mono.go

     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.
     4  
     5  package types2
     6  
     7  import (
     8  	"cmd/compile/internal/syntax"
     9  )
    10  
    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
    14  // monomorphization).
    15  //
    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.
    20  //
    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:
    24  //
    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.
    30  //
    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.
    34  //
    35  // For example, given:
    36  //
    37  //	func f[A, B any]() {
    38  //		type T int
    39  //		f[T, map[A]B]()
    40  //	}
    41  //
    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.
    46  //
    47  // Finally, we look for any positive-weight cycles. Zero-weight cycles
    48  // are allowed because static instantiation will reach a fixed point.
    49  
    50  type monoGraph struct {
    51  	vertices []monoVertex
    52  	edges    []monoEdge
    53  
    54  	// canon maps method receiver type parameters to their respective
    55  	// receiver type's type parameters.
    56  	canon map[*TypeParam]*TypeParam
    57  
    58  	// nameIdx maps a defined type or (canonical) type parameter to its
    59  	// vertex index.
    60  	nameIdx map[*TypeName]int
    61  }
    62  
    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
    67  
    68  	// obj is the defined type or type parameter represented by this
    69  	// vertex.
    70  	obj *TypeName
    71  }
    72  
    73  type monoEdge struct {
    74  	dst, src int
    75  	weight   int
    76  
    77  	pos syntax.Pos
    78  	typ Type
    79  }
    80  
    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.
    87  
    88  	again := true
    89  	for again {
    90  		again = false
    91  
    92  		for i, edge := range check.mono.edges {
    93  			src := &check.mono.vertices[edge.src]
    94  			dst := &check.mono.vertices[edge.dst]
    95  
    96  			// N.B., we're looking for the greatest weight paths, unlike
    97  			// typical Bellman-Ford.
    98  			w := src.weight + edge.weight
    99  			if w <= dst.weight {
   100  				continue
   101  			}
   102  
   103  			dst.pre = i
   104  			dst.len = src.len + 1
   105  			if dst.len == len(check.mono.vertices) {
   106  				check.reportInstanceLoop(edge.dst)
   107  				return
   108  			}
   109  
   110  			dst.weight = w
   111  			again = true
   112  		}
   113  	}
   114  }
   115  
   116  func (check *Checker) reportInstanceLoop(v int) {
   117  	var stack []int
   118  	seen := make([]bool, len(check.mono.vertices))
   119  
   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.
   124  	for !seen[v] {
   125  		stack = append(stack, v)
   126  		seen[v] = true
   127  		v = check.mono.edges[check.mono.vertices[v].pre].src
   128  	}
   129  
   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.
   133  	for stack[0] != v {
   134  		stack = stack[1:]
   135  	}
   136  
   137  	// TODO(mdempsky): Pivot stack so we report the cycle from the top?
   138  
   139  	var err error_
   140  	obj0 := check.mono.vertices[v].obj
   141  	err.errorf(obj0, "instantiation cycle:")
   142  
   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
   147  
   148  		switch obj.Type().(type) {
   149  		default:
   150  			panic("unexpected type")
   151  		case *Named:
   152  			err.errorf(edge.pos, "%s implicitly parameterized by %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
   153  		case *TypeParam:
   154  			err.errorf(edge.pos, "%s instantiated as %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
   155  		}
   156  	}
   157  	check.report(&err)
   158  }
   159  
   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) {
   163  	if w.canon == nil {
   164  		w.canon = make(map[*TypeParam]*TypeParam)
   165  	}
   166  	w.canon[mpar] = tpar
   167  }
   168  
   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 {
   173  		pos := pos
   174  		if i < len(xlist) {
   175  			pos = syntax.StartPos(xlist[i])
   176  		}
   177  		w.assign(pkg, pos, tpar, targs[i])
   178  	}
   179  }
   180  
   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
   187  	// type parameters.
   188  	//
   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 {
   192  		return
   193  	}
   194  
   195  	// flow adds an edge from vertex src representing that typ flows to tpar.
