cmd/internal/obj: combine Pcrel and Comefrom

They're each architecture-specific.
Let them share.

Reduces Prog size to 288, which is the
next smaller malloc class.

Reduces inuse_space while compiling the
rotate tests by ~3.2%.

Change-Id: Ica8ec90e466c97b569745fffff0e5acd364e55fa
Reviewed-on: https://go-review.googlesource.com/10514Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/arm/asm5.go

@@ -900,7 +900,7 @@ func addpool(ctxt *obj.Link, p *obj.Prog, a *obj.Addr) {
 		t.To.Name = a.Name
 
 		if ctxt.Flag_shared != 0 && t.To.Sym != nil {
-			t.Pcrel = p
+			t.Rel = p
 		}
 
 	case C_SROREG,
@@ -917,9 +917,9 @@ func addpool(ctxt *obj.Link, p *obj.Prog, a *obj.Addr) {
 		t.To.Offset = ctxt.Instoffset
 	}
 
-	if t.Pcrel == nil {
+	if t.Rel == nil {
 		for q := ctxt.Blitrl; q != nil; q = q.Link { /* could hash on t.t0.offset */
-			if q.Pcrel == nil && q.To == t.To {
+			if q.Rel == nil && q.To == t.To {
 				p.Pcond = q
 				return
 			}
@@ -1671,11 +1671,11 @@ func asmout(ctxt *obj.Link, p *obj.Prog, o *Optab, out []uint32) {
 			if rel.Sym == ctxt.Tlsg && ctxt.Tlsg.Type == 0 {
 				rel.Type = obj.R_TLS
 				if ctxt.Flag_shared != 0 {
-					rel.Add += ctxt.Pc - p.Pcrel.Pc - 8 - int64(rel.Siz)
+					rel.Add += ctxt.Pc - p.Rel.Pc - 8 - int64(rel.Siz)
 				}
 			} else if ctxt.Flag_shared != 0 {
 				rel.Type = obj.R_PCREL
-				rel.Add += ctxt.Pc - p.Pcrel.Pc - 8
+				rel.Add += ctxt.Pc - p.Rel.Pc - 8
 			} else {
 				rel.Type = obj.R_ADDR
 			}
@@ -2062,7 +2062,7 @@ func asmout(ctxt *obj.Link, p *obj.Prog, o *Optab, out []uint32) {
 
 			if o.flag&LPCREL != 0 {
 				rel.Type = obj.R_PCREL
-				rel.Add += ctxt.Pc - p.Pcrel.Pc - 16 + int64(rel.Siz)
+				rel.Add += ctxt.Pc - p.Rel.Pc - 16 + int64(rel.Siz)
 			} else {
 				rel.Type = obj.R_ADDR
 			}

src/cmd/internal/obj/arm/obj5.go

@@ -160,7 +160,7 @@ func linkcase(casep *obj.Prog) {
 	for p := casep; p != nil; p = p.Link {
 		if p.As == ABCASE {
 			for ; p != nil && p.As == ABCASE; p = p.Link {
-				p.Pcrel = casep
+				p.Rel = casep
 			}
 			break
 		}

src/cmd/internal/obj/link.go

@@ -200,30 +200,29 @@ const (
 // TODO(rsc): Describe prog.
 // TODO(rsc): Describe TEXT/GLOBL flag in from3, DATA width in from3.
 type Prog struct {
-	Ctxt     *Link
-	Link     *Prog
-	From     Addr
-	From3    *Addr // optional
-	To       Addr
-	Opt      interface{}
-	Forwd    *Prog
-	Pcond    *Prog
-	Comefrom *Prog
-	Pcrel    *Prog
-	Pc       int64
-	Lineno   int32
-	Spadj    int32
-	As       int16
-	Reg      int16
-	RegTo2   int16 // 2nd register output operand
-	Mark     uint16
-	Optab    uint16
-	Scond    uint8
-	Back     uint8
-	Ft       uint8
-	Tt       uint8
-	Isize    uint8
-	Mode     int8
+	Ctxt   *Link
+	Link   *Prog
+	From   Addr
+	From3  *Addr // optional
+	To     Addr
+	Opt    interface{}
+	Forwd  *Prog
+	Pcond  *Prog
+	Rel    *Prog // Source of forward jumps on x86; pcrel on arm
+	Pc     int64
+	Lineno int32
+	Spadj  int32
+	As     int16
+	Reg    int16
+	RegTo2 int16 // 2nd register output operand
+	Mark   uint16
+	Optab  uint16
+	Scond  uint8
+	Back   uint8
+	Ft     uint8
+	Tt     uint8
+	Isize  uint8
+	Mode   int8
 
 	Info ProgInfo
 }

src/cmd/internal/obj/x86/asm6.go

@@ -1690,7 +1690,7 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 			p.Pc = int64(c)
 
 			// process forward jumps to p
-			for q = p.Comefrom; q != nil; q = q.Forwd {
+			for q = p.Rel; q != nil; q = q.Forwd {
 				v = int32(p.Pc - (q.Pc + int64(q.Mark)))
 				if q.Back&2 != 0 { // short
 					if v > 127 {
@@ -1715,7 +1715,7 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 				}
 			}
 
-			p.Comefrom = nil
+			p.Rel = nil
 
 			p.Pc = int64(c)
 			asmins(ctxt, p)
@@ -3547,9 +3547,9 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 				}
 
 				// Annotate target; will fill in later.
-				p.Forwd = q.Comefrom
+				p.Forwd = q.Rel
 
-				q.Comefrom = p
+				q.Rel = p
 				if p.Back&2 != 0 { // short
 					if p.As == AJCXZL {
 						ctxt.Andptr[0] = 0x67