cmd/compile: replace CallImport with go:wasmimport directive

This change replaces the special assembler instruction CallImport
of the wasm architecture with a new go:wasmimport directive. This new
directive is cleaner and has more flexibility with regards to how
parameters get passed to WebAssembly function imports. This is a
preparation for adding support for wasi (WebAssembly System Interface).

The default mode of the directive passes Go parameters as individual
WebAssembly parameters. This mode will be used with wasi. The second
mode "abi0" only passes the current SP as a single parameter. The
called function then reads its arguments from memory. This is the
method currently used by wasm_exec.js and the goal is to eventually
remove this mode.

* Fixes golang#38248

Co-authored-by: Vedant Roy <vroy101@gmail.com>
Co-authored-by: Richard Musiol <mail@richard-musiol.de>
Co-authored-by: David Blyth <davidcooperblyth@gmail.com>
Change-Id: I2baee4cca5d6c6ecfa26042a5aa233e33ea6f06f

misc/wasm/go_js_wasm_exec

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Copyright 2018 The Go Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.

misc/wasm/wasm_exec.js

@@ -206,6 +206,9 @@
 
 			const timeOrigin = Date.now() - performance.now();
 			this.importObject = {
+				_gotest: {
+					add: (a, b) => a + b,
+				},
 				go: {
 					// Go's SP does not change as long as no Go code is running. Some operations (e.g. calls, getters and setters)
 					// may synchronously trigger a Go event handler. This makes Go code get executed in the middle of the imported

src/cmd/compile/internal/gc/compile.go

@@ -42,6 +42,10 @@ func enqueueFunc(fn *ir.Func) {
 		return // we'll get this as part of its enclosing function
 	}
 
+	if ssagen.CreateWasmImportWrapper(fn) {
+		return
+	}
+
 	if len(fn.Body) == 0 {
 		// Initialize ABI wrappers if necessary.
 		ir.InitLSym(fn, false)

src/cmd/compile/internal/ir/func.go

@@ -133,6 +133,10 @@ type Func struct {
 	// For wrapper functions, WrappedFunc point to the original Func.
 	// Currently only used for go/defer wrappers.
 	WrappedFunc *Func
+
+	// WasmImport is used by the //go:wasmimport directive to store info about
+	// a WebAssembly function import.
+	WasmImport *WasmImport
 }
 
 func NewFunc(pos src.XPos) *Func {

src/cmd/compile/internal/ir/node.go

@@ -462,6 +462,12 @@ const (
 
 )
 
+// WasmImport stores metadata associated with the //go:wasmimport pragma
+type WasmImport struct {
+	Module string
+	Name   string
+}
+
 func AsNode(n types.Object) Node {
 	if n == nil {
 		return nil

src/cmd/compile/internal/ir/sizeof_test.go

@@ -20,8 +20,8 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{Func{}, 184, 320},
-		{Name{}, 100, 176},
+		{Func{}, 196, 328},
+		{Name{}, 112, 176},
 	}
 
 	for _, tt := range tests {

src/cmd/compile/internal/noder/decl.go

@@ -125,6 +125,22 @@ func (g *irgen) funcDecl(out *ir.Nodes, decl *syntax.FuncDecl) {
 		}
 	}
 
+	if p, ok := decl.Pragma.(*pragmas); ok && p.WasmImport != nil {
+		if decl.Body != nil {
+			base.ErrorfAt(fn.Pos(), "can only use //go:wasmimport with external func implementations")
+		}
+		name := typecheck.Lookup(decl.Name.Value).Def.(*ir.Name)
+		f := name.Defn.(*ir.Func)
+		f.WasmImport = &ir.WasmImport{
+			Module: p.WasmImport.Module,
+			Name:   p.WasmImport.Name,
+		}
+		// While functions annotated with //go:wasmimport are
+		// bodyless, the compiler generates a WebAssembly body for
+		// them. However, the body will never grow the Go stack.
+		f.Pragma |= ir.Nosplit
+	}
+
 	if decl.Body != nil {
 		if fn.Pragma&ir.Noescape != 0 {
 			base.ErrorfAt(fn.Pos(), "can only use //go:noescape with external func implementations")
@@ -349,4 +365,7 @@ func (g *irgen) reportUnused(pragma *pragmas) {
 			base.ErrorfAt(g.makeXPos(e.Pos), "misplaced go:embed directive")
 		}
 	}
+	if pragma.WasmImport != nil {
+		base.ErrorfAt(g.makeXPos(pragma.WasmImport.Pos), "misplaced go:wasmimport directive")
+	}
 }

src/cmd/compile/internal/noder/irgen.go

@@ -368,7 +368,7 @@ Outer:
 	if base.Flag.Complete {
 		for _, n := range g.target.Decls {
 			if fn, ok := n.(*ir.Func); ok {
-				if fn.Body == nil && fn.Nname.Sym().Linkname == "" {
+				if fn.Body == nil && fn.Nname.Sym().Linkname == "" && fn.WasmImport == nil {
 					base.ErrorfAt(fn.Pos(), "missing function body")
 				}
 			}

src/cmd/compile/internal/noder/noder.go

@@ -7,6 +7,7 @@ package noder
 import (
 	"errors"
 	"fmt"
+	"internal/buildcfg"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -219,9 +220,17 @@ var allowedStdPragmas = map[string]bool{
 
 // *pragmas is the value stored in a syntax.pragmas during parsing.
 type pragmas struct {
-	Flag   ir.PragmaFlag // collected bits
-	Pos    []pragmaPos   // position of each individual flag
-	Embeds []pragmaEmbed
+	Flag       ir.PragmaFlag // collected bits
+	Pos        []pragmaPos   // position of each individual flag
+	Embeds     []pragmaEmbed
+	WasmImport *WasmImport
+}
+
+// WasmImport stores metadata associated with the //go:wasmimport pragma
+type WasmImport struct {
+	Pos    syntax.Pos
+	Module string
+	Name   string
 }
 
 type pragmaPos struct {
@@ -245,6 +254,9 @@ func (p *noder) checkUnusedDuringParse(pragma *pragmas) {
 			p.error(syntax.Error{Pos: e.Pos, Msg: "misplaced go:embed directive"})
 		}
 	}
+	if pragma.WasmImport != nil {
+		p.error(syntax.Error{Pos: pragma.WasmImport.Pos, Msg: "misplaced go:wasmimport directive"})
+	}
 }
 
 // pragma is called concurrently if files are parsed concurrently.
@@ -272,6 +284,22 @@ func (p *noder) pragma(pos syntax.Pos, blankLine bool, text string, old syntax.P
 	}
 
 	switch {
+	case strings.HasPrefix(text, "go:wasmimport "):
+		if buildcfg.GOARCH == "wasm" {
+			f := strings.Fields(text)
+			if len(f) != 3 {
+				p.error(syntax.Error{Pos: pos, Msg: "usage: //go:wasmimport module_name import_name"})
+			}
+			if !base.Flag.CompilingRuntime && base.Ctxt.Pkgpath != "syscall/js" && base.Ctxt.Pkgpath != "syscall/js_test" {
+				p.error(syntax.Error{Pos: pos, Msg: "//go:wasmimport directive cannot be used outside of runtime or syscall/js"})
+			}
+			pragma.WasmImport = &WasmImport{
+				Pos:    pos,
+				Module: f[1],
+				Name:   f[2],
+			}
+		}
+
 	case strings.HasPrefix(text, "go:linkname "):
 		f := strings.Fields(text)
 		if !(2 <= len(f) && len(f) <= 3) {

src/cmd/compile/internal/noder/writer.go

@@ -1003,11 +1003,15 @@ func (w *writer) funcExt(obj *types2.Func) {
 	if pragma&ir.Systemstack != 0 && pragma&ir.Nosplit != 0 {
 		w.p.errorf(decl, "go:nosplit and go:systemstack cannot be combined")
 	}
+	wi := asWasmImport(decl.Pragma)
 
 	if decl.Body != nil {
 		if pragma&ir.Noescape != 0 {
 			w.p.errorf(decl, "can only use //go:noescape with external func implementations")
 		}
+		if wi != nil {
+			w.p.errorf(decl, "can only use //go:wasmimport with external func implementations")
+		}
 		if (pragma&ir.UintptrKeepAlive != 0 && pragma&ir.UintptrEscapes == 0) && pragma&ir.Nosplit == 0 {
 			// Stack growth can't handle uintptr arguments that may
 			// be pointers (as we don't know which are pointers
@@ -1028,7 +1032,8 @@ func (w *writer) funcExt(obj *types2.Func) {
 		if base.Flag.Complete || decl.Name.Value == "init" {
 			// Linknamed functions are allowed to have no body. Hopefully
 			// the linkname target has a body. See issue 23311.
-			if _, ok := w.p.linknames[obj]; !ok {
+			// Wasmimport functions are also allowed to have no body.
+			if _, ok := w.p.linknames[obj]; !ok && wi == nil {
 				w.p.errorf(decl, "missing function body")
 			}
 		}
@@ -2728,6 +2733,13 @@ func asPragmaFlag(p syntax.Pragma) ir.PragmaFlag {
 	return p.(*pragmas).Flag
 }
 
+func asWasmImport(p syntax.Pragma) *WasmImport {
+	if p == nil {
+		return nil
+	}
+	return p.(*pragmas).WasmImport
+}
+
 // isPtrTo reports whether from is the type *to.
 func isPtrTo(from, to types2.Type) bool {
 	ptr, ok := from.(*types2.Pointer)

src/cmd/compile/internal/ssagen/abi.go

@@ -11,8 +11,10 @@ import (
 	"os"
 	"strings"
 
