all: implement wasmimport directive

Go programs can now use the //go:wasmimport module_name function_name
directive to import functions from the WebAssembly runtime.

For now, the directive is restricted to the runtime and syscall/js
packages.

* Derived from CL 350737
* Original work modified to work with changes to the IR conversion code.
* Modification of CL 350737 changes to fully exist in Unified IR path (emp)
* Original work modified to work with changes to the ABI configuration code.
* Fixes #38248

Co-authored-by: Vedant Roy <vroy101@gmail.com>
Co-authored-by: Richard Musiol <mail@richard-musiol.de>
Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Change-Id: I740719735d91c306ac718a435a78e1ee9686bc16
Reviewed-on: https://go-review.googlesource.com/c/go/+/463018
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

17 files changed, 533 insertions, 43 deletions

diff --git a/src/cmd/compile/internal/gc/compile.go b/src/cmd/compile/internal/gc/compile.go
index cfce77d828..4795297e7e 100644
--- a/src/cmd/compile/internal/gc/compile.go
+++ b/src/cmd/compile/internal/gc/compile.go
@@ -43,6 +43,10 @@ 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)
diff --git a/src/cmd/compile/internal/ir/func.go b/src/cmd/compile/internal/ir/func.go
index 967ebb02c2..76ab952157 100644
--- a/src/cmd/compile/internal/ir/func.go
+++ b/src/cmd/compile/internal/ir/func.go
@@ -133,6 +133,16 @@ 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
+}
+
+// WasmImport stores metadata associated with the //go:wasmimport pragma.
+type WasmImport struct {
+	Module string
+	Name   string
 }
 
 func NewFunc(pos src.XPos) *Func {
diff --git a/src/cmd/compile/internal/ir/sizeof_test.go b/src/cmd/compile/internal/ir/sizeof_test.go
index 754d1a8de0..307f40d484 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{}, 184, 320},
+		{Func{}, 188, 328},
 		{Name{}, 100, 176},
 	}
 
diff --git a/src/cmd/compile/internal/noder/linker.go b/src/cmd/compile/internal/noder/linker.go
index 0f39fdec05..44de017ae5 100644
--- a/src/cmd/compile/internal/noder/linker.go
+++ b/src/cmd/compile/internal/noder/linker.go
@@ -5,6 +5,7 @@
 package noder
 
 import (
+	"internal/buildcfg"
 	"internal/pkgbits"
 	"io"
 
@@ -269,6 +270,16 @@ func (l *linker) relocFuncExt(w *pkgbits.Encoder, name *ir.Name) {
 	l.pragmaFlag(w, name.Func.Pragma)
 	l.linkname(w, name)
 
+	if buildcfg.GOARCH == "wasm" {
+		if name.Func.WasmImport != nil {
+			w.String(name.Func.WasmImport.Module)
+			w.String(name.Func.WasmImport.Name)
+		} else {
+			w.String("")
+			w.String("")
+		}
+	}
+
 	// Relocated extension data.
 	w.Bool(true)
 
diff --git a/src/cmd/compile/internal/noder/noder.go b/src/cmd/compile/internal/noder/noder.go
index 10619bf569..16113e37a3 100644
--- a/src/cmd/compile/internal/noder/noder.go
+++ b/src/cmd/compile/internal/noder/noder.go
@@ -7,6 +7,7 @@ package noder
 import (
 	"errors"
 	"fmt"
+	"internal/buildcfg"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -166,9 +167,17 @@ 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 {
@@ -192,6 +201,9 @@ 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.
@@ -219,6 +231,25 @@ func (p *noder) pragma(pos syntax.Pos, blankLine bool, text string, old syntax.P
 	}
 
 	switch {
+	case strings.HasPrefix(text, "go:wasmimport "):
+		f := strings.Fields(text)
+		if len(f) != 3 {
+			p.error(syntax.Error{Pos: pos, Msg: "usage: //go:wasmimport importmodule importname"})
+			break
+		}
+		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"})
+			break
+		}
+
+		if buildcfg.GOARCH == "wasm" {
+			// Only actually use them if we're compiling to WASM though.
+			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) {
diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index b7605e9317..e4ab80b2d0 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -1081,6 +1081,18 @@ func (r *reader) funcExt(name *ir.Name, method *types.Sym) {
 	fn.Pragma = r.pragmaFlag()
 	r.linkname(name)
 
