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, 27 insertions, 0 deletions

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) {