go/types: implement SizesFor convenience function

SizesFor returns a Sizes implementation for a supported architecture.
Use functionality in srcimporter.

Change-Id: I197e641b419c678030dfaab5c5b8c569fd0410f3
Reviewed-on: https://go-review.googlesource.com/37583
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/internal/srcimporter/srcimporter.go

@@ -34,7 +34,7 @@ func New(ctxt *build.Context, fset *token.FileSet, packages map[string]*types.Pa
 	return &Importer{
 		ctxt:     ctxt,
 		fset:     fset,
-		sizes:    archSizes[ctxt.GOARCH], // use go/types default if GOARCH not found (map access returns nil)
+		sizes:    types.SizesFor(ctxt.GOARCH), // uses go/types default if GOARCH not found
 		packages: packages,
 	}
 }
@@ -180,20 +180,3 @@ func (p *Importer) joinPath(elem ...string) string {
 	}
 	return filepath.Join(elem...)
 }
-
-// common architecture word sizes and alignments
-// TODO(gri) consider making this available via go/types
-var archSizes = map[string]*types.StdSizes{
-	"386":      {WordSize: 4, MaxAlign: 4},
-	"arm":      {WordSize: 4, MaxAlign: 4},
-	"arm64":    {WordSize: 8, MaxAlign: 8},
-	"amd64":    {WordSize: 8, MaxAlign: 8},
-	"amd64p32": {WordSize: 4, MaxAlign: 8},
-	"mips":     {WordSize: 4, MaxAlign: 4},
-	"mipsle":   {WordSize: 4, MaxAlign: 4},
-	"mips64":   {WordSize: 8, MaxAlign: 8},
-	"mips64le": {WordSize: 8, MaxAlign: 8},
-	"ppc64":    {WordSize: 8, MaxAlign: 8},
-	"ppc64le":  {WordSize: 8, MaxAlign: 8},
-	"s390x":    {WordSize: 8, MaxAlign: 8},
-}

src/go/types/api.go

@@ -121,7 +121,7 @@ type Config struct {
 	Importer Importer
 
 	// If Sizes != nil, it provides the sizing functions for package unsafe.
-	// Otherwise &StdSizes{WordSize: 8, MaxAlign: 8} is used instead.
+	// Otherwise SizesFor("amd64") is used instead.
 	Sizes Sizes
 
 	// If DisableUnusedImportCheck is set, packages are not checked

src/go/types/sizes.go

@@ -153,8 +153,34 @@ func (s *StdSizes) Sizeof(T Type) int64 {
 	return s.WordSize // catch-all
 }
 
+// common architecture word sizes and alignments
+var archSizes = map[string]*StdSizes{
+	"386":      {4, 4},
+	"arm":      {4, 4},
+	"arm64":    {8, 8},
+	"amd64":    {8, 8},
+	"amd64p32": {4, 8},
+	"mips":     {4, 4},
+	"mipsle":   {4, 4},
+	"mips64":   {8, 8},
+	"mips64le": {8, 8},
+	"ppc64":    {8, 8},
+	"ppc64le":  {8, 8},
+	"s390x":    {8, 8},
+	// When adding more architectures here,
+	// update the doc string of SizesFor below.
+}
+
+// SizesFor returns the Sizes for one of these architectures:
+// "386", "arm", "arm64", "amd64", "amd64p32", "mips", "mipsle",
+// "mips64", "mips64le", "ppc64", "ppc64le", "s390x".
+// The result is nil if an architecture is not known.
+func SizesFor(arch string) Sizes {
+	return archSizes[arch]
+}
+
 // stdSizes is used if Config.Sizes == nil.
-var stdSizes = StdSizes{8, 8}
+var stdSizes = SizesFor("amd64")
 
 func (conf *Config) alignof(T Type) int64 {
 	if s := conf.Sizes; s != nil {