runtime: mapiter linkname compatibility layer

This CL reintroduces the various mapiter* linkname functions with a
compatibility layer that is careful to maintain compatibility with users
of the linkname.

The wrappers are straightforward. Callers of these APIs get an extra
layer of indirection, with their hiter containing a pointer to the real
maps.Iter. These users will take a minor performance hit from the extra
allocation, but this approach should have good long-term
maintainability.

Fixes #71408.

Change-Id: I6a6a636c7574bbd670ff5243dfeb63dfba6dc611
Reviewed-on: https://go-review.googlesource.com/c/go/+/643899
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

diff --git a/src/runtime/linkname_swiss.go b/src/runtime/linkname_swiss.go
new file mode 100644 (file)
index 0000000..1be7244
--- /dev/null
+++ b/src/runtime/linkname_swiss.go
@@ -0,0 +1,211 @@
+// 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.
+
+//go:build goexperiment.swissmap
+
+package runtime
+
+import (
+       "internal/abi"
+       "internal/runtime/maps"
+       "internal/runtime/sys"
+       "unsafe"
+)
+
+// Legacy //go:linkname compatibility shims
+//
+// The functions below are unused by the toolchain, and exist only for
+// compatibility with existing //go:linkname use in the ecosystem (and in
+// map_noswiss.go for normal use via GOEXPERIMENT=noswissmap).
+
+// linknameIter is the it argument to mapiterinit and mapiternext.
+//
+// Callers of mapiterinit allocate their own iter structure, which has the
+// layout of the pre-Go 1.24 hiter structure, shown here for posterity:
+//
+//     type hiter struct {
+//             key         unsafe.Pointer
+//             elem        unsafe.Pointer
+//             t           *maptype
+//             h           *hmap
+//             buckets     unsafe.Pointer
+//             bptr        *bmap
+//             overflow    *[]*bmap
+//             oldoverflow *[]*bmap
+//             startBucket uintptr
+//             offset      uint8
+//             wrapped     bool
+//             B           uint8
+//             i           uint8
+//             bucket      uintptr
+//             checkBucket uintptr
+//     }
+//
+// Our structure must maintain compatibility with the old structure. This
+// means:
+//
+//   - Our structure must be the same size or smaller than hiter. Otherwise we
+//     may write outside the caller's hiter allocation.
+//   - Our structure must have the same pointer layout as hiter, so that the GC
+//     tracks pointers properly.
+//
+// Based on analysis of the "hall of shame" users of these linknames:
+//
+//   - The key and elem fields must be kept up to date with the current key/elem.
+//     Some users directly access the key and elem fields rather than calling
+//     reflect.mapiterkey/reflect.mapiterelem.
+//   - The t field must be non-nil after mapiterinit. gonum.org/v1/gonum uses
+//     this to verify the iterator is initialized.
+//   - github.com/segmentio/encoding and github.com/RomiChan/protobuf check if h
+//     is non-nil, but the code has no effect. Thus the value of h does not
+//     matter. See internal/runtime_reflect/map.go.
+type linknameIter struct {
+       // Fields from hiter.
+       key  unsafe.Pointer
+       elem unsafe.Pointer
+       typ  *abi.SwissMapType
+
+       // The real iterator.
+       it *maps.Iter
+}
+
+// mapiterinit is a compatibility wrapper for map iterator for users of
+// //go:linkname from before Go 1.24. It is not used by Go itself. New users
+// should use reflect or the maps package.
+//
+// mapiterinit should be an internal detail,
+// but widely used packages access it using linkname.
+// Notable members of the hall of shame include:
+//   - github.com/bytedance/sonic
+//   - github.com/goccy/go-json
+//   - github.com/RomiChan/protobuf
+//   - github.com/segmentio/encoding
+//   - github.com/ugorji/go/codec
+//   - github.com/wI2L/jettison
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+//go:linkname mapiterinit