+	if buildcfg.GOARCH == "wasm" {
+		xmod := r.String()
+		xname := r.String()
+
+		if xmod != "" && xname != "" {
+			fn.WasmImport = &ir.WasmImport{
+				Module: xmod,
+				Name:   xname,
+			}
+		}
+	}
+
 	typecheck.Func(fn)
 
 	if r.Bool() {
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index da5c1e910d..5dd8d1de2d 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -6,6 +6,7 @@ package noder
 
 import (
 	"fmt"
+	"internal/buildcfg"
 	"internal/pkgbits"
 
 	"cmd/compile/internal/base"
@@ -1003,11 +1004,15 @@ 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 +1033,8 @@ 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")
 			}
 		}
@@ -1041,6 +1047,17 @@ func (w *writer) funcExt(obj *types2.Func) {
 	w.Sync(pkgbits.SyncFuncExt)
 	w.pragmaFlag(pragma)
 	w.linkname(obj)
+
+	if buildcfg.GOARCH == "wasm" {
+		if wi != nil {
+			w.String(wi.Module)
+			w.String(wi.Name)
+		} else {
+			w.String("")
+			w.String("")
+		}
+	}
+
 	w.Bool(false) // stub extension
 	w.Reloc(pkgbits.RelocBody, body)
 	w.Sync(pkgbits.SyncEOF)
@@ -2728,6 +2745,13 @@ 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)
diff --git a/src/cmd/compile/internal/ssagen/abi.go b/src/cmd/compile/internal/ssagen/abi.go
index fa26ae1f06..9c725b898d 100644
--- a/src/cmd/compile/internal/ssagen/abi.go
+++ b/src/cmd/compile/internal/ssagen/abi.go
@@ -11,11 +11,14 @@ 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"
+	"cmd/internal/obj/wasm"
 )
 
 // SymABIs records information provided by the assembler about symbol
@@ -336,3 +339,88 @@ 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)
+
+	setupWasmABI(fn)
+
+	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 setupWasmABI(f *ir.Func) {
+	wi := obj.WasmImport{
+		Module: f.WasmImport.Module,
+		Name:   f.WasmImport.Name,
+	}
+	if wi.Module == wasm.GojsModule {
+		// Functions that are imported from the "gojs" module use a special
+		// ABI that just accepts the stack pointer.
+		// Example:
+		//
+		// 	//go:wasmimport gojs add
+		// 	func importedAdd(a, b uint) uint
+		//
+		// will roughly become
+		//
+		// 	(import "gojs" "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
+}
diff --git a/src/cmd/internal/goobj/objfile.go b/src/cmd/internal/goobj/objfile.go
index 547b826495..0364f856cf 100644
--- a/src/cmd/internal/goobj/objfile.go
+++ b/src/cmd/internal/goobj/objfile.go
@@ -442,6 +442,7 @@ const (
 	AuxPcline
 	AuxPcinline
 	AuxPcdata
+	AuxWasmImport
 )
 
 func (a *Aux) Type() uint8 { return a[0] }
diff --git a/src/cmd/internal/obj/link.go b/src/cmd/internal/obj/link.go
index d153afbfae..077562a267 100644
--- a/src/cmd/internal/obj/link.go
+++ b/src/cmd/internal/obj/link.go
@@ -37,6 +37,7 @@ import (
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"cmd/internal/sys"
+	"encoding/binary"
 	"fmt"
 	"sync"
 	"sync/atomic"
@@ -499,7 +500,9 @@ type FuncInfo struct {
 	WrapInfo           *LSym // for wrapper, info of wrapped function
 	JumpTables         []JumpTable
 
-	FuncInfoSym *LSym
+	FuncInfoSym   *LSym
+	WasmImportSym *LSym
+	WasmImport    *WasmImport
 }
 
 // JumpTable represents a table used for implementing multi-way
@@ -558,6 +561,75 @@ func (s *LSym) File() *FileInfo {
 	return f
 }
 
+// WasmImport represents a WebAssembly (WASM) imported function with
+// parameters and results translated into WASM types based on the Go function
+// declaration.
+type WasmImport struct {
+	// Module holds the WASM module name specified by the //go:wasmimport
+	// directive.
+	Module string
+	// Name holds the WASM imported function name specified by the
+	// //go:wasmimport directive.
+	Name string
+	// Params holds the imported function parameter fields.
+	Params []WasmField
+	// Results holds the imported function result fields.
+	Results []WasmField
+}
+
+func (wi *WasmImport) CreateSym(ctxt *Link) *LSym {
+	var sym LSym
+
+	var b [8]byte
+	writeByte := func(x byte) {
+		sym.WriteBytes(ctxt, sym.Size, []byte{x})
+	}
+	writeUint32 := func(x uint32) {
+		binary.LittleEndian.PutUint32(b[:], x)
+		sym.WriteBytes(ctxt, sym.Size, b[:4])
+	}
+	writeInt64 := func(x int64) {
+		binary.LittleEndian.PutUint64(b[:], uint64(x))
+		sym.WriteBytes(ctxt, sym.Size, b[:])
+	}
+	writeString := func(s string) {
+		writeUint32(uint32(len(s)))
+		sym.WriteString(ctxt, sym.Size, len(s), s)
+	}
+	writeString(wi.Module)
+	writeString(wi.Name)
+	writeUint32(uint32(len(wi.Params)))
+	for _, f := range wi.Params {
+		writeByte(byte(f.Type))
+		writeInt64(f.Offset)
+	}
+	writeUint32(uint32(len(wi.Results)))
+	for _, f := range wi.Results {
+		writeByte(byte(f.Type))
+		writeInt64(f.Offset)
+	}
+
+	return &sym
+}
+
+type WasmField struct {
+	Type WasmFieldType
+	// Offset holds the frame-pointer-relative locations for Go's stack-based
+	// ABI. This is used by the src/cmd/internal/wasm package to map WASM
+	// import parameters to the Go stack in a wrapper function.
+	Offset int64
+}
+
+type WasmFieldType byte
+
+const (
+	WasmI32 WasmFieldType = iota
+	WasmI64
+	WasmF32
+	WasmF64
+	WasmPtr
+)
+
 type InlMark struct {
 	// When unwinding from an instruction in an inlined body, mark
 	// where we should unwind to.
diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index 73c29d9686..78fa4c1076 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -605,7 +605,12 @@ func (w *writer) Aux(s *LSym) {
 		for _, pcSym := range fn.Pcln.Pcdata {
 			w.aux1(goobj.AuxPcdata, pcSym)
 		}
-
+		if fn.WasmImportSym != nil {
+			if fn.WasmImportSym.Size == 0 {
+				panic("wasmimport aux sym must have non-zero size")
+			}
+			w.aux1(goobj.AuxWasmImport, fn.WasmImportSym)
+		}
 	}
 }
 
@@ -703,6 +708,12 @@ func nAuxSym(s *LSym) int {
 			n++
 		}
 		n += len(fn.Pcln.Pcdata)
+		if fn.WasmImport != nil {
+			if fn.WasmImportSym == nil || fn.WasmImportSym.Size == 0 {
+				panic("wasmimport aux sym must exist and have non-zero size")
+			}
+			n++
+		}
 	}
 	return n
 }
@@ -759,8 +770,8 @@ func genFuncInfoSyms(ctxt *Link) {
 		fn.FuncInfoSym = isym
 		b.Reset()
 
-		dwsyms := []*LSym{fn.dwarfRangesSym, fn.dwarfLocSym, fn.dwarfDebugLinesSym, fn.dwarfInfoSym}
-		for _, s := range dwsyms {
+		auxsyms := []*LSym{fn.dwarfRangesSym, fn.dwarfLocSym, fn.dwarfDebugLinesSym, fn.dwarfInfoSym, fn.WasmImportSym}
+		for _, s := range auxsyms {
 			if s == nil || s.Size == 0 {
 				continue
 			}
diff --git a/src/cmd/internal/obj/sym.go b/src/cmd/internal/obj/sym.go
index e0817d5f74..4a01af3927 100644
--- a/src/cmd/internal/obj/sym.go
+++ b/src/cmd/internal/obj/sym.go
@@ -416,16 +416,16 @@ func (ctxt *Link) traverseFuncAux(flag traverseFlag, fsym *LSym, fn func(parent
 		}
 	}
 
