cmd/ld: remove Plan 9 symbol table

Update issue 6853

Nothing reads the Plan 9 symbol table anymore.
The last holdout was 'go tool nm', but since being rewritten in Go
it uses the standard symbol table for the binary format
(ELF, Mach-O, PE) instead.

Removing the Plan 9 symbol table saves ~15% disk space
on most binaries.

Two supporting changes included in this CL:

debug/gosym: use Go 1.2 pclntab to synthesize func-only
symbol table when there is no Plan 9 symbol table

debug/elf, debug/macho, debug/pe: ignore final EOF from ReadAt

LGTM=r
R=r, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/65740045

6 files changed, 46 insertions, 170 deletions

diff --git a/src/cmd/ld/symtab.c b/src/cmd/ld/symtab.c
index 0789c8a3e..bd14d9be7 100644
--- a/src/cmd/ld/symtab.c
+++ b/src/cmd/ld/symtab.c
@@ -270,47 +270,6 @@ asmplan9sym(void)
 
 static LSym *symt;
 
-static void
-scput(int b)
-{
-	uchar *p;
-
-	symgrow(ctxt, symt, symt->size+1);
-	p = symt->p + symt->size;
-	*p = b;
-	symt->size++;
-}
-
-static void
-slputb(int32 v)
-{
-	uchar *p;
-
-	symgrow(ctxt, symt, symt->size+4);
-	p = symt->p + symt->size;
-	*p++ = v>>24;
-	*p++ = v>>16;
-	*p++ = v>>8;
-	*p = v;
-	symt->size += 4;
-}
-
-static void
-slputl(int32 v)
-{
-	uchar *p;
-
-	symgrow(ctxt, symt, symt->size+4);
-	p = symt->p + symt->size;
-	*p++ = v;
-	*p++ = v>>8;
-	*p++ = v>>16;
-	*p = v>>24;
-	symt->size += 4;
-}
-
-static void (*slput)(int32);
-
 void
 wputl(ushort w)
 {
@@ -357,108 +316,6 @@ vputl(uint64 v)
 	lputl(v >> 32);
 }
 
-// Emit symbol table entry.
-// The table format is described at the top of ../../pkg/runtime/symtab.c.
-void
-putsymb(LSym *s, char *name, int t, vlong v, vlong size, int ver, LSym *typ)
-{
-	int i, f, c;
-	vlong v1;
-	Reloc *rel;
-
-	USED(size);
-
-	// type byte
-	if('A' <= t && t <= 'Z')
-		c = t - 'A' + (ver ? 26 : 0);
-	else if('a' <= t && t <= 'z')
-		c = t - 'a' + 26;
-	else {
-		diag("invalid symbol table type %c", t);
-		errorexit();
-		return;
-	}
-	
-	if(s != nil)
-		c |= 0x40; // wide value
-	if(typ != nil)
-		c |= 0x80; // has go type
-	scput(c);
-
-	// value
-	if(s != nil) {
-		// full width
-		rel = addrel(symt);
-		rel->siz = PtrSize;
-		rel->sym = s;
-		rel->type = D_ADDR;
-		rel->off = symt->size;
-		if(PtrSize == 8)
-			slput(0);
-		slput(0);
-	} else {
-		// varint
-		if(v < 0) {
-			diag("negative value in symbol table: %s %lld", name, v);
-			errorexit();
-		}
-		v1 = v;
-		while(v1 >= 0x80) {
-			scput(v1 | 0x80);
-			v1 >>= 7;
-		}
-		scput(v1);
-	}
-
-	// go type if present
-	if(typ != nil) {
-		if(!typ->reachable)
-			diag("unreachable type %s", typ->name);
-		rel = addrel(symt);
-		rel->siz = PtrSize;
-		rel->sym = typ;
-		rel->type = D_ADDR;
-		rel->off = symt->size;
-		if(PtrSize == 8)
-			slput(0);
-		slput(0);
-	}
-	
-	// name	
-	if(t == 'f')
-		name++;
-
-	if(t == 'Z' || t == 'z') {
-		scput(name[0]);
-		for(i=1; name[i] != 0 || name[i+1] != 0; i += 2) {
-			scput(name[i]);
-			scput(name[i+1]);
-		}
-		scput(0);
-		scput(0);
-	} else {
-		for(i=0; name[i]; i++)
-			scput(name[i]);
-		scput(0);
-	}
-
-	if(debug['n']) {
-		if(t == 'z' || t == 'Z') {
-			Bprint(&bso, "%c %.8llux ", t, v);
-			for(i=1; name[i] != 0 || name[i+1] != 0; i+=2) {
-				f = ((name[i]&0xff) << 8) | (name[i+1]&0xff);
-				Bprint(&bso, "/%x", f);
-			}
-			Bprint(&bso, "\n");
-			return;
-		}
-		if(ver)
-			Bprint(&bso, "%c %.8llux %s<%d> %s\n", t, v, name, ver, typ ? typ->name : "");
-		else
-			Bprint(&bso, "%c %.8llux %s %s\n", t, v, name, typ ? typ->name : "");
-	}
-}
-
 void
 symtab(void)
 {
@@ -553,31 +410,4 @@ symtab(void)
 			s->outer = symgofunc;
 		}
 	}
-
-	if(debug['s'])
-		return;
-
-	switch(thechar) {
-	default:
-		diag("unknown architecture %c", thechar);
-		errorexit();
-	case '5':
-	case '6':
-	case '8':
-		// little-endian symbol table
-		slput = slputl;
-		break;
-	case 'v':
-		// big-endian symbol table
-		slput = slputb;
-		break;
-	}
-	// new symbol table header.
-	slput(0xfffffffd);
-	scput(0);
-	scput(0);
-	scput(0);
-	scput(PtrSize);
-
-	genasmsym(putsymb);
 }
diff --git a/src/pkg/debug/elf/file.go b/src/pkg/debug/elf/file.go
index a40617099..2840f0767 100644
--- a/src/pkg/debug/elf/file.go
+++ b/src/pkg/debug/elf/file.go
@@ -76,6 +76,9 @@ type Section struct {
 func (s *Section) Data() ([]byte, error) {
 	dat := make([]byte, s.sr.Size())
 	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
 	return dat[0:n], err
 }
 