+func mapiterinit(t *abi.SwissMapType, m *maps.Map, it *linknameIter) {
+       if raceenabled && m != nil {
+               callerpc := sys.GetCallerPC()
+               racereadpc(unsafe.Pointer(m), callerpc, abi.FuncPCABIInternal(mapiterinit))
+       }
+
+       it.typ = t
+
+       it.it = new(maps.Iter)
+       it.it.Init(t, m)
+       it.it.Next()
+
+       it.key = it.it.Key()
+       it.elem = it.it.Elem()
+}
+
+// reflect_mapiterinit is a compatibility wrapper for map iterator for users of
+// //go:linkname from before Go 1.24. It is not used by Go itself. New users
+// should use reflect or the maps package.
+//
+// reflect_mapiterinit should be an internal detail,
+// but widely used packages access it using linkname.
+// Notable members of the hall of shame include:
+//   - github.com/modern-go/reflect2
+//   - gitee.com/quant1x/gox
+//   - github.com/v2pro/plz
+//   - github.com/wI2L/jettison
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+//go:linkname reflect_mapiterinit reflect.mapiterinit
+func reflect_mapiterinit(t *abi.SwissMapType, m *maps.Map, it *linknameIter) {
+       mapiterinit(t, m, it)
+}
+
+// mapiternext is a compatibility wrapper for map iterator for users of
+// //go:linkname from before Go 1.24. It is not used by Go itself. New users
+// should use reflect or the maps package.
+//
+// mapiternext should be an internal detail,
+// but widely used packages access it using linkname.
+// Notable members of the hall of shame include:
+//   - github.com/bytedance/sonic
+//   - github.com/RomiChan/protobuf
+//   - github.com/segmentio/encoding
+//   - github.com/ugorji/go/codec
+//   - gonum.org/v1/gonum
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+//go:linkname mapiternext
+func mapiternext(it *linknameIter) {
+       if raceenabled {
+               callerpc := sys.GetCallerPC()
+               racereadpc(unsafe.Pointer(it.it.Map()), callerpc, abi.FuncPCABIInternal(mapiternext))
+       }
+
+       it.it.Next()
+
+       it.key = it.it.Key()
+       it.elem = it.it.Elem()
+}
+
+// reflect_mapiternext is a compatibility wrapper for map iterator for users of
+// //go:linkname from before Go 1.24. It is not used by Go itself. New users
+// should use reflect or the maps package.
+//
+// reflect_mapiternext is for package reflect,
+// but widely used packages access it using linkname.
+// Notable members of the hall of shame include:
+//   - gitee.com/quant1x/gox
+//   - github.com/modern-go/reflect2
+//   - github.com/goccy/go-json
+//   - github.com/v2pro/plz
+//   - github.com/wI2L/jettison
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+//go:linkname reflect_mapiternext reflect.mapiternext
+func reflect_mapiternext(it *linknameIter) {
+       mapiternext(it)
+}
+
+// reflect_mapiterkey is a compatibility wrapper for map iterator for users of
+// //go:linkname from before Go 1.24. It is not used by Go itself. New users
+// should use reflect or the maps package.
+//
+// reflect_mapiterkey should be an internal detail,
+// but widely used packages access it using linkname.
+// Notable members of the hall of shame include:
+//   - github.com/goccy/go-json
+//   - gonum.org/v1/gonum
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+//go:linkname reflect_mapiterkey reflect.mapiterkey
+func reflect_mapiterkey(it *linknameIter) unsafe.Pointer {
+       return it.it.Key()
+}
+
+// reflect_mapiterelem is a compatibility wrapper for map iterator for users of
+// //go:linkname from before Go 1.24. It is not used by Go itself. New users
+// should use reflect or the maps package.
+//
+// reflect_mapiterelem should be an internal detail,
+// but widely used packages access it using linkname.
+// Notable members of the hall of shame include:
+//   - github.com/goccy/go-json
+//   - gonum.org/v1/gonum
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+//go:linkname reflect_mapiterelem reflect.mapiterelem
+func reflect_mapiterelem(it *linknameIter) unsafe.Pointer {
+       return it.it.Elem()
+}

