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, 0 deletions

diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index cbdc5a3486..de38349930 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -361,6 +361,9 @@ func (w *writer) Sym(s *LSym) {
 	if s.ABIWrapper() {
 		flag2 |= goobj.SymFlagABIWrapper
 	}
+	if s.Func() != nil && s.Func().WasmExport != nil {
+		flag2 |= goobj.SymFlagWasmExport
+	}
 	if strings.HasPrefix(name, "gofile..") {
 		name = filepath.ToSlash(name)
 	}
@@ -627,6 +630,9 @@ func (w *writer) Aux(s *LSym) {
 			}
 			w.aux1(goobj.AuxWasmImport, fn.WasmImport.AuxSym)
 		}
+		if fn.WasmExport != nil {
+			w.aux1(goobj.AuxWasmType, fn.WasmExport.AuxSym)
+		}
 	} else if v := s.VarInfo(); v != nil {
 		if v.dwarfInfoSym != nil && v.dwarfInfoSym.Size != 0 {
 			w.aux1(goobj.AuxDwarfInfo, v.dwarfInfoSym)
@@ -737,6 +743,9 @@ func nAuxSym(s *LSym) int {
 			}
 			n++
 		}
+		if fn.WasmExport != nil {
+			n++
+		}
 	} else if v := s.VarInfo(); v != nil {
 		if v.dwarfInfoSym != nil && v.dwarfInfoSym.Size != 0 {
 			n++
@@ -801,6 +810,9 @@ func genFuncInfoSyms(ctxt *Link) {
 		if wi := fn.WasmImport; wi != nil {
 			auxsyms = append(auxsyms, wi.AuxSym)
 		}
+		if we := fn.WasmExport; we != nil {
+			auxsyms = append(auxsyms, we.AuxSym)
+		}
 		for _, s := range auxsyms {
 			if s == nil || s.Size == 0 {
 				continue