diff --git a/src/pkg/debug/gosym/pclntab.go b/src/pkg/debug/gosym/pclntab.go
index 3e6a8046b..6620aefb0 100644
--- a/src/pkg/debug/gosym/pclntab.go
+++ b/src/pkg/debug/gosym/pclntab.go
@@ -196,6 +196,33 @@ func (t *LineTable) go12Init() {
 	t.go12 = 1 // so far so good
 }
 
+// go12Funcs returns a slice of Funcs derived from the Go 1.2 pcln table.
+func (t *LineTable) go12Funcs() []Func {
+	// Assume it is malformed and return nil on error.
+	defer func() {
+		recover()
+	}()
+
+	n := len(t.functab) / int(t.ptrsize) / 2
+	funcs := make([]Func, n)
+	for i := range funcs {
+		f := &funcs[i]
+		f.Entry = uint64(t.uintptr(t.functab[2*i*int(t.ptrsize):]))
+		f.End = uint64(t.uintptr(t.functab[(2*i+2)*int(t.ptrsize):]))
+		info := t.Data[t.uintptr(t.functab[(2*i+1)*int(t.ptrsize):]):]
+		f.LineTable = t
+		f.FrameSize = int(t.binary.Uint32(info[t.ptrsize+2*4:]))
+		f.Sym = &Sym{
+			Value:  f.Entry,
+			Type:   'T',
+			Name:   t.string(t.binary.Uint32(info[t.ptrsize:])),
+			GoType: 0,
+			Func:   f,
+		}
+	}
+	return funcs
+}
+
 // findFunc returns the func corresponding to the given program counter.
 func (t *LineTable) findFunc(pc uint64) []byte {
 	if pc < t.uintptr(t.functab) || pc >= t.uintptr(t.functab[len(t.functab)-int(t.ptrsize):]) {
diff --git a/src/pkg/debug/gosym/symtab.go b/src/pkg/debug/gosym/symtab.go
index 9ab05bac2..3864e3cb4 100644
--- a/src/pkg/debug/gosym/symtab.go
+++ b/src/pkg/debug/gosym/symtab.go
@@ -129,6 +129,9 @@ var (
 )
 
 func walksymtab(data []byte, fn func(sym) error) error {
+	if len(data) == 0 { // missing symtab is okay
+		return nil
+	}
 	var order binary.ByteOrder = binary.BigEndian
 	newTable := false
 	switch {
@@ -455,6 +458,10 @@ func NewTable(symtab []byte, pcln *LineTable) (*Table, error) {
 			i = end - 1 // loop will i++
 		}
 	}
+
+	if t.go12line != nil && nf == 0 {
+		t.Funcs = t.go12line.go12Funcs()
+	}
 	if obj != nil {
 		obj.Funcs = t.Funcs[lastf:]
 	}
diff --git a/src/pkg/debug/macho/file.go b/src/pkg/debug/macho/file.go
index c799fa49d..2b19f7f65 100644
--- a/src/pkg/debug/macho/file.go
+++ b/src/pkg/debug/macho/file.go
@@ -74,6 +74,9 @@ type Segment struct {
 func (s *Segment) Data() ([]byte, error) {
 	dat := make([]byte, s.sr.Size())
 	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
 	return dat[0:n], err
 }
 
@@ -109,6 +112,9 @@ type Section struct {
 func (s *Section) Data() ([]byte, error) {
 	dat := make([]byte, s.sr.Size())
 	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
 	return dat[0:n], err
 }
 
diff --git a/src/pkg/debug/pe/file.go b/src/pkg/debug/pe/file.go
index a2859bf37..d0005bacf 100644
--- a/src/pkg/debug/pe/file.go
+++ b/src/pkg/debug/pe/file.go
@@ -72,6 +72,9 @@ type ImportDirectory struct {
 func (s *Section) Data() ([]byte, error) {
 	dat := make([]byte, s.sr.Size())
 	n, err := s.sr.ReadAt(dat, 0)
+	if n == len(dat) {
+		err = nil
+	}
 	return dat[0:n], err
 }