diff --git a/src/runtime/map_swiss.go b/src/runtime/map_swiss.go
index f4b4062dd9d44ef35c85a46581a16ec5cdabf6a6..a8fe87257acd280e864631714acf796189fb65b0 100644 (file)
--- a/src/runtime/map_swiss.go
+++ b/src/runtime/map_swiss.go
@@ -158,31 +158,6 @@ func mapdelete(t *abi.SwissMapType, m *maps.Map, key unsafe.Pointer) {
        m.Delete(t, key)
 }
 
-// mapiterinit initializes the Iter struct used for ranging over maps.
-// The Iter struct pointed to by 'it' is allocated on the stack
-// by the compilers order pass or on the heap by reflect_mapiterinit.
-// Both need to have zeroed hiter since the struct contains pointers.
-//
-// mapiterinit should be an internal detail,
-// but widely used packages access it using linkname.
-// Notable members of the hall of shame include:
-//   - github.com/bytedance/sonic
-//   - github.com/goccy/go-json
-//   - github.com/RomiChan/protobuf
-//   - github.com/segmentio/encoding
-//   - github.com/ugorji/go/codec
-//   - github.com/wI2L/jettison
-//
-// Do not remove or change the type signature.
-// See go.dev/issue/67401.
-//
-// TODO go:linkname mapiterinit
-func mapiterinit(t *abi.SwissMapType, m *maps.Map, it *maps.Iter) {
-       // N.B. This is required by the builtin list in internal/goobj because
-       // it is a builtin for old maps.
-       throw("unreachable")
-}
-
 // mapIterStart initializes the Iter struct used for ranging over maps and
 // performs the first step of iteration. The Iter struct pointed to by 'it' is
 // allocated on the stack by the compilers order pass or on the heap by
@@ -197,25 +172,6 @@ func mapIterStart(t *abi.SwissMapType, m *maps.Map, it *maps.Iter) {
        it.Next()
 }
 
-// mapiternext should be an internal detail,
-// but widely used packages access it using linkname.
-// Notable members of the hall of shame include:
-//   - github.com/bytedance/sonic
-//   - github.com/RomiChan/protobuf
-//   - github.com/segmentio/encoding
-//   - github.com/ugorji/go/codec
-//   - gonum.org/v1/gonum
-//
-// Do not remove or change the type signature.
-// See go.dev/issue/67401.
-//
-// TODO go:linkname mapiternext
-func mapiternext(it *maps.Iter) {
-       // N.B. This is required by the builtin list in internal/goobj because
-       // it is a builtin for old maps.
-       throw("unreachable")
-}
-
 // mapIterNext performs the next step of iteration. Afterwards, the next
 // key/elem are in it.Key()/it.Elem().
 func mapIterNext(it *maps.Iter) {
@@ -324,67 +280,6 @@ func reflect_mapdelete_faststr(t *abi.SwissMapType, m *maps.Map, key string) {
        mapdelete_faststr(t, m, key)
 }
 
-// reflect_mapiterinit is for package reflect,
-// but widely used packages access it using linkname.
-// Notable members of the hall of shame include:
-//   - github.com/modern-go/reflect2
-//   - gitee.com/quant1x/gox
-//   - github.com/v2pro/plz
-//   - github.com/wI2L/jettison
-//
-// Do not remove or change the type signature.
-// See go.dev/issue/67401.
-//
-// TODO go:linkname reflect_mapiterinit reflect.mapiterinit
-//func reflect_mapiterinit(t *abi.SwissMapType, m *maps.Map, it *maps.Iter) {
-//     mapiterinit(t, m, it)
-//}
-
-// reflect_mapiternext is for package reflect,
-// but widely used packages access it using linkname.
-// Notable members of the hall of shame include:
-//   - gitee.com/quant1x/gox
-//   - github.com/modern-go/reflect2
-//   - github.com/goccy/go-json
-//   - github.com/v2pro/plz
-//   - github.com/wI2L/jettison
-//
-// Do not remove or change the type signature.
-// See go.dev/issue/67401.
-//
-// TODO go:linkname reflect_mapiternext reflect.mapiternext
-//func reflect_mapiternext(it *maps.Iter) {
-//     mapiternext(it)
-//}
-
-// reflect_mapiterkey was for package reflect,
-// but widely used packages access it using linkname.
-// Notable members of the hall of shame include:
-//   - github.com/goccy/go-json
-//   - gonum.org/v1/gonum
-//
-// Do not remove or change the type signature.
-// See go.dev/issue/67401.
-//
-// TODO go:linkname reflect_mapiterkey reflect.mapiterkey
-//func reflect_mapiterkey(it *maps.Iter) unsafe.Pointer {
-//     return it.Key()
-//}
-
-// reflect_mapiterelem was for package reflect,
-// but widely used packages access it using linkname.
-// Notable members of the hall of shame include:
-//   - github.com/goccy/go-json
-//   - gonum.org/v1/gonum
-//
-// Do not remove or change the type signature.
-// See go.dev/issue/67401.
-//
-// TODO go:linkname reflect_mapiterelem reflect.mapiterelem
-//func reflect_mapiterelem(it *maps.Iter) unsafe.Pointer {
-//     return it.Elem()
-//}
-
 // reflect_maplen is for package reflect,
 // but widely used packages access it using linkname.
 // Notable members of the hall of shame include: