cmd/link: rewrite pe symbol table generating code

Every go executable has COFF symbol table appended at the end. The table is
used by nm and addr2line and contains all symbols present in the executable.
The table is quite large. For example, my go.exe has 11736 records.

To generate symbol table:
1) we walk "all symbols" list to count symbols we want for the table;
2) we allocate large global array of COFFSym structs (32 bytes each)
   to fit our symbols;
3) we walk "all symbols" list again to fill our array with contents;
4) we iterate over our global array to write all records to the file.

This CL changes all these steps with single step:
- walk "all symbols" list and write each COFF symbol table record to
  the file as we go.

I hope new version is faster and uses less garbage, but I don't know
how to benchmark this.

Change-Id: Ie4870583250131ea4428e0e83a0696c9df1794e0
Reviewed-on: https://go-review.googlesource.com/20580Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>

src/cmd/link/internal/ld/pe.go

@@ -375,18 +375,6 @@ var dexport [1024]*LSym
 
 var nexport int
 
-type COFFSym struct {
-	sym       *LSym
-	strtbloff int
-	sect      int
-	value     int64
-	typ       uint16
-}
-
-var coffsym []COFFSym
-
-var ncoffsym int
-
 func addpesection(name string, sectsize int, filesize int) *IMAGE_SECTION_HEADER {
 	if pensect == 16 {
 		Diag("too many sections")
@@ -922,59 +910,73 @@ func newPEDWARFSection(name string, size int64) *IMAGE_SECTION_HEADER {
 	return h
 }
 
-func addpesym(s *LSym, name string, type_ int, addr int64, size int64, ver int, gotype *LSym) {
-	if s == nil {
-		return
-	}
+// writePESymTableRecords writes all COFF symbol table records.
+// It returns number of records written.
+func writePESymTableRecords() int {
+	var symcnt int
 
-	if s.Sect == nil && type_ != 'U' {
-		return
-	}
-
-	switch type_ {
-	default:
-		return
-
-	case 'D', 'B', 'T', 'U':
-		break
-	}
+	put := func(s *LSym, name string, type_ int, addr int64, size int64, ver int, gotype *LSym) {
+		if s == nil {
+			return
+		}
+		if s.Sect == nil && type_ != 'U' {
+			return
+		}
+		switch type_ {
+		default:
+			return
+		case 'D', 'B', 'T', 'U':
+		}
 
-	if coffsym != nil {
 		// only windows/386 requires underscore prefix on external symbols
 		if Thearch.Thechar == '8' && Linkmode == LinkExternal && (s.Type == obj.SHOSTOBJ || s.Attr.CgoExport()) && s.Name == s.Extname {
 			s.Name = "_" + s.Name
 		}
-		cs := &coffsym[ncoffsym]
-		cs.sym = s
-		if len(s.Name) > 8 {
-			cs.strtbloff = strtbladd(s.Name)
-		}
+
+		var typ uint16
+		var sect int
+		var value int64
 		// Note: although address of runtime.edata (type SDATA) is at the start of .bss section
 		// it still belongs to the .data section, not the .bss section.
 		if uint64(s.Value) >= Segdata.Vaddr+Segdata.Filelen && s.Type != obj.SDATA && Linkmode == LinkExternal {
-			cs.value = int64(uint64(s.Value) - Segdata.Vaddr - Segdata.Filelen)
-			cs.sect = bsssect
+			value = int64(uint64(s.Value) - Segdata.Vaddr - Segdata.Filelen)
+			sect = bsssect
 		} else if uint64(s.Value) >= Segdata.Vaddr {
-			cs.value = int64(uint64(s.Value) - Segdata.Vaddr)
-			cs.sect = datasect
+			value = int64(uint64(s.Value) - Segdata.Vaddr)
+			sect = datasect
 		} else if uint64(s.Value) >= Segtext.Vaddr {
-			cs.value = int64(uint64(s.Value) - Segtext.Vaddr)
-			cs.sect = textsect
+			value = int64(uint64(s.Value) - Segtext.Vaddr)
+			sect = textsect
 		} else if type_ == 'U' {
-			cs.value = 0
-			cs.typ = IMAGE_SYM_DTYPE_FUNCTION
+			typ = IMAGE_SYM_DTYPE_FUNCTION
 		} else {
-			cs.value = 0
-			cs.sect = 0
 			Diag("addpesym %#x", addr)
 		}
-	}
 