-	dwsyms := []*LSym{fninfo.dwarfRangesSym, fninfo.dwarfLocSym, fninfo.dwarfDebugLinesSym, fninfo.dwarfInfoSym}
-	for _, dws := range dwsyms {
-		if dws == nil || dws.Size == 0 {
+	auxsyms := []*LSym{fninfo.dwarfRangesSym, fninfo.dwarfLocSym, fninfo.dwarfDebugLinesSym, fninfo.dwarfInfoSym, fninfo.WasmImportSym}
+	for _, s := range auxsyms {
+		if s == nil || s.Size == 0 {
 			continue
 		}
-		fn(fsym, dws)
+		fn(fsym, s)
 		if flag&traverseRefs != 0 {
-			for _, r := range dws.R {
+			for _, r := range s.R {
 				if r.Sym != nil {
-					fn(dws, r.Sym)
+					fn(s, r.Sym)
 				}
 			}
 		}
diff --git a/src/cmd/internal/obj/wasm/a.out.go b/src/cmd/internal/obj/wasm/a.out.go
index 83ce0a6738..0262630d5a 100644
--- a/src/cmd/internal/obj/wasm/a.out.go
+++ b/src/cmd/internal/obj/wasm/a.out.go
@@ -18,8 +18,7 @@ const (
  *	wasm
  */
 const (
-	ACallImport = obj.ABaseWasm + obj.A_ARCHSPECIFIC + iota
-	AGet
+	AGet = obj.ABaseWasm + obj.A_ARCHSPECIFIC + iota
 	ASet
 	ATee
 	ANot // alias for I32Eqz
diff --git a/src/cmd/internal/obj/wasm/anames.go b/src/cmd/internal/obj/wasm/anames.go
index c9bc15d270..6f1a662960 100644
--- a/src/cmd/internal/obj/wasm/anames.go
+++ b/src/cmd/internal/obj/wasm/anames.go
@@ -5,8 +5,7 @@ package wasm
 import "cmd/internal/obj"
 
 var Anames = []string{
-	obj.A_ARCHSPECIFIC: "CallImport",
-	"Get",
+	obj.A_ARCHSPECIFIC: "Get",
 	"Set",
 	"Tee",
 	"Not",
diff --git a/src/cmd/internal/obj/wasm/wasmobj.go b/src/cmd/internal/obj/wasm/wasmobj.go
index 96a2ef4a6f..fd0faec84b 100644
--- a/src/cmd/internal/obj/wasm/wasmobj.go
+++ b/src/cmd/internal/obj/wasm/wasmobj.go
@@ -100,7 +100,6 @@ var unaryDst = map[obj.As]bool{
 	ATee:          true,
 	ACall:         true,
 	ACallIndirect: true,
-	ACallImport:   true,
 	ABr:           true,
 	ABrIf:         true,
 	ABrTable:      true,
@@ -135,6 +134,14 @@ const (
 	WasmImport = 1 << 0
 )
 
+const (
+	// This is a special wasm module name that when used as the module name
+	// in //go:wasmimport will cause the generated code to pass the stack pointer
+	// directly to the imported function. In other words, any function that
+	// uses the gojs module understands the internal Go WASM ABI directly.
+	GojsModule = "gojs"
+)
+
 func instinit(ctxt *obj.Link) {
 	morestack = ctxt.Lookup("runtime.morestack")
 	morestackNoCtxt = ctxt.Lookup("runtime.morestack_noctxt")
@@ -177,7 +184,121 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 	s.Func().Args = s.Func().Text.To.Val.(int32)
 	s.Func().Locals = int32(framesize)
 