+	"cmd/compile/internal/abi"
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
+	"cmd/compile/internal/objw"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
@@ -339,3 +341,129 @@ func makeABIWrapper(f *ir.Func, wrapperABI obj.ABI) {
 	typecheck.DeclContext = savedclcontext
 	ir.CurFunc = savedcurfn
 }
+
+// CreateWasmImportWrapper creates a wrapper for imported WASM functions to
+// adapt them to the Go calling convention. The body for this function is
+// generated in cmd/internal/obj/wasm/wasmobj.go
+func CreateWasmImportWrapper(fn *ir.Func) bool {
+	if fn.WasmImport == nil {
+		return false
+	}
+	if buildcfg.GOARCH != "wasm" {
+		base.FatalfAt(fn.Pos(), "CreateWasmImportWrapper call not supported on %s: func was %v", buildcfg.GOARCH, fn)
+	}
+
+	ir.InitLSym(fn, true)
+
+	pp := objw.NewProgs(fn, 0)
+	defer pp.Free()
+	pp.Text.To.Type = obj.TYPE_TEXTSIZE
+	pp.Text.To.Val = int32(types.RoundUp(fn.Type().ArgWidth(), int64(types.RegSize)))
+	// Wrapper functions never need their own stack frame
+	pp.Text.To.Offset = 0
+	pp.Flush()
+
+	return true
+}
+
+func toWasmFields(result *abi.ABIParamResultInfo, abiParams []abi.ABIParamAssignment) []obj.WasmField {
+	wfs := make([]obj.WasmField, len(abiParams))
+	for i, p := range abiParams {
+		t := p.Type
+		switch {
+		case t.IsInteger() && t.Size() == 4:
+			wfs[i].Type = obj.WasmI32
+		case t.IsInteger() && t.Size() == 8:
+			wfs[i].Type = obj.WasmI64
+		case t.IsFloat() && t.Size() == 4:
+			wfs[i].Type = obj.WasmF32
+		case t.IsFloat() && t.Size() == 8:
+			wfs[i].Type = obj.WasmF64
+		case t.IsPtr():
+			wfs[i].Type = obj.WasmPtr
+		default:
+			base.Fatalf("wasm import has bad function signature")
+		}
+		wfs[i].Offset = p.FrameOffset(result)
+	}
+	return wfs
+}
+
+// setupTextLSym initializes the LSym for a with-body text symbol.
+func setupTextLSym(f *ir.Func, flag int) {
+	if f.Dupok() {
+		flag |= obj.DUPOK
+	}
+	if f.Wrapper() {
+		flag |= obj.WRAPPER
+	}
+	if f.ABIWrapper() {
+		flag |= obj.ABIWRAPPER
+	}
+	if f.Needctxt() {
+		flag |= obj.NEEDCTXT
+	}
+	if f.Pragma&ir.Nosplit != 0 {
+		flag |= obj.NOSPLIT
+	}
+	if f.ReflectMethod() {
+		flag |= obj.REFLECTMETHOD
+	}
+
+	// Clumsy but important.
+	// For functions that could be on the path of invoking a deferred
+	// function that can recover (runtime.reflectcall, reflect.callReflect,
+	// and reflect.callMethod), we want the panic+recover special handling.
+	// See test/recover.go for test cases and src/reflect/value.go
+	// for the actual functions being considered.
+	//
+	// runtime.reflectcall is an assembly function which tailcalls
+	// WRAPPER functions (runtime.callNN). Its ABI wrapper needs WRAPPER
+	// flag as well.
+	fnname := f.Sym().Name
+	if base.Ctxt.Pkgpath == "runtime" && fnname == "reflectcall" {
+		flag |= obj.WRAPPER
+	} else if base.Ctxt.Pkgpath == "reflect" {
+		switch fnname {
+		case "callReflect", "callMethod":
+			flag |= obj.WRAPPER
+		}
+	}
+
+	base.Ctxt.InitTextSym(f.LSym, flag, f.Pos())
+
+	if f.WasmImport != nil {
+		wi := obj.WasmImport{
+			Module: f.WasmImport.Module,
+			Name:   f.WasmImport.Name,
+		}
+		if wi.Module == "go" {
+			// Functions that are imported from the "go" module use a special
+			// ABI that just accepts the stack pointer.
+			// Example:
+			//
+			// 	//go:wasmimport go add
+			// 	func importedAdd(a, b uint) uint
+			//
+			// will roughly become
+			//
+			// 	(import "go" "add" (func (param i32)))
+			wi.Params = []obj.WasmField{{Type: obj.WasmI32}}
+		} else {
+			// All other imported functions use the normal WASM ABI.
+			// Example:
+			//
+			// 	//go:wasmimport a_module add
+			// 	func importedAdd(a, b uint) uint
+			//
+			// will roughly become
+			//
+			// 	(import "a_module" "add" (func (param i32 i32) (result i32)))
+			abiConfig := AbiForBodylessFuncStackMap(f)
+			abiInfo := abiConfig.ABIAnalyzeFuncType(f.Type().FuncType())
+			wi.Params = toWasmFields(abiInfo, abiInfo.InParams())
+			wi.Results = toWasmFields(abiInfo, abiInfo.OutParams())
+		}
+		f.LSym.Func().WasmImport = &wi
+	}
+}

src/cmd/internal/goobj/objfile.go

@@ -440,6 +440,7 @@ const (
 	AuxPcline
 	AuxPcinline
 	AuxPcdata
+	AuxWasmImport
 )
 
 func (a *Aux) Type() uint8 { return a[0] }