[dev.typeparams] cmd/compile: refactor import reading

This CL restructures the gcimports importer to mmap the export data
into memory as a string, and then pass that same string to both the
typecheck and types2 importers.

This is primarily motivated by preparation for unified IR; but it
should also improve performance (fewer string copies) and reduces
divergance between the two importers.

Passes toolstash -cmp.

Change-Id: I397f720693e9e6360bfcb5acb12609ab339d251f
Reviewed-on: https://go-review.googlesource.com/c/go/+/325210
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 3 insertions, 9 deletions

diff --git a/src/cmd/compile/internal/noder/decl.go b/src/cmd/compile/internal/noder/decl.go
index 5c80b20671..96abbe66ae 100644
--- a/src/cmd/compile/internal/noder/decl.go
+++ b/src/cmd/compile/internal/noder/decl.go
@@ -41,21 +41,15 @@ func (g *irgen) decls(decls []syntax.Decl) []ir.Node {
 }
 
 func (g *irgen) importDecl(p *noder, decl *syntax.ImportDecl) {
-	// TODO(mdempsky): Merge with gcimports so we don't have to import
-	// packages twice.
-
 	g.pragmaFlags(decl.Pragma, 0)
 
 	// Get the imported package's path, as resolved already by types2
 	// and gcimporter. This is the same path as would be computed by
 	// parseImportPath.
-	path := pkgNameOf(g.info, decl).Imported().Path()
-
-	ipkg := readImportFile(g.target, path)
-	if ipkg == ir.Pkgs.Unsafe {
+	switch pkgNameOf(g.info, decl).Imported().Path() {
+	case "unsafe":
 		p.importedUnsafe = true
-	}
-	if ipkg.Path == "embed" {
+	case "embed":
 		p.importedEmbed = true
 	}
 }