-	s.Dynid = int32(ncoffsym)
-	ncoffsym++
-}
+		// write COFF symbol table record
+		if len(s.Name) > 8 {
+			Lputl(0)
+			Lputl(uint32(strtbladd(s.Name)))
+		} else {
+			strnput(s.Name, 8)
+		}
+		Lputl(uint32(value))
+		Wputl(uint16(sect))
+		if typ != 0 {
+			Wputl(typ)
+		} else if Linkmode == LinkExternal {
+			Wputl(0)
+		} else {
+			Wputl(0x0308) // "array of structs"
+		}
+		Cput(2) // storage class: external
+		Cput(0) // no aux entries
+
+		s.Dynid = int32(symcnt)
+
+		symcnt++
+	}
 
-func pegenasmsym(put func(*LSym, string, int, int64, int64, int, *LSym)) {
 	if Linkmode == LinkExternal {
 		for d := dr; d != nil; d = d.next {
 			for m := d.ms; m != nil; m = m.next {
@@ -983,57 +985,36 @@ func pegenasmsym(put func(*LSym, string, int, int64, int64, int, *LSym)) {
 			}
 		}
 	}
+
 	genasmsym(put)
+
+	return symcnt
 }
 
 func addpesymtable() {
+	symtabStartPos := Cpos()
+
+	// write COFF symbol table
+	var symcnt int
 	if Debug['s'] == 0 || Linkmode == LinkExternal {
-		ncoffsym = 0
-		pegenasmsym(addpesym)
-		coffsym = make([]COFFSym, ncoffsym)
-		ncoffsym = 0
-		pegenasmsym(addpesym)
+		symcnt = writePESymTableRecords()
 	}
-	size := len(strtbl) + 4 + 18*ncoffsym
 
+	// update COFF file header and section table
+	size := len(strtbl) + 4 + 18*symcnt
 	var h *IMAGE_SECTION_HEADER
 	if Linkmode != LinkExternal {
 		// We do not really need .symtab for go.o, and if we have one, ld
 		// will also include it in the exe, and that will confuse windows.
 		h = addpesection(".symtab", size, size)
 		h.Characteristics = IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_DISCARDABLE
-		chksectoff(h, Cpos())
-	}
-	fh.PointerToSymbolTable = uint32(Cpos())
-	fh.NumberOfSymbols = uint32(ncoffsym)
-
-	// put COFF symbol table
-	var s *COFFSym
-	for i := 0; i < ncoffsym; i++ {
-		s = &coffsym[i]
-		if s.strtbloff == 0 {
-			strnput(s.sym.Name, 8)
-		} else {
-			Lputl(0)
-			Lputl(uint32(s.strtbloff))
-		}
-
-		Lputl(uint32(s.value))
-		Wputl(uint16(s.sect))
-		if s.typ != 0 {
-			Wputl(s.typ)
-		} else if Linkmode == LinkExternal {
-			Wputl(0)
-		} else {
-			Wputl(0x0308) // "array of structs"
-		}
-		Cput(2) // storage class: external
-		Cput(0) // no aux entries
+		chksectoff(h, symtabStartPos)
 	}
+	fh.PointerToSymbolTable = uint32(symtabStartPos)
+	fh.NumberOfSymbols = uint32(symcnt)
 
-	// put COFF string table
+	// write COFF string table
 	Lputl(uint32(len(strtbl)) + 4)
-
 	for i := 0; i < len(strtbl); i++ {
 		Cput(uint8(strtbl[i]))
 	}