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>

8 files changed, 138 insertions, 15 deletions

diff --git a/src/cmd/compile/internal/gc/compile.go b/src/cmd/compile/internal/gc/compile.go
index 81a6023e47..5ade700d46 100644
--- a/src/cmd/compile/internal/gc/compile.go
+++ b/src/cmd/compile/internal/gc/compile.go
@@ -105,6 +105,11 @@ func prepareFunc(fn *ir.Func) {
 	// Calculate parameter offsets.
 	types.CalcSize(fn.Type())
 
+	// Generate wrappers between Go ABI and Wasm ABI, for a wasmexport
+	// function.
+	// Must be done after InitLSym and CalcSize.
+	ssagen.GenWasmExportWrapper(fn)
+
 	ir.CurFunc = fn
 	walk.Walk(fn)
 	ir.CurFunc = nil // enforce no further uses of CurFunc
diff --git a/src/cmd/compile/internal/ir/func.go b/src/cmd/compile/internal/ir/func.go
index e005ef7a7f..0675150b2d 100644
--- a/src/cmd/compile/internal/ir/func.go
+++ b/src/cmd/compile/internal/ir/func.go
@@ -142,6 +142,9 @@ type Func struct {
 	// WasmImport is used by the //go:wasmimport directive to store info about
 	// a WebAssembly function import.
 	WasmImport *WasmImport
+	// WasmExport is used by the //go:wasmexport directive to store info about
+	// a WebAssembly function import.
+	WasmExport *WasmExport
 }
 
 // WasmImport stores metadata associated with the //go:wasmimport pragma.
@@ -150,6 +153,11 @@ type WasmImport struct {
 	Name   string
 }
 
+// WasmExport stores metadata associated with the //go:wasmexport pragma.
+type WasmExport struct {
+	Name string
+}
+
 // NewFunc returns a new Func with the given name and type.
 //
 // fpos is the position of the "func" token, and npos is the position
diff --git a/src/cmd/compile/internal/ir/sizeof_test.go b/src/cmd/compile/internal/ir/sizeof_test.go
index 68d2865595..6331cceb4a 100644
--- a/src/cmd/compile/internal/ir/sizeof_test.go
+++ b/src/cmd/compile/internal/ir/sizeof_test.go
@@ -20,7 +20,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{Func{}, 176, 296},
+		{Func{}, 180, 304},
 		{Name{}, 96, 168},
 	}
 
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)
diff --git a/src/cmd/compile/internal/ssagen/abi.go b/src/cmd/compile/internal/ssagen/abi.go
index d5ae3b1793..0c42c84312 100644
--- a/src/cmd/compile/internal/ssagen/abi.go
+++ b/src/cmd/compile/internal/ssagen/abi.go
@@ -19,6 +19,8 @@ import (
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
 	"cmd/internal/obj/wasm"
+
+	rtabi "internal/abi"
 )
 
 // SymABIs records information provided by the assembler about symbol
@@ -347,7 +349,7 @@ func CreateWasmImportWrapper(fn *ir.Func) bool {
 
 	ir.InitLSym(fn, true)
 
-	setupWasmABI(fn)
+	setupWasmImport(fn)
 
 	pp := objw.NewProgs(fn, 0)
 	defer pp.Free()
@@ -360,7 +362,49 @@ func CreateWasmImportWrapper(fn *ir.Func) bool {
 	return true
 }
 
