cmd/compile, unique: model data flow of non-string pointers

Currently, hash/maphash.Comparable escapes its parameter if it
contains non-string pointers, but does not escape strings or types
that contain strings but no other pointers. This is achieved by a
compiler intrinsic.

unique.Make does something similar: it stores its parameter to a
central map, with strings cloned. So from the escape analysis's
perspective, the non-string pointers are passed through, whereas
string pointers are not. We currently cannot model this type of
type-dependent data flow directly in Go. So we do this with a
compiler intrinsic. In fact, we can unify this and the intrinsic
above.

Tests are from Jake Bailey's CL 671955 (thanks!).

Fixes #73680.

Change-Id: Ia6a78e09dee39f8d9198a16758e4b5322ee2c56a
Reviewed-on: https://go-review.googlesource.com/c/go/+/675156
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Jake Bailey <jacob.b.bailey@gmail.com>

Author: cherrymui

src/cmd/compile/internal/escape/call.go

@@ -84,15 +84,19 @@ func (e *escape) call(ks []hole, call ir.Node) {
 			argument(e.tagHole(ks, fn, param), arg)
 		}
 
-		// hash/maphash.escapeForHash forces its argument to be on
-		// the heap, if it contains a non-string pointer. We cannot
+		// internal/abi.EscapeNonString forces its argument to be on
+		// the heap, if it contains a non-string pointer.
+		// This is used in hash/maphash.Comparable, where we cannot
 		// hash pointers to local variables, as the address of the
 		// local variable might change on stack growth.
 		// Strings are okay as the hash depends on only the content,
 		// not the pointer.
+		// This is also used in unique.clone, to model the data flow
+		// edge on the value with strings excluded, because strings
+		// are cloned (by content).
 		// The actual call we match is
-		//   hash/maphash.escapeForHash[go.shape.T](dict, go.shape.T)
-		if fn != nil && fn.Sym().Pkg.Path == "hash/maphash" && strings.HasPrefix(fn.Sym().Name, "escapeForHash[") {
+		//   internal/abi.EscapeNonString[go.shape.T](dict, go.shape.T)
+		if fn != nil && fn.Sym().Pkg.Path == "internal/abi" && strings.HasPrefix(fn.Sym().Name, "EscapeNonString[") {
 			ps := fntype.Params()
 			if len(ps) == 2 && ps[1].Type.IsShape() {
 				if !hasNonStringPointers(ps[1].Type) {

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

@@ -454,6 +454,11 @@ opSwitch:
 						// generate code.
 						cheap = true
 					}
+					if strings.HasPrefix(fn, "EscapeNonString[") {
+						// internal/abi.EscapeNonString[T] is a compiler intrinsic
+						// implemented in the escape analysis phase.
+						cheap = true
+					}
 				case "internal/runtime/sys":
 					switch fn {
 					case "GetCallerPC", "GetCallerSP":
@@ -472,12 +477,6 @@ opSwitch:
 					case "panicrangestate":
 						cheap = true
 					}
-				case "hash/maphash":
-					if strings.HasPrefix(fn, "escapeForHash[") {
-						// hash/maphash.escapeForHash[T] is a compiler intrinsic
-						// implemented in the escape analysis phase.
-						cheap = true
-					}
 				}
 			}
 			// Special case for coverage counter updates; although
@@ -801,10 +800,10 @@ func inlineCallCheck(callerfn *ir.Func, call *ir.CallExpr) (bool, bool) {
 		}
 	}
 
-	// hash/maphash.escapeForHash[T] is a compiler intrinsic implemented
+	// internal/abi.EscapeNonString[T] is a compiler intrinsic implemented
 	// in the escape analysis phase.
-	if fn := ir.StaticCalleeName(call.Fun); fn != nil && fn.Sym().Pkg.Path == "hash/maphash" &&
-		strings.HasPrefix(fn.Sym().Name, "escapeForHash[") {
+	if fn := ir.StaticCalleeName(call.Fun); fn != nil && fn.Sym().Pkg.Path == "internal/abi" &&
+		strings.HasPrefix(fn.Sym().Name, "EscapeNonString[") {
 		return false, true
 	}
 

src/cmd/compile/internal/walk/expr.go

@@ -594,8 +594,8 @@ func walkCall(n *ir.CallExpr, init *ir.Nodes) ir.Node {
 
 	if n.Op() == ir.OCALLFUNC {
 		fn := ir.StaticCalleeName(n.Fun)
-		if fn != nil && fn.Sym().Pkg.Path == "hash/maphash" && strings.HasPrefix(fn.Sym().Name, "escapeForHash[") {
-			// hash/maphash.escapeForHash[T] is a compiler intrinsic
+		if fn != nil && fn.Sym().Pkg.Path == "internal/abi" && strings.HasPrefix(fn.Sym().Name, "EscapeNonString[") {
+			// internal/abi.EscapeNonString[T] is a compiler intrinsic
 			// for the escape analysis to escape its argument based on
 			// the type. The call itself is no-op. Just walk the
 			// argument.

src/hash/maphash/maphash.go

@@ -14,6 +14,7 @@ package maphash
 
 import (
 	"hash"
+	"internal/abi"
 	"internal/byteorder"
 	"math"
 )
@@ -293,26 +294,13 @@ func (h *Hash) Clone() (hash.Cloner, error) {
 // such that Comparable(s, v1) == Comparable(s, v2) if v1 == v2.
 // If v != v, then the resulting hash is randomly distributed.
 func Comparable[T comparable](seed Seed, v T) uint64 {
-	escapeForHash(v)
+	abi.EscapeNonString(v)
 	return comparableHash(v, seed)
 }
 
