cmd/link, runtime: remove typelinks

Instead of adding a typelinks section to a Go binary,
mark the start and end of the typelinked type descriptors.
The runtime can then step through the descriptors to find them all,
rather than relying on the extra linker-generated offset list.

The runtime steps through the type descriptors lazily,
as many Go programs don't need the typelinks list at all.

This reduces the size of cmd/go by 15K bytes, which isn't much
but it's not nothing.

A future CL will change the reflect package to use the type pointers
directly rather than converting to offsets and then back to type pointers.

For #6853

Change-Id: Id0af4ce81c5b1cea899fc92b6ff9d2db8ce4c267
Reviewed-on: https://go-review.googlesource.com/c/go/+/724261
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index 965ff8ab51..9b1dd585ca 100644
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -630,11 +630,21 @@ func releasem(mp *m) {
 //go:linkname reflect_typelinks reflect.typelinks
 func reflect_typelinks() ([]unsafe.Pointer, [][]int32) {
 	modules := activeModules()
+
+	typesToOffsets := func(md *moduledata) []int32 {
+		types := moduleTypelinks(md)
+		ret := make([]int32, 0, len(types))
+		for _, typ := range types {
+			ret = append(ret, int32(uintptr(unsafe.Pointer(typ))-md.types))
+		}
+		return ret
+	}
+
 	sections := []unsafe.Pointer{unsafe.Pointer(modules[0].types)}
-	ret := [][]int32{modules[0].typelinks}
+	ret := [][]int32{typesToOffsets(modules[0])}
 	for _, md := range modules[1:] {
 		sections = append(sections, unsafe.Pointer(md.types))
-		ret = append(ret, md.typelinks)
+		ret = append(ret, typesToOffsets(md))
 	}
 	return sections, ret
 }