 0, range_files[:1]},
+	{"2", "transformed loop variable i ", 0, range_files},
+
+	{"1", "", 0, []string{"for_nested.go"}},
+}
+
+// TestLoopVar checks that the GOEXPERIMENT and debug flags behave as expected.
+func TestLoopVar(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)
+	tmpdir := t.TempDir()
+	output := filepath.Join(tmpdir, "foo.exe")
+
+	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.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)
+			b, e := cmd.CombinedOutput()
+			if e != nil {
+				t.Error(e)
+			}
+			if tc.buildExpect != "" {
+				s := string(b)
+				if !strings.Contains(s, tc.buildExpect) {
+					t.Errorf("File %s test %d expected to match '%s' with \n-----\n%s\n-----", f, i, tc.buildExpect, s)
+				}
+			}
+			// run what we just built.
+			cmd = testenv.Command(t, output)
+			b, e = cmd.CombinedOutput()
+			if tc.expectRC != 0 {
+				if e == nil {
+					t.Errorf("Missing expected error, file %s, case %d", f, i)
+				} else if ee, ok := (e).(*exec.ExitError); !ok || ee.ExitCode() != tc.expectRC {
+					t.Error(e)
+				} else {
+					// okay
+				}
+			} else if e != nil {
+				t.Error(e)
+			}
+		}
+	}
+}
+
+func TestLoopVarHashes(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)
+	tmpdir := t.TempDir()
+
+	root := "cmd/compile/internal/loopvar/testdata/inlines"
+
+	f := func(hash string) string {
+		// 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", root)
+		cmd.Env = append(cmd.Env, "GOCOMPILEDEBUG=loopvarhash=FS"+hash, "HOME="+tmpdir)
+		cmd.Dir = filepath.Join("testdata", "inlines")
+
+		b, _ := cmd.CombinedOutput()
+		// Ignore the error, sometimes it's supposed to fail, the output test will catch it.
+		return string(b)
+	}
+
+	m := f("000100000010011111101100")
+	t.Logf(m)
+
+	mCount := strings.Count(m, "loopvarhash triggered POS=main.go:27:6")
+	otherCount := strings.Count(m, "loopvarhash")
+	if mCount < 1 {
+		t.Errorf("Did not see expected value of m compile")
+	}
+	if mCount != otherCount {
+		t.Errorf("Saw extraneous hash matches")
+	}
+	// This next test carefully dodges a bug-to-be-fixed with inlined locations for ir.Names.
+	if !strings.Contains(m, ", 100, 100, 100, 100") {
+		t.Errorf("Did not see expected value of m run")
+	}
+
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/for_complicated_esc_address.go b/src/cmd/compile/internal/loopvar/testdata/for_complicated_esc_address.go
new file mode 100644
index 0000000000..c658340fd5
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/for_complicated_esc_address.go
@@ -0,0 +1,115 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	ss, sa := shared(23)
+	ps, pa := private(23)
+	es, ea := experiment(23)
+
+	fmt.Printf("shared s, a; private, s, a; experiment s, a = %d, %d;  %d, %d;  %d, %d\n", ss, sa, ps, pa, es, ea)
+
+	if ss != ps || ss != es || ea != pa || sa == pa {
+		os.Exit(11)
+	} else {
+		fmt.Println("PASS")
+	}
+}
+
+func experiment(x int) (int, int) {
+	sum := 0
+	var is []*int
+	for i := x; i != 1; i = i / 2 {
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		i = i*3 + 1
+		if i&1 == 0 {
+			is = append(is, &i)
+			for i&2 == 0 {
+				i = i >> 1
+			}
+		} else {
+			i = i + i
+		}
+	}
+
+	asum := 0
+	for _, pi := range is {
+		asum += *pi
+	}
+
+	return sum, asum
+}
+
+func private(x int) (int, int) {
+	sum := 0
+	var is []*int
+	I := x
+	for ; I != 1; I = I / 2 {
+		i := I
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				I = i
+				continue
+			}
+			sum++
+		}
+		i = i*3 + 1
+		if i&1 == 0 {
+			is = append(is, &i)
+			for i&2 == 0 {
+				i = i >> 1
+			}
+		} else {
+			i = i + i
+		}
+		I = i
+	}
+
+	asum := 0
+	for _, pi := range is {
+		asum += *pi
+	}
+
+	return sum, asum
+}
+
+func shared(x int) (int, int) {
+	sum := 0
+	var is []*int
+	i := x
+	for ; i != 1; i = i / 2 {
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		i = i*3 + 1
+		if i&1 == 0 {
+			is = append(is, &i)
+			for i&2 == 0 {
+				i = i >> 1
+			}
+		} else {
+			i = i + i
+		}
+	}
+
+	asum := 0
+	for _, pi := range is {
+		asum += *pi
+	}
+	return sum, asum
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/for_esc_address.go b/src/cmd/compile/internal/loopvar/testdata/for_esc_address.go
new file mode 100644
index 0000000000..beaefb10ab
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/for_esc_address.go