-	if s.Func().Text.From.Sym.Wrapper() {
+	// If the function exits just to call out to a wasmimport, then
+	// generate the code to translate from our internal Go-stack
+	// based call convention to the native webassembly call convention.
+	if wi := s.Func().WasmImport; wi != nil {
+		s.Func().WasmImportSym = wi.CreateSym(ctxt)
+		p := s.Func().Text
+		if p.Link != nil {
+			panic("wrapper functions for WASM imports should not have a body")
+		}
+		to := obj.Addr{
+			Type: obj.TYPE_MEM,
+			Name: obj.NAME_EXTERN,
+			Sym:  s,
+		}
+
+		// If the module that the import is for is our magic "gojs" module, then this
+		// indicates that the called function understands the Go stack-based call convention
+		// so we just pass the stack pointer to it, knowing it will read the params directly
+		// off the stack and push the results into memory based on the stack pointer.
+		if wi.Module == GojsModule {
+			// The called function has a signature of 'func(sp int)'. It has access to the memory
+			// value somewhere to be able to address the memory based on the "sp" value.
+
+			p = appendp(p, AGet, regAddr(REG_SP))
+			p = appendp(p, ACall, to)
+
+			p.Mark = WasmImport
+		} else {
+			if len(wi.Results) > 1 {
+				// TODO(evanphx) implement support for the multi-value proposal:
+				// https://github.com/WebAssembly/multi-value/blob/master/proposals/multi-value/Overview.md
+				panic("invalid results type") // impossible until multi-value proposal has landed
+			}
+			if len(wi.Results) == 1 {
+				// If we have a result (rather than returning nothing at all), then
+				// we'll write the result to the Go stack relative to the current stack pointer.
+				// We cache the current stack pointer value on the wasm stack here and then use
+				// it after the Call instruction to store the result.
+				p = appendp(p, AGet, regAddr(REG_SP))
+			}
+			for _, f := range wi.Params {
+				// Each load instructions will consume the value of sp on the stack, so
+				// we need to read sp for each param. WASM appears to not have a stack dup instruction
+				// (a strange ommission for a stack-based VM), if it did, we'd be using the dup here.
+				p = appendp(p, AGet, regAddr(REG_SP))
+
+				// Offset is the location of the param on the Go stack (ie relative to sp).
+				// Because of our call convention, the parameters are located an additional 8 bytes
+				// from sp because we store the return address as a int64 at the bottom of the stack.
+				// Ie the stack looks like [return_addr, param3, param2, param1, etc]
+
+				// Ergo, we add 8 to the true byte offset of the param to skip the return address.
+				loadOffset := f.Offset + 8
+
+				// We're reading the value from the Go stack onto the WASM stack and leaving it there
+				// for CALL to pick them up.
+				switch f.Type {
+				case obj.WasmI32:
+					p = appendp(p, AI32Load, constAddr(loadOffset))
+				case obj.WasmI64:
+					p = appendp(p, AI64Load, constAddr(loadOffset))
+				case obj.WasmF32:
+					p = appendp(p, AF32Load, constAddr(loadOffset))
+				case obj.WasmF64:
+					p = appendp(p, AF64Load, constAddr(loadOffset))
+				case obj.WasmPtr:
+					p = appendp(p, AI64Load, constAddr(loadOffset))
+					p = appendp(p, AI32WrapI64)
+				default:
+					panic("bad param type")
+				}
+			}
+
+			// The call instruction is marked as being for a wasm import so that a later phase
+			// will generate relocation information that allows us to patch this with then
+			// offset of the imported function in the wasm imports.
+			p = appendp(p, ACall, to)
+			p.Mark = WasmImport
+
+			if len(wi.Results) == 1 {
+				f := wi.Results[0]
+
+				// Much like with the params, we need to adjust the offset we store the result value
+				// to by 8 bytes to account for the return address on the Go stack.
+				storeOffset := f.Offset + 8
+
+				// This code is paired the code above that reads the stack pointer onto the wasm
+				// stack. We've done this so we have a consistent view of the sp value as it might
+				// be manipulated by the call and we want to ignore that manipulation here.
+				switch f.Type {
+				case obj.WasmI32:
+					p = appendp(p, AI32Store, constAddr(storeOffset))
+				case obj.WasmI64:
+					p = appendp(p, AI64Store, constAddr(storeOffset))
+				case obj.WasmF32:
+					p = appendp(p, AF32Store, constAddr(storeOffset))
+				case obj.WasmF64:
+					p = appendp(p, AF64Store, constAddr(storeOffset))
+				case obj.WasmPtr:
+					p = appendp(p, AI64ExtendI32U)
+					p = appendp(p, AI64Store, constAddr(storeOffset))
+				default:
+					panic("bad result type")
+				}
+			}
+		}
+
+		p = appendp(p, obj.ARET)
+
+		// It should be 0 already, but we'll set it to 0 anyway just to be sure
+		// that the code below which adds frame expansion code to the function body
+		// isn't run. We don't want the frame expansion code because our function
+		// body is just the code to translate and call the imported function.
+		framesize = 0
+	} else if s.Func().Text.From.Sym.Wrapper() {
 		// if g._panic != nil && g._panic.argp == FP {
 		//   g._panic.argp = bottom-of-frame
 		// }
@@ -241,7 +362,9 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 		p.Spadj = int32(framesize)
 	}
 