-// escapeForHash forces v to be on the heap, if v contains a
-// non-string pointer. We cannot hash pointers to local variables,
-// as the address of the local variable might change on stack growth.
-// Strings are okay as the hash depends on only the content, not
-// the pointer.
-//
-// This is essentially
-//
-//	if hasNonStringPointers(T) { abi.Escape(v) }
-//
-// Implemented as a compiler intrinsic.
-func escapeForHash[T comparable](v T) { panic("intrinsic") }
-
 // WriteComparable adds x to the data hashed by h.
 func WriteComparable[T comparable](h *Hash, x T) {
-	escapeForHash(x)
+	abi.EscapeNonString(x)
 	// writeComparable (not in purego mode) directly operates on h.state
 	// without using h.buf. Mix in the buffer length so it won't
 	// commute with a buffered write, which either changes h.n or changes

src/internal/abi/escape.go

@@ -31,3 +31,35 @@ func Escape[T any](x T) T {
 	}
 	return x
 }
+
+// EscapeNonString forces v to be on the heap, if v contains a
+// non-string pointer.
+//
+// This is used in hash/maphash.Comparable. We cannot hash pointers
+// to local variables on stack, as their addresses might change on
+// stack growth. Strings are okay as the hash depends on only the
+// content, not the pointer.
+//
+// This is essentially
+//
+//	if hasNonStringPointers(T) { Escape(v) }
+//
+// Implemented as a compiler intrinsic.
+func EscapeNonString[T any](v T) { panic("intrinsic") }
+
+// EscapeToResultNonString models a data flow edge from v to the result,
+// if v contains a non-string pointer. If v contains only string pointers,
+// it returns a copy of v, but is not modeled as a data flow edge
+// from the escape analysis's perspective.
+//
+// This is used in unique.clone, to model the data flow edge on the
+// value with strings excluded, because strings are cloned (by
+// content).
+//
+// TODO: probably we should define this as a intrinsic and EscapeNonString
+// could just be "heap = EscapeToResultNonString(v)". This way we can model
+// an edge to the result but not necessarily heap.
+func EscapeToResultNonString[T any](v T) T {
+	EscapeNonString(v)
+	return *(*T)(NoEscape(unsafe.Pointer(&v)))
+}

src/unique/clone.go

@@ -23,7 +23,7 @@ func clone[T comparable](value T, seq *cloneSeq) T {
 		ps := (*string)(unsafe.Pointer(uintptr(unsafe.Pointer(&value)) + offset))
 		*ps = stringslite.Clone(*ps)
 	}
-	return value
+	return abi.EscapeToResultNonString(value)
 }
 
 // singleStringClone describes how to clone a single string.

src/unique/handle_test.go

@@ -227,7 +227,7 @@ func TestMakeAllocs(t *testing.T) {
 			stringHandle = Make(heapString)
 		}},
 
-		{name: "stack string", allocs: 1, f: func() {
+		{name: "stack string", allocs: 0, f: func() {
 			var b [16]byte
 			b[8] = 'a'
 			stringHandle = Make(string(b[:]))
@@ -237,7 +237,7 @@ func TestMakeAllocs(t *testing.T) {
 			stringHandle = Make(string(heapBytes))
 		}},
 
-		{name: "bytes truncated short", allocs: 1, f: func() {
+		{name: "bytes truncated short", allocs: 0, f: func() {
 			stringHandle = Make(string(heapBytes[:16]))
 		}},
 
@@ -261,7 +261,7 @@ func TestMakeAllocs(t *testing.T) {
 			pairHandle = Make([2]string{heapString, heapString})
 		}},
 
-		{name: "pair from stack", allocs: 2, f: func() {
+		{name: "pair from stack", allocs: 0, f: func() {
 			var b [16]byte
 			b[8] = 'a'
 			pairHandle = Make([2]string{string(b[:]), string(b[:])})

test/escape_unique.go

@@ -0,0 +1,62 @@
+// errorcheck -0 -m -l
+
+// Copyright 2025 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.
+
+// Test escape analysis for unique.
+
+package escape
+
+import "unique"
+
+type T string
+
+func f1(s string) unique.Handle[string] { // ERROR "s does not escape$"
+	return unique.Make(s)
+}
+
+func f1a(s []byte) unique.Handle[string] { // ERROR "s does not escape$"
+	return unique.Make(string(s)) // ERROR "string\(s\) does not escape$"
+}
+
+func gen[S ~string](s S) unique.Handle[S] {
+	return unique.Make(s)
+}
+
+func f2(s T) unique.Handle[T] { // ERROR "s does not escape$"
+	return unique.Make(s)
+}
+
+func f3(s T) unique.Handle[T] { // ERROR "s does not escape$"
+	return gen(s)
+}
+
+type pair struct {
+	s1 string
+	s2 string
+}
+
+func f4(s1 string, s2 string) unique.Handle[pair] { // ERROR "s1 does not escape$" "s2 does not escape$"
+	return unique.Make(pair{s1, s2})
+}
+
+type viaInterface struct {
+	s any
+}
+
+func f5(s string) unique.Handle[viaInterface] { // ERROR "leaking param: s$"
+	return unique.Make(viaInterface{s}) // ERROR "s escapes to heap$"
+}
+
+var sink any
+
+func f6(s string) unique.Handle[string] { // ERROR "leaking param: s$"
+	sink = s // ERROR "s escapes to heap$"
+	return unique.Make(s)
+}
+
+func f6a(s []byte) unique.Handle[string] { // ERROR "leaking param: s$"
+	sink = s                      // ERROR "s escapes to heap$"
+	return unique.Make(string(s)) // ERROR "string\(s\) does not escape$"
+}