cmd/compile: add //go:embed support

This commit contains the compiler support for //go:embed lines.
The go command passes to the compiler an "embed config"
that maps literal patterns like *.txt to the set of files to embed.
The compiler then lays out the content of those files as static data
in the form of an embed.Files or string or []byte in the final object file.

The test for this code is the end-to-end test hooking up the
embed, cmd/compile, and cmd/go changes, in the next CL.

For #41191.

Change-Id: I916e57f8cc65871dc0044c13d3f90c252a3fe1bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/243944
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -14,6 +14,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"io/ioutil"
+	"os"
 	"sort"
 	"strconv"
 )
@@ -125,6 +127,7 @@ func dumpdata() {
 	itabsLen := len(itabs)
 	dumpimportstrings()
 	dumpbasictypes()
+	dumpembeds()
 
 	// Calls to dumpsignats can generate functions,
 	// like method wrappers and hash and equality routines.
@@ -358,28 +361,31 @@ func dbvec(s *obj.LSym, off int, bv bvec) int {
 	return off
 }
 
+const (
+	stringSymPrefix  = "go.string."
+	stringSymPattern = ".gostring.%d.%x"
+)
+
+// stringsym returns a symbol containing the string s.
+// The symbol contains the string data, not a string header.
 func stringsym(pos src.XPos, s string) (data *obj.LSym) {
 	var symname string
 	if len(s) > 100 {
 		// Huge strings are hashed to avoid long names in object files.
 		// Indulge in some paranoia by writing the length of s, too,
 		// as protection against length extension attacks.
+		// Same pattern is known to fileStringSym below.
 		h := sha256.New()
 		io.WriteString(h, s)
-		symname = fmt.Sprintf(".gostring.%d.%x", len(s), h.Sum(nil))
+		symname = fmt.Sprintf(stringSymPattern, len(s), h.Sum(nil))
 	} else {
 		// Small strings get named directly by their contents.
 		symname = strconv.Quote(s)
 	}
 
-	const prefix = "go.string."
-	symdataname := prefix + symname
-
-	symdata := Ctxt.Lookup(symdataname)
-
+	symdata := Ctxt.Lookup(stringSymPrefix + symname)
 	if !symdata.OnList() {
-		// string data
-		off := dsname(symdata, 0, s, pos, "string")
+		off := dstringdata(symdata, 0, s, pos, "string")
 		ggloblsym(symdata, int32(off), obj.DUPOK|obj.RODATA|obj.LOCAL)
 		symdata.Set(obj.AttrContentAddressable, true)
 	}
@@ -387,26 +393,122 @@ func stringsym(pos src.XPos, s string) (data *obj.LSym) {
 	return symdata
 }
 
-var slicebytes_gen int
+// fileStringSym returns a symbol for the contents and the size of file.
+// If readonly is true, the symbol shares storage with any literal string
+// or other file with the same content and is placed in a read-only section.
+// If readonly is false, the symbol is a read-write copy separate from any other,
+// for use as the backing store of a []byte.
+// The content hash of file is copied into hash. (If hash is nil, nothing is copied.)
+// The returned symbol contains the data itself, not a string header.
+func fileStringSym(pos src.XPos, file string, readonly bool, hash []byte) (*obj.LSym, int64, error) {
+	f, err := os.Open(file)
+	if err != nil {
+		return nil, 0, err
+	}
+	defer f.Close()
+	info, err := f.Stat()
+	if err != nil {
+		return nil, 0, err
+	}
+	if !info.Mode().IsRegular() {
+		return nil, 0, fmt.Errorf("not a regular file")
+	}
+	size := info.Size()
+	if size <= 1*1024 {
+		data, err := ioutil.ReadAll(f)
+		if err != nil {
+			return nil, 0, err
+		}
+		if int64(len(data)) != size {
+			return nil, 0, fmt.Errorf("file changed between reads")
+		}
+		var sym *obj.LSym
+		if readonly {
+			sym = stringsym(pos, string(data))
+		} else {
+			sym = slicedata(pos, string(data)).Sym.Linksym()
+		}
+		if len(hash) > 0 {
+			sum := sha256.Sum256(data)
+			copy(hash, sum[:])
+		}
+		return sym, size, nil
+	}
+	if size > 2e9 {
+		// ggloblsym takes an int32,
+		// and probably the rest of the toolchain
+		// can't handle such big symbols either.
+		// See golang.org/issue/9862.
+		return nil, 0, fmt.Errorf("file too large")
+	}
 
-func slicebytes(nam *Node, s string) {
-	slicebytes_gen++
-	symname := fmt.Sprintf(".gobytes.%d", slicebytes_gen)
+	// File is too big to read and keep in memory.
+	// Compute hash if needed for read-only content hashing or if the caller wants it.
+	var sum []byte
+	if readonly || len(hash) > 0 {
+		h := sha256.New()
+		n, err := io.Copy(h, f)
+		if err != nil {
+			return nil, 0, err
+		}
+		if n != size {
+			return nil, 0, fmt.Errorf("file changed between reads")
+		}
+		sum = h.Sum(nil)
+		copy(hash, sum)
+	}
+
+	var symdata *obj.LSym
+	if readonly {
+		symname := fmt.Sprintf(stringSymPattern, size, sum)
+		symdata = Ctxt.Lookup(stringSymPrefix + symname)
+		if !symdata.OnList() {
+			info := symdata.NewFileInfo()
+			info.Name = file
+			info.Size = size
+			ggloblsym(symdata, int32(size), obj.DUPOK|obj.RODATA|obj.LOCAL)
+			// Note: AttrContentAddressable cannot be set here,
+			// because the content-addressable-handling code
+			// does not know about file symbols.
+		}
+	} else {
+		// Emit a zero-length data symbol
+		// and then fix up length and content to use file.
+		symdata = slicedata(pos, "").Sym.Linksym()
+		symdata.Size = size
+		symdata.Type = objabi.SNOPTRDATA
+		info := symdata.NewFileInfo()
+		info.Name = file
+		info.Size = size
+	}
+
+	return symdata, size, nil
+}
+
+var slicedataGen int
+
+func slicedata(pos src.XPos, s string) *Node {
+	slicedataGen++
+	symname := fmt.Sprintf(".gobytes.%d", slicedataGen)
 	sym := localpkg.Lookup(symname)
 	symnode := newname(sym)
 	sym.Def = asTypesNode(symnode)
 
 	lsym := sym.Linksym()
-	off := dsname(lsym, 0, s, nam.Pos, "slice")
+	off := dstringdata(lsym, 0, s, pos, "slice")
 	ggloblsym(lsym, int32(off), obj.NOPTR|obj.LOCAL)
 
+	return symnode
+}
+
+func slicebytes(nam *Node, s string) {
 	if nam.Op != ONAME {
 		Fatalf("slicebytes %v", nam)
 	}
-	slicesym(nam, symnode, int64(len(s)))
+	slicesym(nam, slicedata(nam.Pos, s), int64(len(s)))
 }
 
-func dsname(s *obj.LSym, off int, t string, pos src.XPos, what string) int {
+func dstringdata(s *obj.LSym, off int, t string, pos src.XPos, what string) int {
 	// Objects that are too large will cause the data section to overflow right away,
 	// causing a cryptic error message by the linker. Check for oversize objects here
 	// and provide a useful error message instead.