asm.go - Android社区 - https://www.androidos.net.cn/

// 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.

package wasm

import (
	"bytes"
	"cmd/internal/objabi"
	"cmd/link/internal/ld"
	"cmd/link/internal/sym"
	"io"
	"regexp"
	"runtime"
)

const (
	I32 = 0x7F
	I64 = 0x7E
	F32 = 0x7D
	F64 = 0x7C
)

const (
	sectionCustom   = 0
	sectionType     = 1
	sectionImport   = 2
	sectionFunction = 3
	sectionTable    = 4
	sectionMemory   = 5
	sectionGlobal   = 6
	sectionExport   = 7
	sectionStart    = 8
	sectionElement  = 9
	sectionCode     = 10
	sectionData     = 11
)

// funcValueOffset is the offset between the PC_F value of a function and the index of the function in WebAssembly
const funcValueOffset = 0x1000 // TODO(neelance): make function addresses play nice with heap addresses

func gentext(ctxt *ld.Link) {
}

type wasmFunc struct {
	Name string
	Type uint32
	Code []byte
}

type wasmFuncType struct {
	Params  []byte
	Results []byte
}

var wasmFuncTypes = map[string]*wasmFuncType{
	"_rt0_wasm_js":           &wasmFuncType{Params: []byte{}},                                         //
	"wasm_export_run":        &wasmFuncType{Params: []byte{I32, I32}},                                 // argc, argv
	"wasm_export_resume":     &wasmFuncType{Params: []byte{}},                                         //
	"wasm_export_getsp":      &wasmFuncType{Results: []byte{I32}},                                     // sp
	"wasm_pc_f_loop":         &wasmFuncType{Params: []byte{}},                                         //
	"runtime.wasmMove":       &wasmFuncType{Params: []byte{I32, I32, I32}},                            // dst, src, len
	"runtime.wasmZero":       &wasmFuncType{Params: []byte{I32, I32}},                                 // ptr, len
	"runtime.wasmDiv":        &wasmFuncType{Params: []byte{I64, I64}, Results: []byte{I64}},           // x, y -> x/y
	"runtime.wasmTruncS":     &wasmFuncType{Params: []byte{F64}, Results: []byte{I64}},                // x -> int(x)
	"runtime.wasmTruncU":     &wasmFuncType{Params: []byte{F64}, Results: []byte{I64}},                // x -> uint(x)
	"runtime.gcWriteBarrier": &wasmFuncType{Params: []byte{I64, I64}},                                 // ptr, val
	"cmpbody":                &wasmFuncType{Params: []byte{I64, I64, I64, I64}, Results: []byte{I64}}, // a, alen, b, blen -> -1/0/1
	"memeqbody":              &wasmFuncType{Params: []byte{I64, I64, I64}, Results: []byte{I64}},      // a, b, len -> 0/1
	"memcmp":                 &wasmFuncType{Params: []byte{I32, I32, I32}, Results: []byte{I32}},      // a, b, len -> <0/0/>0
	"memchr":                 &wasmFuncType{Params: []byte{I32, I32, I32}, Results: []byte{I32}},      // s, c, len -> index
}

func assignAddress(ctxt *ld.Link, sect *sym.Section, n int, s *sym.Symbol, va uint64, isTramp bool) (*sym.Section, int, uint64) {
	// WebAssembly functions do not live in the same address space as the linear memory.
	// Instead, WebAssembly automatically assigns indices. Imported functions (section "import")
	// have indices 0 to n. They are followed by native functions (sections "function" and "code")
	// with indices n+1 and following.
	//
	// The following rules describe how wasm handles function indices and addresses:
	//   PC_F = funcValueOffset + WebAssembly function index (not including the imports)
	//   s.Value = PC = PC_F<<16 + PC_B
	//
	// The funcValueOffset is necessary to avoid conflicts with expectations
	// that the Go runtime has about function addresses.
	// The field "s.Value" corresponds to the concept of PC at runtime.
	// However, there is no PC register, only PC_F and PC_B. PC_F denotes the function,
	// PC_B the resume point inside of that function. The entry of the function has PC_B = 0.
	s.Sect = sect
	s.Value = int64(funcValueOffset+va/ld.MINFUNC) << 16 // va starts at zero
	va += uint64(ld.MINFUNC)
	return sect, n, va
}

// asmb writes the final WebAssembly module binary.
// Spec: https://webassembly.github.io/spec/core/binary/modules.html
func asmb(ctxt *ld.Link) {
	if ctxt.Debugvlog != 0 {
		ctxt.Logf("%5.2f asmb\n", ld.Cputime())
	}

types := []*wasmFuncType{
		// For normal Go functions the return value is
		// 0 if the function returned normally or
		// 1 if the stack needs to be unwound.
		&wasmFuncType{Results: []byte{I32}},
	}

