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>

2 files changed, 126 insertions, 15 deletions

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
+}