1 // Copyright 2021 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
8 "cmd/compile/internal/syntax"
11 // This file implements a check to validate that a Go package doesn't
12 // have unbounded recursive instantiation, which is not compatible
13 // with compilers using static instantiation (such as
16 // It implements a sort of "type flow" analysis by detecting which
17 // type parameters are instantiated with other type parameters (or
18 // types derived thereof). A package cannot be statically instantiated
19 // if the graph has any cycles involving at least one derived type.
21 // Concretely, we construct a directed, weighted graph. Vertices are
22 // used to represent type parameters as well as some defined
23 // types. Edges are used to represent how types depend on each other:
25 // * Everywhere a type-parameterized function or type is instantiated,
26 // we add edges to each type parameter from the vertices (if any)
27 // representing each type parameter or defined type referenced by
28 // the type argument. If the type argument is just the referenced
29 // type itself, then the edge has weight 0, otherwise 1.
31 // * For every defined type declared within a type-parameterized
32 // function or method, we add an edge of weight 1 to the defined
33 // type from each ambient type parameter.
35 // For example, given:
37 // func f[A, B any]() {
42 // we construct vertices representing types A, B, and T. Because of
43 // declaration "type T int", we construct edges T<-A and T<-B with
44 // weight 1; and because of instantiation "f[T, map[A]B]" we construct
45 // edges A<-T with weight 0, and B<-A and B<-B with weight 1.
47 // Finally, we look for any positive-weight cycles. Zero-weight cycles
48 // are allowed because static instantiation will reach a fixed point.
50 type monoGraph struct {
54 // canon maps method receiver type parameters to their respective
55 // receiver type's type parameters.
56 canon map[*TypeParam]*TypeParam
58 // nameIdx maps a defined type or (canonical) type parameter to its
60 nameIdx map[*TypeName]int
63 type monoVertex struct {
64 weight int // weight of heaviest known path to this vertex
65 pre int // previous edge (if any) in the above path
66 len int // length of the above path
68 // obj is the defined type or type parameter represented by this
73 type monoEdge struct {
81 func (check *Checker) monomorph() {
82 // We detect unbounded instantiation cycles using a variant of
83 // Bellman-Ford's algorithm. Namely, instead of always running |V|
84 // iterations, we run until we either reach a fixed point or we've
85 // found a path of length |V|. This allows us to terminate earlier
86 // when there are no cycles, which should be the common case.
92 for i, edge := range check.mono.edges {
93 src := &check.mono.vertices[edge.src]
94 dst := &check.mono.vertices[edge.dst]
96 // N.B., we're looking for the greatest weight paths, unlike
97 // typical Bellman-Ford.
98 w := src.weight + edge.weight
104 dst.len = src.len + 1
105 if dst.len == len(check.mono.vertices) {
106 check.reportInstanceLoop(edge.dst)
116 func (check *Checker) reportInstanceLoop(v int) {
118 seen := make([]bool, len(check.mono.vertices))
120 // We have a path that contains a cycle and ends at v, but v may
121 // only be reachable from the cycle, not on the cycle itself. We
122 // start by walking backwards along the path until we find a vertex
123 // that appears twice.
125 stack = append(stack, v)
127 v = check.mono.edges[check.mono.vertices[v].pre].src
130 // Trim any vertices we visited before visiting v the first
131 // time. Since v is the first vertex we found within the cycle, any
132 // vertices we visited earlier cannot be part of the cycle.
137 // TODO(mdempsky): Pivot stack so we report the cycle from the top?
140 obj0 := check.mono.vertices[v].obj
141 err.errorf(obj0, "instantiation cycle:")
143 qf := RelativeTo(check.pkg)
144 for _, v := range stack {
145 edge := check.mono.edges[check.mono.vertices[v].pre]
146 obj := check.mono.vertices[edge.dst].obj
148 switch obj.Type().(type) {
150 panic("unexpected type")
152 err.errorf(edge.pos, "%s implicitly parameterized by %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
154 err.errorf(edge.pos, "%s instantiated as %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
160 // recordCanon records that tpar is the canonical type parameter
161 // corresponding to method type parameter mpar.