-	needMoreStack := !s.Func().Text.From.Sym.NoSplit()
+	// If the framesize is 0, then imply nosplit because it's a specially
+	// generated function.
+	needMoreStack := framesize > 0 && !s.Func().Text.From.Sym.NoSplit()
 
 	// If the maymorestack debug option is enabled, insert the
 	// call to maymorestack *before* processing resume points so
@@ -707,12 +830,6 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			default:
 				panic("bad MOV type")
 			}
-
-		case ACallImport:
-			p.As = obj.ANOP
-			p = appendp(p, AGet, regAddr(REG_SP))
-			p = appendp(p, ACall, obj.Addr{Type: obj.TYPE_MEM, Name: obj.NAME_EXTERN, Sym: s})
-			p.Mark = WasmImport
 		}
 	}
 
diff --git a/src/cmd/link/internal/loader/loader.go b/src/cmd/link/internal/loader/loader.go
index e3ee819a9d..fa8c0c6b20 100644
--- a/src/cmd/link/internal/loader/loader.go
+++ b/src/cmd/link/internal/loader/loader.go
@@ -1618,6 +1618,29 @@ func (l *Loader) Aux(i Sym, j int) Aux {
 	return Aux{r.Aux(li, j), r, l}
 }
 
+// WasmImportSym returns the auxiliary WebAssembly import symbol associated with
+// a given function symbol. The aux sym only exists for Go function stubs that
+// have been annotated with the //go:wasmimport directive.  The aux sym
+// contains the information necessary for the linker to add a WebAssembly
+// import statement.
+// (https://webassembly.github.io/spec/core/syntax/modules.html#imports)
+func (l *Loader) WasmImportSym(fnSymIdx Sym) (Sym, bool) {
+	if l.SymType(fnSymIdx) != sym.STEXT {
+		log.Fatalf("error: non-function sym %d/%s t=%s passed to WasmImportSym", fnSymIdx, l.SymName(fnSymIdx), l.SymType(fnSymIdx).String())
+	}
+	r, li := l.toLocal(fnSymIdx)
+	auxs := r.Auxs(li)
+	for i := range auxs {
+		a := &auxs[i]
+		switch a.Type() {
+		case goobj.AuxWasmImport:
+			return l.resolve(r, a.Sym()), true
+		}
+	}
+
+	return 0, false
+}
+
 // GetFuncDwarfAuxSyms collects and returns the auxiliary DWARF
 // symbols associated with a given function symbol.  Prior to the
 // introduction of the loader, this was done purely using name
diff --git a/src/cmd/link/internal/wasm/asm.go b/src/cmd/link/internal/wasm/asm.go
index b5685701f2..30d0dc7ff2 100644
--- a/src/cmd/link/internal/wasm/asm.go
+++ b/src/cmd/link/internal/wasm/asm.go
@@ -6,10 +6,14 @@ package wasm
 
 import (
 	"bytes"
+	"cmd/internal/obj"
+	"cmd/internal/obj/wasm"
 	"cmd/internal/objabi"
 	"cmd/link/internal/ld"
 	"cmd/link/internal/loader"
 	"cmd/link/internal/sym"
+	"encoding/binary"
+	"fmt"
 	"internal/buildcfg"
 	"io"
 	"regexp"
@@ -44,9 +48,10 @@ func gentext(ctxt *ld.Link, ldr *loader.Loader) {
 }
 
 type wasmFunc struct {
-	Name string
-	Type uint32
-	Code []byte
+	Module string
+	Name   string
+	Type   uint32
+	Code   []byte
 }
 
 type wasmFuncType struct {
@@ -54,6 +59,59 @@ type wasmFuncType struct {
 	Results []byte
 }
 
