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>

4 files changed, 57 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/noder/linker.go b/src/cmd/compile/internal/noder/linker.go
index f5667f57ab..486013c7df 100644
--- a/src/cmd/compile/internal/noder/linker.go
+++ b/src/cmd/compile/internal/noder/linker.go
@@ -278,6 +278,11 @@ func (l *linker) relocFuncExt(w *pkgbits.Encoder, name *ir.Name) {
 			w.String("")
 			w.String("")
 		}
+		if name.Func.WasmExport != nil {
+			w.String(name.Func.WasmExport.Name)
+		} else {
+			w.String("")
+		}
 	}
 
 	// Relocated extension data.
diff --git a/src/cmd/compile/internal/noder/noder.go b/src/cmd/compile/internal/noder/noder.go
index 1652dc6618..7905c374c5 100644
--- a/src/cmd/compile/internal/noder/noder.go
+++ b/src/cmd/compile/internal/noder/noder.go
@@ -171,6 +171,7 @@ type pragmas struct {
 	Pos        []pragmaPos   // position of each individual flag
 	Embeds     []pragmaEmbed
 	WasmImport *WasmImport
+	WasmExport *WasmExport
 }
 
 // WasmImport stores metadata associated with the //go:wasmimport pragma
@@ -180,6 +181,12 @@ type WasmImport struct {
 	Name   string
 }
 
+// WasmExport stores metadata associated with the //go:wasmexport pragma
+type WasmExport struct {
+	Pos  syntax.Pos
+	Name string
+}
+
 type pragmaPos struct {
 	Flag ir.PragmaFlag
 	Pos  syntax.Pos
@@ -204,6 +211,9 @@ func (p *noder) checkUnusedDuringParse(pragma *pragmas) {
 	if pragma.WasmImport != nil {
 		p.error(syntax.Error{Pos: pragma.WasmImport.Pos, Msg: "misplaced go:wasmimport directive"})
 	}
+	if pragma.WasmExport != nil {
+		p.error(syntax.Error{Pos: pragma.WasmExport.Pos, Msg: "misplaced go:wasmexport directive"})
+	}
 }
 
 // pragma is called concurrently if files are parsed concurrently.
@@ -246,6 +256,23 @@ func (p *noder) pragma(pos syntax.Pos, blankLine bool, text string, old syntax.P
 				Name:   f[2],
 			}
 		}
+
+	case strings.HasPrefix(text, "go:wasmexport "):
+		f := strings.Fields(text)
+		if len(f) != 2 {
+			// TODO: maybe make the name optional? It was once mentioned on proposal 65199.
+			p.error(syntax.Error{Pos: pos, Msg: "usage: //go:wasmexport exportname"})
+			break
+		}
+
+		if buildcfg.GOARCH == "wasm" {
+			// Only actually use them if we're compiling to WASM though.
+			pragma.WasmExport = &WasmExport{
+				Pos:  pos,
+				Name: f[1],
+			}
+		}
+
 	case strings.HasPrefix(text, "go:linkname "):
 		f := strings.Fields(text)
 		if !(2 <= len(f) && len(f) <= 3) {
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() {
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index c1560941b8..9f862f9a4c 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -1050,6 +1050,7 @@ func (w *writer) funcExt(obj *types2.Func) {
 		w.p.errorf(decl, "go:nosplit and go:systemstack cannot be combined")
 	}
 	wi := asWasmImport(decl.Pragma)
+	we := asWasmExport(decl.Pragma)
 
 	if decl.Body != nil {
 		if pragma&ir.Noescape != 0 {
@@ -1104,6 +1105,11 @@ func (w *writer) funcExt(obj *types2.Func) {
 			w.String("")
 			w.String("")
 		}
+		if we != nil {
+			w.String(we.Name)
+		} else {
+			w.String("")
+		}
 	}
 
 	w.Bool(false) // stub extension
@@ -3011,6 +3017,13 @@ func asWasmImport(p syntax.Pragma) *WasmImport {
 	return p.(*pragmas).WasmImport
 }
 
+func asWasmExport(p syntax.Pragma) *WasmExport {
+	if p == nil {
+		return nil
+	}
+	return p.(*pragmas).WasmExport
+}
+
 // isPtrTo reports whether from is the type *to.
 func isPtrTo(from, to types2.Type) bool {
 	ptr, ok := types2.Unalias(from).(*types2.Pointer)