cmd/compile: add basic wasmexport support

This CL adds a compiler directive go:wasmexport, which applies to
a Go function and makes it an exported function of the Wasm module
being built, so it can be called directly from the host. As
proposed in #65199, parameter and result types are limited to
32-bit and 64-bit integers and floats, and there can be at most
one result.

As the Go and Wasm calling conventions are different, for a
wasmexport function we generate a wrapper function does the ABI
conversion at compile time.

Currently this CL only adds basic support. In particular,
- it only supports executable mode, i.e. the Go wasm module calls
  into the host via wasmimport, which then calls back to Go via
  wasmexport. Library (c-shared) mode is not implemented yet.
- only supports wasip1, not js.
- if the exported function unwinds stacks (goroutine switch, stack
growth, etc.), it probably doesn't work.

TODO: support stack unwinding, c-shared mode, js.

For #65199.

Change-Id: Id1777c2d44f7d51942c1caed3173c0a82f120cc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/603055
Reviewed-by: Than McIntosh <thanm@golang.org>
Reviewed-by: Randy Reddig <randy.reddig@fastly.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 12 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index ff44adedb4..1dd2e09b0d 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -1132,15 +1132,22 @@ func (r *reader) funcExt(name *ir.Name, method *types.Sym) {
 	r.linkname(name)
 
 	if buildcfg.GOARCH == "wasm" {
-		xmod := r.String()
-		xname := r.String()
+		importmod := r.String()
+		importname := r.String()
+		exportname := r.String()
 
-		if xmod != "" && xname != "" {
+		if importmod != "" && importname != "" {
 			fn.WasmImport = &ir.WasmImport{
-				Module: xmod,
-				Name:   xname,
+				Module: importmod,
+				Name:   importname,
 			}
 		}
+		if exportname != "" {
+			if method != nil {
+				base.ErrorfAt(fn.Pos(), 0, "cannot use //go:wasmexport on a method")
+			}
+			fn.WasmExport = &ir.WasmExport{Name: exportname}
+		}
 	}
 
 	if r.Bool() {