[dev.typeparams] cmd/compile: scaffolding for export data experiments

This CL adds a simple framework for augmenting the current export data
format by writing out additional data *after* the existing data, with
an extra header before it that current readers ignore.

In particular, this is used by unified IR to be able to experiment and
iterate on export data designs without having to keep the
go/internal/gcimporter and x/tools/go/gcexportdata importers in
sync. Instead, they simply continue reading the existing data written
out by typecheck/iexport.go.

Change-Id: I883211c2892e2c7dec758b85ff6bc31b244440a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/327169
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

1 files changed, 43 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/noder/export.go b/src/cmd/compile/internal/noder/export.go
index 9fb3b4da10..1a296e22c8 100644
--- a/src/cmd/compile/internal/noder/export.go
+++ b/src/cmd/compile/internal/noder/export.go
@@ -5,22 +5,61 @@
 package noder
 
 import (
+	"bytes"
 	"fmt"
+	"io"
 
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/typecheck"
 	"cmd/internal/bio"
 )
 
+// writeNewExportFunc is a hook that can be added to append extra
+// export data after the normal export data section. It allows
+// experimenting with new export data format designs without requiring
+// immediate support in the go/internal or x/tools importers.
+var writeNewExportFunc func(out io.Writer)
+
 func WriteExports(out *bio.Writer) {
+	// When unified IR exports are enable, we simply append it to the
+	// end of the normal export data (with compiler extensions
+	// disabled), and write an extra header giving its size.
+	//
+	// If the compiler sees this header, it knows to read the new data
+	// instead; meanwhile the go/types importers will silently ignore it
+	// and continue processing the old export instead.
+	//
+	// This allows us to experiment with changes to the new export data
+	// format without needing to update the go/internal/gcimporter or
+	// (worse) x/tools/go/gcexportdata.
+
+	useNewExport := writeNewExportFunc != nil
+
+	var old, new bytes.Buffer
+
+	typecheck.WriteExports(&old, !useNewExport)
+
+	if useNewExport {
+		writeNewExportFunc(&new)
+	}
+
+	oldLen := old.Len()
+	newLen := new.Len()
+
+	if useNewExport {
+		fmt.Fprintf(out, "\nnewexportsize %v\n", newLen)
+	}
+
 	// The linker also looks for the $$ marker - use char after $$ to distinguish format.
 	out.WriteString("\n$$B\n") // indicate binary export format
-	off := out.Offset()
-	typecheck.WriteExports(out, true)
-	size := out.Offset() - off
+	io.Copy(out, &old)
 	out.WriteString("\n$$\n")
+	io.Copy(out, &new)
 
 	if base.Debug.Export != 0 {
-		fmt.Printf("BenchmarkExportSize:%s 1 %d bytes\n", base.Ctxt.Pkgpath, size)
+		fmt.Printf("BenchmarkExportSize:%s 1 %d bytes\n", base.Ctxt.Pkgpath, oldLen)
+		if useNewExport {
+			fmt.Printf("BenchmarkNewExportSize:%s 1 %d bytes\n", base.Ctxt.Pkgpath, newLen)
+		}
 	}
 }