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)
+		}
 	}
 }