// collect host imports (functions that get imported from the WebAssembly host, usually JavaScript)
	hostImports := []*wasmFunc{
		&wasmFunc{
			Name: "debug",
			Type: lookupType(&wasmFuncType{Params: []byte{I32}}, &types),
		},
	}
	hostImportMap := make(map[*sym.Symbol]int64)
	for _, fn := range ctxt.Textp {
		for _, r := range fn.R {
			if r.Type == objabi.R_WASMIMPORT {
				hostImportMap[r.Sym] = int64(len(hostImports))
				hostImports = append(hostImports, &wasmFunc{
					Name: r.Sym.Name,
					Type: lookupType(&wasmFuncType{Params: []byte{I32}}, &types),
				})
			}
		}
	}

// collect functions with WebAssembly body
	var buildid []byte
	fns := make([]*wasmFunc, len(ctxt.Textp))
	for i, fn := range ctxt.Textp {
		wfn := new(bytes.Buffer)
		if fn.Name == "go.buildid" {
			writeUleb128(wfn, 0) // number of sets of locals
			writeI32Const(wfn, 0)
			wfn.WriteByte(0x0b) // end
			buildid = fn.P
		} else {
			// Relocations have variable length, handle them here.
			off := int32(0)
			for _, r := range fn.R {
				wfn.Write(fn.P[off:r.Off])
				off = r.Off
				switch r.Type {
				case objabi.R_ADDR:
					writeSleb128(wfn, r.Sym.Value+r.Add)
				case objabi.R_CALL:
					writeSleb128(wfn, int64(len(hostImports))+r.Sym.Value>>16-funcValueOffset)
				case objabi.R_WASMIMPORT:
					writeSleb128(wfn, hostImportMap[r.Sym])
				default:
					ld.Errorf(fn, "bad reloc type %d (%s)", r.Type, sym.RelocName(ctxt.Arch, r.Type))
					continue
				}
			}
			wfn.Write(fn.P[off:])
		}

typ := uint32(0)
		if sig, ok := wasmFuncTypes[fn.Name]; ok {
			typ = lookupType(sig, &types)
		}

name := nameRegexp.ReplaceAllString(fn.Name, "_")
		fns[i] = &wasmFunc{Name: name, Type: typ, Code: wfn.Bytes()}
	}

ctxt.Out.Write([]byte{0x00, 0x61, 0x73, 0x6d}) // magic
	ctxt.Out.Write([]byte{0x01, 0x00, 0x00, 0x00}) // version

// Add any buildid early in the binary:
	if len(buildid) != 0 {
		writeBuildID(ctxt, buildid)
	}

writeGoVersion(ctxt)
	writeTypeSec(ctxt, types)
	writeImportSec(ctxt, hostImports)
	writeFunctionSec(ctxt, fns)
	writeTableSec(ctxt, fns)
	writeMemorySec(ctxt)
	writeGlobalSec(ctxt)
	writeExportSec(ctxt, len(hostImports))
	writeElementSec(ctxt, uint64(len(hostImports)), uint64(len(fns)))
	writeCodeSec(ctxt, fns)
	writeDataSec(ctxt)
	if !*ld.FlagS {
		writeNameSec(ctxt, len(hostImports), fns)
	}

ctxt.Out.Flush()
}

func lookupType(sig *wasmFuncType, types *[]*wasmFuncType) uint32 {
	for i, t := range *types {
		if bytes.Equal(sig.Params, t.Params) && bytes.Equal(sig.Results, t.Results) {
			return uint32(i)
		}
	}
	*types = append(*types, sig)
	return uint32(len(*types) - 1)
}

func writeSecHeader(ctxt *ld.Link, id uint8) int64 {
	ctxt.Out.WriteByte(id)
	sizeOffset := ctxt.Out.Offset()
	ctxt.Out.Write(make([]byte, 5)) // placeholder for length
	return sizeOffset
}

func writeSecSize(ctxt *ld.Link, sizeOffset int64) {
	endOffset := ctxt.Out.Offset()
	ctxt.Out.SeekSet(sizeOffset)
	writeUleb128FixedLength(ctxt.Out, uint64(endOffset-sizeOffset-5), 5)
	ctxt.Out.SeekSet(endOffset)
}

func writeBuildID(ctxt *ld.Link, buildid []byte) {
	sizeOffset := writeSecHeader(ctxt, sectionCustom)
	writeName(ctxt.Out, "go.buildid")
	ctxt.Out.Write(buildid)
	writeSecSize(ctxt, sizeOffset)
}

func writeGoVersion(ctxt *ld.Link) {
	sizeOffset := writeSecHeader(ctxt, sectionCustom)
	writeName(ctxt.Out, "go.version")
	ctxt.Out.Write([]byte(runtime.Version()))
	writeSecSize(ctxt, sizeOffset)
}

// writeTypeSec writes the section that declares all function types
// so they can be referenced by index.
func writeTypeSec(ctxt *ld.Link, types []*wasmFuncType) {
	sizeOffset := writeSecHeader(ctxt, sectionType)

writeUleb128(ctxt.Out, uint64(len(types)))

for _, t := range types {
		ctxt.Out.WriteByte(0x60) // functype
		writeUleb128(ctxt.Out, uint64(len(t.Params)))
		for _, v := range t.Params {
			ctxt.Out.WriteByte(byte(v))
		}
		writeUleb128(ctxt.Out, uint64(len(t.Results)))
		for _, v := range t.Results {
			ctxt.Out.WriteByte(byte(v))
		}
	}