cmd/compile/internal/types2: use a global atomic counter for type parameter ids

This avoids a dependency on a *Checker when we create type parameters
outside the type checker proper, e.g. in an importer. There may be
better solutions but this does the trick for now.

Change-Id: Icf22c934970cb04c88c2729555ae6a79ef5a2245
Reviewed-on: https://go-review.googlesource.com/c/go/+/309830
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/api_test.go

@@ -349,6 +349,7 @@ func TestTypesInfo(t *testing.T) {
 	}
 
 	for _, test := range tests {
+		ResetId() // avoid renumbering of type parameter ids when adding tests
 		info := Info{Types: make(map[syntax.Expr]TypeAndValue)}
 		var name string
 		if strings.HasPrefix(test.src, brokenPkg) {

src/cmd/compile/internal/types2/check.go

@@ -83,7 +83,6 @@ type Checker struct {
 	pkg  *Package
 	*Info
 	version version                     // accepted language version
-	nextId  uint64                      // unique Id for type parameters (first valid Id is 1)
 	objMap  map[Object]*declInfo        // maps package-level objects and (non-interface) methods to declaration info
 	impMap  map[importKey]*Package      // maps (import path, source directory) to (complete or fake) package
 	posMap  map[*Interface][]syntax.Pos // maps interface types to lists of embedded interface positions
@@ -178,7 +177,6 @@ func NewChecker(conf *Config, pkg *Package, info *Info) *Checker {
 		pkg:     pkg,
 		Info:    info,
 		version: version,
-		nextId:  1,
 		objMap:  make(map[Object]*declInfo),
 		impMap:  make(map[importKey]*Package),
 		posMap:  make(map[*Interface][]syntax.Pos),

src/cmd/compile/internal/types2/type.go

@@ -7,6 +7,7 @@ package types2
 import (
 	"cmd/compile/internal/syntax"
 	"fmt"
+	"sync/atomic"
 )
 
 // A Type represents a type of Go.
@@ -718,6 +719,15 @@ func (t *Named) AddMethod(m *Func) {
 	}
 }
 
+// Note: This is a uint32 rather than a uint64 because the
+// respective 64 bit atomic instructions are not available
+// on all platforms.
+var lastId uint32
+
+// nextId returns a value increasing monotonically by 1 with
+// each call, starting with 1. It may be called concurrently.
+func nextId() uint64 { return uint64(atomic.AddUint32(&lastId, 1)) }
+
 // A TypeParam represents a type parameter type.
 type TypeParam struct {
 	check *Checker  // for lazy type bound completion
@@ -733,8 +743,7 @@ func (t *TypeParam) Obj() *TypeName { return t.obj }
 // NewTypeParam returns a new TypeParam.
 func (check *Checker) NewTypeParam(obj *TypeName, index int, bound Type) *TypeParam {
 	assert(bound != nil)
-	typ := &TypeParam{check: check, id: check.nextId, obj: obj, index: index, bound: bound}
-	check.nextId++
+	typ := &TypeParam{check: check, id: nextId(), obj: obj, index: index, bound: bound}
 	if obj.typ == nil {
 		obj.typ = typ
 	}

src/cmd/compile/internal/types2/types_test.go

@@ -4,6 +4,14 @@
 
 package types2
 
+import "sync/atomic"
+
 func init() {
 	acceptMethodTypeParams = true
 }
+
+// Upon calling ResetId, nextId starts with 1 again.
+// It may be called concurrently. This is only needed
+// for tests where we may want to have a consistent
+// numbering for each individual test case.
+func ResetId() { atomic.StoreUint32(&lastId, 0) }