go/types: add a compiler param to SizesFor

The current StdSizes most closely matches
the gc compiler, and the uses I know of that care
which compiler the sizes are for are all for
the gc compiler, so call the existing
implementation "gc".

Updates #17586
Fixes #19351

Change-Id: I2bdd694518fbe233473896321a1f9758b46ed79b
Reviewed-on: https://go-review.googlesource.com/37666
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/internal/srcimporter/srcimporter.go

@@ -35,7 +35,7 @@ func New(ctxt *build.Context, fset *token.FileSet, packages map[string]*types.Pa
 	return &Importer{
 		ctxt:     ctxt,
 		fset:     fset,
-		sizes:    types.SizesFor(ctxt.GOARCH), // uses go/types default if GOARCH not found
+		sizes:    types.SizesFor(ctxt.Compiler, ctxt.GOARCH), // uses go/types default if GOARCH not found
 		packages: packages,
 	}
 }

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 SizesFor("amd64") is used instead.
+	// Otherwise SizesFor("gc", "amd64") is used instead.
 	Sizes Sizes
 
 	// If DisableUnusedImportCheck is set, packages are not checked

src/go/types/gotype.go

@@ -247,7 +247,7 @@ func checkPkgFiles(files []*ast.File) {
 			report(err)
 		},
 		Importer: importer.For(*compiler, nil),
-		Sizes:    types.SizesFor(build.Default.GOARCH),
+		Sizes:    types.SizesFor(build.Default.Compiler, build.Default.GOARCH),
 	}
 
 	defer func() {

src/go/types/sizes.go

@@ -154,7 +154,7 @@ func (s *StdSizes) Sizeof(T Type) int64 {
 }
 
 // common architecture word sizes and alignments
-var archSizes = map[string]*StdSizes{
+var gcArchSizes = map[string]*StdSizes{
 	"386":      {4, 4},
 	"arm":      {4, 4},
 	"arm64":    {8, 8},
@@ -171,16 +171,21 @@ var archSizes = map[string]*StdSizes{
 	// update the doc string of SizesFor below.
 }
 
-// SizesFor returns the Sizes for one of these architectures:
+// SizesFor returns the Sizes used by a compiler for an architecture.
+// The result is nil if a compiler/architecture pair is not known.
+//
+// Supported architectures for compiler "gc":
 // "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]
+func SizesFor(compiler, arch string) Sizes {
+	if compiler != "gc" {
+		return nil
+	}
+	return gcArchSizes[arch]
 }
 
 // stdSizes is used if Config.Sizes == nil.
-var stdSizes = SizesFor("amd64")
+var stdSizes = SizesFor("gc", "amd64")
 
 func (conf *Config) alignof(T Type) int64 {
 	if s := conf.Sizes; s != nil {