   196  	flow := func(src int, typ Type) {
   197  		weight := 1
   198  		if typ == targ {
   199  			weight = 0
   200  		}
   201  
   202  		w.addEdge(w.typeParamVertex(tpar), src, weight, pos, targ)
   203  	}
   204  
   205  	// Recursively walk the type argument to find any defined types or
   206  	// type parameters.
   207  	var do func(typ Type)
   208  	do = func(typ Type) {
   209  		switch typ := typ.(type) {
   210  		default:
   211  			panic("unexpected type")
   212  
   213  		case *TypeParam:
   214  			assert(typ.Obj().Pkg() == pkg)
   215  			flow(w.typeParamVertex(typ), typ)
   216  
   217  		case *Named:
   218  			if src := w.localNamedVertex(pkg, typ.Origin()); src >= 0 {
   219  				flow(src, typ)
   220  			}
   221  
   222  			targs := typ.TypeArgs()
   223  			for i := 0; i < targs.Len(); i++ {
   224  				do(targs.At(i))
   225  			}
   226  
   227  		case *Array:
   228  			do(typ.Elem())
   229  		case *Basic:
   230  			// ok
   231  		case *Chan:
   232  			do(typ.Elem())
   233  		case *Map:
   234  			do(typ.Key())
   235  			do(typ.Elem())
   236  		case *Pointer:
   237  			do(typ.Elem())
   238  		case *Slice:
   239  			do(typ.Elem())
   240  
   241  		case *Interface:
   242  			for i := 0; i < typ.NumMethods(); i++ {
   243  				do(typ.Method(i).Type())
   244  			}
   245  		case *Signature:
   246  			tuple := func(tup *Tuple) {
   247  				for i := 0; i < tup.Len(); i++ {
   248  					do(tup.At(i).Type())
   249  				}
   250  			}
   251  			tuple(typ.Params())
   252  			tuple(typ.Results())
   253  		case *Struct:
   254  			for i := 0; i < typ.NumFields(); i++ {
   255  				do(typ.Field(i).Type())
   256  			}
   257  		}
   258  	}
   259  	do(targ)
   260  }
   261  
   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 {
   265  	obj := named.Obj()
   266  	if obj.Pkg() != pkg {
   267  		return -1 // imported type
   268  	}
   269  
   270  	root := pkg.Scope()
   271  	if obj.Parent() == root {
   272  		return -1 // package scope, no ambient type parameters
   273  	}
   274  
   275  	if idx, ok := w.nameIdx[obj]; ok {
   276  		return idx
   277  	}
   278  
   279  	idx := -1
   280  
   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 {
   287  					if idx < 0 {
   288  						idx = len(w.vertices)
   289  						w.vertices = append(w.vertices, monoVertex{obj: obj})
   290  					}
   291  
   292  					w.addEdge(idx, w.typeParamVertex(tpar), 1, obj.Pos(), tpar)
   293  				}
   294  			}
   295  		}
   296  	}
   297  
   298  	if w.nameIdx == nil {
   299  		w.nameIdx = make(map[*TypeName]int)
   300  	}
   301  	w.nameIdx[obj] = idx
   302  	return idx
   303  }
   304  
   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 {
   308  		tpar = x
   309  	}
   310  
   311  	obj := tpar.Obj()
   312  
   313  	if idx, ok := w.nameIdx[obj]; ok {
   314  		return idx
   315  	}
   316  
   317  	if w.nameIdx == nil {
   318  		w.nameIdx = make(map[*TypeName]int)
   319  	}
   320  
   321  	idx := len(w.vertices)
   322  	w.vertices = append(w.vertices, monoVertex{obj: obj})
   323  	w.nameIdx[obj] = idx
   324  	return idx
   325  }
   326  
   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{
   330  		dst:    dst,
   331  		src:    src,
   332  		weight: weight,
   333  
   334  		pos: pos,
   335  		typ: typ,
   336  	})
   337  }
   338
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