-func paramsToWasmFields(f *ir.Func, result *abi.ABIParamResultInfo, abiParams []abi.ABIParamAssignment) []obj.WasmField {
+func GenWasmExportWrapper(wrapped *ir.Func) {
+	if wrapped.WasmExport == nil {
+		return
+	}
+	if buildcfg.GOARCH != "wasm" {
+		base.FatalfAt(wrapped.Pos(), "GenWasmExportWrapper call not supported on %s: func was %v", buildcfg.GOARCH, wrapped)
+	}
+
+	pos := base.AutogeneratedPos
+	sym := &types.Sym{
+		Name:     wrapped.WasmExport.Name,
+		Linkname: wrapped.WasmExport.Name,
+	}
+	ft := wrapped.Nname.Type()
+	fn := ir.NewFunc(pos, pos, sym, types.NewSignature(nil,
+		typecheck.NewFuncParams(ft.Params()),
+		typecheck.NewFuncParams(ft.Results())))
+	fn.ABI = obj.ABI0 // actually wasm ABI
+	// The wrapper function has a special calling convention that
+	// morestack currently doesn't handle. For now we require that
+	// the argument size fits in StackSmall, which we know we have
+	// on stack, so we don't need to split stack.
+	// cmd/internal/obj/wasm supports only 16 argument "registers"
+	// anyway.
+	if ft.ArgWidth() > rtabi.StackSmall {
+		base.ErrorfAt(wrapped.Pos(), 0, "wasmexport function argument too large")
+	}
+	fn.Pragma |= ir.Nosplit
+
+	ir.InitLSym(fn, true)
+
+	setupWasmExport(fn, wrapped)
+
+	pp := objw.NewProgs(fn, 0)
+	defer pp.Free()
+	pp.Text.To.Type = obj.TYPE_TEXTSIZE
+	pp.Text.To.Val = int32(0)
+	pp.Text.To.Offset = types.RoundUp(ft.ArgWidth(), int64(types.RegSize))
+	pp.Flush()
+	// Actual code geneneration is in cmd/internal/obj/wasm.
+}
+
+func paramsToWasmFields(f *ir.Func, pragma string, result *abi.ABIParamResultInfo, abiParams []abi.ABIParamAssignment) []obj.WasmField {
 	wfs := make([]obj.WasmField, len(abiParams))
 	for i, p := range abiParams {
 		t := p.Type
@@ -376,16 +420,16 @@ func paramsToWasmFields(f *ir.Func, result *abi.ABIParamResultInfo, abiParams []
 		case types.TUNSAFEPTR:
 			wfs[i].Type = obj.WasmPtr
 		default:
-			base.ErrorfAt(f.Pos(), 0, "go:wasmimport %s %s: unsupported parameter type %s", f.WasmImport.Module, f.WasmImport.Name, t.String())
+			base.ErrorfAt(f.Pos(), 0, "%s: unsupported parameter type %s", pragma, t.String())
 		}
 		wfs[i].Offset = p.FrameOffset(result)
 	}
 	return wfs
 }
 
-func resultsToWasmFields(f *ir.Func, result *abi.ABIParamResultInfo, abiParams []abi.ABIParamAssignment) []obj.WasmField {
+func resultsToWasmFields(f *ir.Func, pragma string, result *abi.ABIParamResultInfo, abiParams []abi.ABIParamAssignment) []obj.WasmField {
 	if len(abiParams) > 1 {
-		base.ErrorfAt(f.Pos(), 0, "go:wasmimport %s %s: too many return values", f.WasmImport.Module, f.WasmImport.Name)
+		base.ErrorfAt(f.Pos(), 0, "%s: too many return values", pragma)
 		return nil
 	}
 	wfs := make([]obj.WasmField, len(abiParams))
@@ -408,8 +452,9 @@ func resultsToWasmFields(f *ir.Func, result *abi.ABIParamResultInfo, abiParams [
 	return wfs
 }
 
-// setupWasmABI calculates the params and results in terms of WebAssembly values for the given function.
-func setupWasmABI(f *ir.Func) {
+// setupWasmImport calculates the params and results in terms of WebAssembly values for the given function,
+// and sets up the wasmimport metadata.
+func setupWasmImport(f *ir.Func) {
 	wi := obj.WasmImport{
 		Module: f.WasmImport.Module,
 		Name:   f.WasmImport.Name,
@@ -438,8 +483,21 @@ func setupWasmABI(f *ir.Func) {
 		// 	(import "a_module" "add" (func (param i32 i32) (result i32)))
 		abiConfig := AbiForBodylessFuncStackMap(f)
 		abiInfo := abiConfig.ABIAnalyzeFuncType(f.Type())
-		wi.Params = paramsToWasmFields(f, abiInfo, abiInfo.InParams())
-		wi.Results = resultsToWasmFields(f, abiInfo, abiInfo.OutParams())
+		wi.Params = paramsToWasmFields(f, "go:wasmimport", abiInfo, abiInfo.InParams())
+		wi.Results = resultsToWasmFields(f, "go:wasmimport", abiInfo, abiInfo.OutParams())
 	}
 	f.LSym.Func().WasmImport = &wi
 }
+
+// setupWasmExport calculates the params and results in terms of WebAssembly values for the given function,
+// and sets up the wasmexport metadata.
+func setupWasmExport(f, wrapped *ir.Func) {
+	we := obj.WasmExport{
+		WrappedSym: wrapped.LSym,
+	}
+	abiConfig := AbiForBodylessFuncStackMap(wrapped)
+	abiInfo := abiConfig.ABIAnalyzeFuncType(wrapped.Type())
+	we.Params = paramsToWasmFields(wrapped, "go:wasmexport", abiInfo, abiInfo.InParams())
+	we.Results = resultsToWasmFields(wrapped, "go:wasmexport", abiInfo, abiInfo.OutParams())
+	f.LSym.Func().WasmExport = &we
+}