+func readWasmImport(ldr *loader.Loader, s loader.Sym) obj.WasmImport {
+	reportError := func(err error) { panic(fmt.Sprintf("failed to read WASM import in sym %v: %v", s, err)) }
+
+	data := ldr.Data(s)
+
+	readUint32 := func() (v uint32) {
+		v = binary.LittleEndian.Uint32(data)
+		data = data[4:]
+		return
+	}
+
+	readUint64 := func() (v uint64) {
+		v = binary.LittleEndian.Uint64(data)
+		data = data[8:]
+		return
+	}
+
+	readByte := func() byte {
+		if len(data) == 0 {
+			reportError(io.EOF)
+		}
+
+		b := data[0]
+		data = data[1:]
+		return b
+	}
+
+	readString := func() string {
+		n := readUint32()
+
+		s := string(data[:n])
+
+		data = data[n:]
+
+		return s
+	}
+
+	var wi obj.WasmImport
+	wi.Module = readString()
+	wi.Name = readString()
+	wi.Params = make([]obj.WasmField, readUint32())
+	for i := range wi.Params {
+		wi.Params[i].Type = obj.WasmFieldType(readByte())
+		wi.Params[i].Offset = int64(readUint64())
+	}
+	wi.Results = make([]obj.WasmField, readUint32())
+	for i := range wi.Results {
+		wi.Results[i].Type = obj.WasmFieldType(readByte())
+		wi.Results[i].Offset = int64(readUint64())
+	}
+	return wi
+}
+
 var wasmFuncTypes = map[string]*wasmFuncType{
 	"_rt0_wasm_js":            {Params: []byte{}},                                         //
 	"wasm_export_run":         {Params: []byte{I32, I32}},                                 // argc, argv
@@ -136,23 +194,30 @@ func asmb2(ctxt *ld.Link, ldr *loader.Loader) {
 	}
 
 	// collect host imports (functions that get imported from the WebAssembly host, usually JavaScript)
-	hostImports := []*wasmFunc{
-		{
-			Name: "debug",
-			Type: lookupType(&wasmFuncType{Params: []byte{I32}}, &types),
-		},
-	}
+	// we store the import index of each imported function, so the R_WASMIMPORT relocation
+	// can write the correct index after a "call" instruction
+	// these are added as import statements to the top of the WebAssembly binary
+	var hostImports []*wasmFunc
 	hostImportMap := make(map[loader.Sym]int64)
 	for _, fn := range ctxt.Textp {
 		relocs := ldr.Relocs(fn)
 		for ri := 0; ri < relocs.Count(); ri++ {
 			r := relocs.At(ri)
 			if r.Type() == objabi.R_WASMIMPORT {
-				hostImportMap[r.Sym()] = int64(len(hostImports))
-				hostImports = append(hostImports, &wasmFunc{
-					Name: ldr.SymName(r.Sym()),
-					Type: lookupType(&wasmFuncType{Params: []byte{I32}}, &types),
-				})
+				if lsym, ok := ldr.WasmImportSym(fn); ok {
+					wi := readWasmImport(ldr, lsym)
+					hostImportMap[fn] = int64(len(hostImports))
+					hostImports = append(hostImports, &wasmFunc{
+						Module: wi.Module,
+						Name:   wi.Name,
+						Type: lookupType(&wasmFuncType{
+							Params:  fieldsToTypes(wi.Params),
+							Results: fieldsToTypes(wi.Results),
+						}, &types),
+					})
+				} else {
+					panic(fmt.Sprintf("missing wasm symbol for %s", ldr.SymName(r.Sym())))
+				}
 			}
 		}
 	}
@@ -288,7 +353,11 @@ func writeImportSec(ctxt *ld.Link, hostImports []*wasmFunc) {
 
 	writeUleb128(ctxt.Out, uint64(len(hostImports))) // number of imports
 	for _, fn := range hostImports {
-		writeName(ctxt.Out, "go") // provided by the import object in wasm_exec.js
+		if fn.Module != "" {
+			writeName(ctxt.Out, fn.Module)
+		} else {
+			writeName(ctxt.Out, wasm.GojsModule) // provided by the import object in wasm_exec.js
+		}
 		writeName(ctxt.Out, fn.Name)
 		ctxt.Out.WriteByte(0x00) // func import
 		writeUleb128(ctxt.Out, uint64(fn.Type))
@@ -610,3 +679,22 @@ func writeSleb128(w io.ByteWriter, v int64) {
 		w.WriteByte(c)
 	}
 }
+
+func fieldsToTypes(fields []obj.WasmField) []byte {
+	b := make([]byte, len(fields))
+	for i, f := range fields {
+		switch f.Type {
+		case obj.WasmI32, obj.WasmPtr:
+			b[i] = I32
+		case obj.WasmI64:
+			b[i] = I64
+		case obj.WasmF32:
+			b[i] = F32
+		case obj.WasmF64:
+			b[i] = F64
+		default:
+			panic(fmt.Sprintf("fieldsToTypes: unknown field type: %d", f.Type))
+		}
+	}
+	return b
+}