@@ -0,0 +1,45 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	sum := 0
+	var is []*int
+	for i := 0; i < 10; i++ {
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		if i&1 == 0 {
+			is = append(is, &i)
+		}
+	}
+
+	bug := false
+	if sum != 100-10 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 90, sum)
+		bug = true
+	}
+	sum = 0
+	for _, pi := range is {
+		sum += *pi
+	}
+	if sum != 2+4+6+8 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 20, sum)
+		bug = true
+	}
+	if !bug {
+		fmt.Printf("PASS\n")
+	} else {
+		os.Exit(11)
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/for_esc_closure.go b/src/cmd/compile/internal/loopvar/testdata/for_esc_closure.go
new file mode 100644
index 0000000000..b60d0007bd
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/for_esc_closure.go
@@ -0,0 +1,51 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var is []func() int
+
+func main() {
+	sum := 0
+	for i := 0; i < 10; i++ {
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		if i&1 == 0 {
+			is = append(is, func() int {
+				if i%17 == 15 {
+					i++
+				}
+				return i
+			})
+		}
+	}
+
+	bug := false
+	if sum != 100-10 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 90, sum)
+		bug = true
+	}
+	sum = 0
+	for _, f := range is {
+		sum += f()
+	}
+	if sum != 2+4+6+8 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 20, sum)
+		bug = true
+	}
+	if !bug {
+		fmt.Printf("PASS\n")
+	} else {
+		os.Exit(11)
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/for_esc_method.go b/src/cmd/compile/internal/loopvar/testdata/for_esc_method.go
new file mode 100644
index 0000000000..0e2f8017bc
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/for_esc_method.go
@@ -0,0 +1,51 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+type I int
+
+func (x *I) method() int {
+	return int(*x)
+}
+
+func main() {
+	sum := 0
+	var is []func() int
+	for i := I(0); int(i) < 10; i++ {
+		for j := 0; j < 10; j++ {
+			if int(i) == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		if i&1 == 0 {
+			is = append(is, i.method)
+		}
+	}
+
+	bug := false
+	if sum != 100-10 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 90, sum)
+		bug = true
+	}
+	sum = 0
+	for _, m := range is {
+		sum += m()
+	}
+	if sum != 2+4+6+8 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 20, sum)
+		bug = true
+	}
+	if !bug {
+		fmt.Printf("PASS\n")
+	} else {
+		os.Exit(11)
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/for_esc_minimal_closure.go b/src/cmd/compile/internal/loopvar/testdata/for_esc_minimal_closure.go
new file mode 100644
index 0000000000..971c91dde1
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/for_esc_minimal_closure.go
@@ -0,0 +1,48 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var is []func() int
+
+func main() {
+	sum := 0
+	for i := 0; i < 10; i++ {
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		if i&1 == 0 {
+			is = append(is, func() int {
+				return i
+			})
+		}
+	}
+
+	bug := false
+	if sum != 100-10 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 90, sum)
+		bug = true
+	}
+	sum = 0
+	for _, f := range is {
+		sum += f()
+	}
+	if sum != 2+4+6+8 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 20, sum)
+		bug = true
+	}
+	if !bug {
+		fmt.Printf("PASS\n")
+	} else {
+		os.Exit(11)
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/for_nested.go b/src/cmd/compile/internal/loopvar/testdata/for_nested.go
new file mode 100644
index 0000000000..4888fabc45
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/for_nested.go
@@ -0,0 +1,47 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	x := f(60)
+	fmt.Println(x)
+	if x != 54 {
+		os.Exit(11)
+	}
+}
+
+var escape *int
+
+func f(i int) int {
+	a := 0
+outer:
+	for {
+		switch {
+		case i > 55:
+			i--
+			continue
+		case i == 55:
+			for j := i; j != 1; j = j / 2 {
+				a++
+				if j == 4 {
+					escape = &j
+					i--
+					continue outer
+				}
+				if j&1 == 1 {
+					j = 2 * (3*j + 1)
+				}
+			}
+			return a
+		case i < 55:
+			return i
+		}
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/inlines/a/a.go b/src/cmd/compile/internal/loopvar/testdata/inlines/a/a.go
new file mode 100644
index 0000000000..0bae36daff
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/inlines/a/a.go
@@ -0,0 +1,20 @@
+// 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.