162 func (w *monoGraph) recordCanon(mpar, tpar *TypeParam) {
164 w.canon = make(map[*TypeParam]*TypeParam)
169 // recordInstance records that the given type parameters were
170 // instantiated with the corresponding type arguments.
171 func (w *monoGraph) recordInstance(pkg *Package, pos syntax.Pos, tparams []*TypeParam, targs []Type, xlist []syntax.Expr) {
172 for i, tpar := range tparams {
175 pos = syntax.StartPos(xlist[i])
177 w.assign(pkg, pos, tpar, targs[i])
181 // assign records that tpar was instantiated as targ at pos.
182 func (w *monoGraph) assign(pkg *Package, pos syntax.Pos, tpar *TypeParam, targ Type) {
183 // Go generics do not have an analog to C++`s template-templates,
184 // where a template parameter can itself be an instantiable
185 // template. So any instantiation cycles must occur within a single
186 // package. Accordingly, we can ignore instantiations of imported
189 // TODO(mdempsky): Push this check up into recordInstance? All type
190 // parameters in a list will appear in the same package.
191 if tpar.Obj().Pkg() != pkg {
195 // flow adds an edge from vertex src representing that typ flows to tpar.
196 flow := func(src int, typ Type) {
202 w.addEdge(w.typeParamVertex(tpar), src, weight, pos, targ)
205 // Recursively walk the type argument to find any defined types or
207 var do func(typ Type)
208 do = func(typ Type) {
209 switch typ := typ.(type) {
211 panic("unexpected type")
214 assert(typ.Obj().Pkg() == pkg)
215 flow(w.typeParamVertex(typ), typ)
218 if src := w.localNamedVertex(pkg, typ.Origin()); src >= 0 {
222 targs := typ.TypeArgs()
223 for i := 0; i < targs.Len(); i++ {
242 for i := 0; i < typ.NumMethods(); i++ {
243 do(typ.Method(i).Type())
246 tuple := func(tup *Tuple) {
247 for i := 0; i < tup.Len(); i++ {
254 for i := 0; i < typ.NumFields(); i++ {
255 do(typ.Field(i).Type())
262 // localNamedVertex returns the index of the vertex representing
263 // named, or -1 if named doesn't need representation.
264 func (w *monoGraph) localNamedVertex(pkg *Package, named *Named) int {
266 if obj.Pkg() != pkg {
267 return -1 // imported type
271 if obj.Parent() == root {
272 return -1 // package scope, no ambient type parameters
275 if idx, ok := w.nameIdx[obj]; ok {
281 // Walk the type definition's scope to find any ambient type
282 // parameters that it's implicitly parameterized by.
283 for scope := obj.Parent(); scope != root; scope = scope.Parent() {
284 for _, elem := range scope.elems {
285 if elem, ok := elem.(*TypeName); ok && !elem.IsAlias() && elem.Pos().Cmp(obj.Pos()) < 0 {
286 if tpar, ok := elem.Type().(*TypeParam); ok {
288 idx = len(w.vertices)
289 w.vertices = append(w.vertices, monoVertex{obj: obj})
292 w.addEdge(idx, w.typeParamVertex(tpar), 1, obj.Pos(), tpar)
298 if w.nameIdx == nil {
299 w.nameIdx = make(map[*TypeName]int)
305 // typeParamVertex returns the index of the vertex representing tpar.
306 func (w *monoGraph) typeParamVertex(tpar *TypeParam) int {
307 if x, ok := w.canon[tpar]; ok {
313 if idx, ok := w.nameIdx[obj]; ok {
317 if w.nameIdx == nil {
318 w.nameIdx = make(map[*TypeName]int)
321 idx := len(w.vertices)
322 w.vertices = append(w.vertices, monoVertex{obj: obj})
327 func (w *monoGraph) addEdge(dst, src, weight int, pos syntax.Pos, typ Type) {
328 // TODO(mdempsky): Deduplicate redundant edges?
329 w.edges = append(w.edges, monoEdge{