+
+package a
+
+import "cmd/compile/internal/loopvar/testdata/inlines/b"
+
+func F() []*int {
+	var s []*int
+	for i := 0; i < 10; i++ {
+		s = append(s, &i)
+	}
+	return s
+}
+
+func Fb() []*int {
+	bf, _ := b.F()
+	return bf
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/inlines/b/b.go b/src/cmd/compile/internal/loopvar/testdata/inlines/b/b.go
new file mode 100644
index 0000000000..7b1d8cede1
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/inlines/b/b.go
@@ -0,0 +1,21 @@
+// 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.
+
+package b
+
+var slice = []int{1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024}
+
+func F() ([]*int, []*int) {
+	return g()
+}
+
+func g() ([]*int, []*int) {
+	var s []*int
+	var t []*int
+	for i, j := range slice {
+		s = append(s, &i)
+		t = append(t, &j)
+	}
+	return s[:len(s)-1], t
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/inlines/c/c.go b/src/cmd/compile/internal/loopvar/testdata/inlines/c/c.go
new file mode 100644
index 0000000000..0405ace9fe
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/inlines/c/c.go
@@ -0,0 +1,14 @@
+// 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.
+
+package c
+
+//go:noinline
+func F() []*int {
+	var s []*int
+	for i := 0; i < 10; i++ {
+		s = append(s, &i)
+	}
+	return s
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/inlines/main.go b/src/cmd/compile/internal/loopvar/testdata/inlines/main.go
new file mode 100644
index 0000000000..46fcee1a6d
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/inlines/main.go
@@ -0,0 +1,53 @@
+// 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.
+
+package main
+
+import (
+	"cmd/compile/internal/loopvar/testdata/inlines/a"
+	"cmd/compile/internal/loopvar/testdata/inlines/b"
+	"cmd/compile/internal/loopvar/testdata/inlines/c"
+	"fmt"
+	"os"
+)
+
+func sum(s []*int) int {
+	sum := 0
+	for _, pi := range s {
+		sum += *pi
+	}
+	return sum
+}
+
+var t []*int
+
+func F() []*int {
+	var s []*int
+	for i, j := 0, 0; j < 10; i, j = i+1, j+1 {
+		s = append(s, &i)
+		t = append(s, &j)
+	}
+	return s
+}
+
+func main() {
+	f := F()
+	af := a.F()
+	bf, _ := b.F()
+	abf := a.Fb()
+	cf := c.F()
+
+	sf, saf, sbf, sabf, scf := sum(f), sum(af), sum(bf), sum(abf), sum(cf)
+
+	fmt.Printf("f, af, bf, abf, cf sums = %d, %d, %d, %d, %d\n", sf, saf, sbf, sabf, scf)
+
+	// Special failure just for use with hash searching, to prove it fires exactly once.
+	// To test: `gossahash -e loopvarhash go run .` in this directory.
+	// This is designed to fail in two different ways, because gossahash searches randomly
+	// it will find both failures over time.
+	if os.Getenv("GOCOMPILEDEBUG") != "" && (sabf == 45 || sf == 45) {
+		os.Exit(11)
+	}
+	os.Exit(0)
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/range_esc_address.go b/src/cmd/compile/internal/loopvar/testdata/range_esc_address.go
new file mode 100644
index 0000000000..79d7f04a0c
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/range_esc_address.go
@@ -0,0 +1,47 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var ints = []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+func main() {
+	sum := 0
+	var is []*int
+	for _, i := range ints {
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		if i&1 == 0 {
+			is = append(is, &i)
+		}
+	}
+
+	bug := false
+	if sum != 100-10 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 90, sum)
+		bug = true
+	}
+	sum = 0
+	for _, pi := range is {
+		sum += *pi
+	}
+	if sum != 2+4+6+8 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 20, sum)
+		bug = true
+	}
+	if !bug {
+		fmt.Printf("PASS\n")
+	} else {
+		os.Exit(11)
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/range_esc_closure.go b/src/cmd/compile/internal/loopvar/testdata/range_esc_closure.go
new file mode 100644
index 0000000000..9bcb5efb09
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/range_esc_closure.go
@@ -0,0 +1,53 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var is []func() int
+
+var ints = []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+func main() {
+	sum := 0
+	for _, i := range ints {
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		if i&1 == 0 {
+			is = append(is, func() int {
+				if i%17 == 15 {
+					i++
+				}
+				return i
+			})
+		}
+	}
+
+	bug := false
+	if sum != 100-10 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 90, sum)
+		bug = true
+	}
+	sum = 0
+	for _, f := range is {
+		sum += f()
+	}
+	if sum != 2+4+6+8 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 20, sum)
+		bug = true
+	}
+	if !bug {
+		fmt.Printf("PASS\n")
+	} else {
+		os.Exit(11)
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/range_esc_method.go b/src/cmd/compile/internal/loopvar/testdata/range_esc_method.go
new file mode 100644
index 0000000000..9a85ab02d3
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/range_esc_method.go
@@ -0,0 +1,53 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+type I int
+
+func (x *I) method() int {
+	return int(*x)
+}
+
+var ints = []I{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+func main() {
+	sum := 0
+	var is []func() int
+	for _, i := range ints {
+		for j := 0; j < 10; j++ {
+			if int(i) == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		if i&1 == 0 {
+			is = append(is, i.method)
+		}
+	}
+
+	bug := false
+	if sum != 100-10 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 90, sum)
+		bug = true
+	}
+	sum = 0
+	for _, m := range is {
+		sum += m()
+	}
+	if sum != 2+4+6+8 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 20, sum)
+		bug = true
+	}
+	if !bug {
+		fmt.Printf("PASS\n")
+	} else {
+		os.Exit(11)
+	}
+}
diff --git a/src/cmd/compile/internal/loopvar/testdata/range_esc_minimal_closure.go b/src/cmd/compile/internal/loopvar/testdata/range_esc_minimal_closure.go
new file mode 100644
index 0000000000..8804d8b789
--- /dev/null
+++ b/src/cmd/compile/internal/loopvar/testdata/range_esc_minimal_closure.go
@@ -0,0 +1,50 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var is []func() int
+
+var ints = []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+func main() {
+	sum := 0
+	for _, i := range ints {
+		for j := 0; j < 10; j++ {
+			if i == j { // 10 skips
+				continue
+			}
+			sum++
+		}
+		if i&1 == 0 {
+			is = append(is, func() int {
+				return i
+			})
+		}
+	}
+
+	bug := false
+	if sum != 100-10 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 90, sum)
+		bug = true
+	}
+	sum = 0
+	for _, f := range is {
+		sum += f()
+	}
+	if sum != 2+4+6+8 {
+		fmt.Printf("wrong sum, expected %d, saw %d\n", 20, sum)
+		bug = true
+	}
+	if !bug {
+		fmt.Printf("PASS\n")
+	} else {
+		os.Exit(11)
+	}
+}
diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index e4ab80b2d0..64173312ac 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -2366,7 +2366,7 @@ func (r *reader) expr() (res ir.Node) {
 				if recv.Type().IsInterface() {
 					// N.B., this happens currently for typeparam/issue51521.go
 					// and typeparam/typeswitch3.go.
-					if base.Flag.LowerM > 0 {
+					if base.Flag.LowerM != 0 {
 						base.WarnfAt(method.Pos(), "imprecise interface call")
 					}
 				}
diff --git a/src/internal/goexperiment/exp_loopvar_off.go b/src/internal/goexperiment/exp_loopvar_off.go
new file mode 100644
index 0000000000..1cd7ee15d7
--- /dev/null
+++ b/src/internal/goexperiment/exp_loopvar_off.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.loopvar
+// +build !goexperiment.loopvar
+
+package goexperiment
+
+const LoopVar = false
+const LoopVarInt = 0
diff --git a/src/internal/goexperiment/exp_loopvar_on.go b/src/internal/goexperiment/exp_loopvar_on.go
new file mode 100644
index 0000000000..e3c8980c1e
--- /dev/null
+++ b/src/internal/goexperiment/exp_loopvar_on.go
@@ -0,0 +1,9 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.loopvar
+// +build goexperiment.loopvar
+
+package goexperiment
+
+const LoopVar = true
+const LoopVarInt = 1
diff --git a/src/internal/goexperiment/flags.go b/src/internal/goexperiment/flags.go
index 07481bcd50..8758505173 100644
--- a/src/internal/goexperiment/flags.go
+++ b/src/internal/goexperiment/flags.go
@@ -101,4 +101,8 @@ type Flags struct {
 	// When this experiment is enabled, cgo rule checks occur regardless
 	// of the GODEBUG=cgocheck setting provided at runtime.
 	CgoCheck2 bool
+
+	// LoopVar changes loop semantics so that each iteration gets its own
+	// copy of the iteration variable.
+	LoopVar bool
 }
diff --git a/src/runtime/race/testdata/mop_test.go b/src/runtime/race/testdata/mop_test.go
index 0d79091df4..0da539fc01 100644
--- a/src/runtime/race/testdata/mop_test.go
+++ b/src/runtime/race/testdata/mop_test.go
@@ -331,7 +331,8 @@ func TestRaceRange(t *testing.T) {
 	var x, y int
 	_ = x + y
 	done := make(chan bool, N)
-	for i, v := range a {
+	var i, v int // declare here (not in for stmt) so that i and v are shared w/ or w/o loop variable sharing change
+	for i, v = range a {
 		go func(i int) {
 			// we don't want a write-vs-write race
 			// so there is no array b here
diff --git a/test/closure2.go b/test/closure2.go
index 812d41f8ce..081d2e2d3d 100644
--- a/test/closure2.go
+++ b/test/closure2.go
@@ -73,7 +73,8 @@ func main() {
 
 	{
 		var g func() int
-		for i := range [2]int{} {
+		var i int
+		for i = range [2]int{} {
 			if i == 0 {
 				g = func() int {
 					return i // test that we capture by ref here, i is mutated on every interaction
diff --git a/test/escape.go b/test/escape.go
index 252a1e59cc..e6103f72c8 100644
--- a/test/escape.go
+++ b/test/escape.go
@@ -126,7 +126,8 @@ func range_escapes2(x, y int) (*int, *int) {
 	var p [2]*int
 	a[0] = x
 	a[1] = y
-	for k, v := range a {
+	var k, v int
+	for k, v = range a {
 		p[k] = &v
 	}
 	return p[0], p[1]
@@ -136,7 +137,8 @@ func range_escapes2(x, y int) (*int, *int) {
 func for_escapes2(x int, y int) (*int, *int) {
 	var p [2]*int
 	n := 0
-	for i := x; n < 2; i = y {
+	i := x
+	for ; n < 2; i = y {
 		p[n] = &i
 		n++
 	}
diff --git a/test/fixedbugs/issue13799.go b/test/fixedbugs/issue13799.go
index c8ecfc54e4..7ab4040434 100644
--- a/test/fixedbugs/issue13799.go
+++ b/test/fixedbugs/issue13799.go
@@ -45,8 +45,9 @@ func test1(iter int) {
 	// Heap -> stack pointer eventually causes badness when stack reallocation
 	// occurs.
 
-	var fn func()               // ERROR "moved to heap: fn$"
-	for i := 0; i < maxI; i++ { // ERROR "moved to heap: i$"
+	var fn func() // ERROR "moved to heap: fn$"
+	i := 0        // ERROR "moved to heap: i$"
+	for ; i < maxI; i++ {
 		// var fn func() // this makes it work, because fn stays off heap
 		j := 0        // ERROR "moved to heap: j$"
 		fn = func() { // ERROR "func literal escapes to heap$"