cmd/internal/obj/x86: factor rewriting to use GOT into separate function

I was prodded into doing this in review comments for the ARM version, and it's
going to make shared libs for 386 easier.

Change-Id: Id12de801b1425b8c6b5736fe91b418fc123a4e40
Reviewed-on: https://go-review.googlesource.com/17012Reviewed-by: David Crawshaw <crawshaw@golang.org>

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

@@ -2090,8 +2090,11 @@ func oclass(ctxt *obj.Link, p *obj.Prog, a *obj.Addr) int {
 
 	case obj.TYPE_ADDR:
 		switch a.Name {
+		case obj.NAME_GOTREF:
+			ctxt.Diag("unexpected TYPE_ADDR with NAME_GOTREF")
+			return Yxxx
+
 		case obj.NAME_EXTERN,
-			obj.NAME_GOTREF,
 			obj.NAME_STATIC:
 			if a.Sym != nil && isextern(a.Sym) || p.Mode == 32 {
 				return Yi32

src/cmd/internal/obj/x86/obj6.go

@@ -308,7 +308,19 @@ func progedit(ctxt *obj.Link, p *obj.Prog) {
 		}
 	}
 
-	if ctxt.Flag_dynlink && (p.As == obj.ADUFFCOPY || p.As == obj.ADUFFZERO) {
+	if ctxt.Flag_dynlink {
+		rewriteToUseGot(ctxt, p)
+	}
+}
+
+// Rewrite p, if necessary, to access global data via the global offset table.
+func rewriteToUseGot(ctxt *obj.Link, p *obj.Prog) {
+	if p.As == obj.ADUFFCOPY || p.As == obj.ADUFFZERO {
+		//     ADUFFxxx $offset
+		// becomes
+		//     MOVQ runtime.duffxxx@GOT, R15
+		//     ADDQ $offset, R15
+		//     CALL R15
 		var sym *obj.LSym
 		if p.As == obj.ADUFFZERO {
 			sym = obj.Linklookup(ctxt, "runtime.duffzero", 0)
@@ -336,12 +348,17 @@ func progedit(ctxt *obj.Link, p *obj.Prog) {
 		p2.To.Reg = REG_R15
 	}
 
-	if ctxt.Flag_dynlink {
+	// We only care about global data: NAME_EXTERN means a global
+	// symbol in the Go sense, and p.Sym.Local is true for a few
+	// internally defined symbols.
 	if p.As == ALEAQ && p.From.Type == obj.TYPE_MEM && p.From.Name == obj.NAME_EXTERN && !p.From.Sym.Local {
+		// LEAQ sym, Rx becomes MOVQ $sym, Rx which will be rewritten below
 		p.As = AMOVQ
 		p.From.Type = obj.TYPE_ADDR
 	}
 	if p.From.Type == obj.TYPE_ADDR && p.From.Name == obj.NAME_EXTERN && !p.From.Sym.Local {
+		// MOVQ $sym, Rx becomes MOVQ sym@GOT, Rx
+		// MOVQ $sym+<off>, Rx becomes MOVQ sym@GOT, Rx; ADDQ <off>, Rx
 		if p.As != AMOVQ {
 			ctxt.Diag("do not know how to handle TYPE_ADDR in %v with -dynlink", p)
 		}
@@ -363,6 +380,9 @@ func progedit(ctxt *obj.Link, p *obj.Prog) {
 		ctxt.Diag("don't know how to handle %v with -dynlink", p)
 	}
 	var source *obj.Addr
+	// MOVx sym, Ry becomes MOVW sym@GOT, R15; MOVx (R15), Ry
+	// MOVx Ry, sym becomes MOVW sym@GOT, R15; MOVx Ry, (R15)
+	// An addition may be inserted between the two MOVs if there is an offset.
 	if p.From.Name == obj.NAME_EXTERN && !p.From.Sym.Local {
 		if p.To.Name == obj.NAME_EXTERN && !p.To.Sym.Local {
 			ctxt.Diag("cannot handle NAME_EXTERN on both sides in %v with -dynlink", p)
@@ -403,13 +423,7 @@ func progedit(ctxt *obj.Link, p *obj.Prog) {
 	} else {
 		return
 	}
-		l := p.Link
-		l2 := p2.Link
-		*p = *p1
-		*p1 = *p2
-		p.Link = l
-		p1.Link = l2
-	}
+	obj.Nopout(p)
 }
 
 func nacladdr(ctxt *obj.Link, p *obj.Prog, a *obj.Addr) {

src/cmd/internal/obj/x86/obj6_test.go

@@ -24,13 +24,13 @@ LEAQ name+10(SB), AX -> MOVQ name@GOT(SB), AX; ADDQ $10, AX
 MOVQ $name(SB), AX -> MOVQ name@GOT(SB), AX
 MOVQ $name+10(SB), AX -> MOVQ name@GOT(SB), AX; ADDQ $10, AX
 
-MOVQ name(SB), AX -> MOVQ name@GOT(SB), R15; MOVQ (R15), AX
-MOVQ name+10(SB), AX -> MOVQ name@GOT(SB), R15; MOVQ 10(R15), AX
+MOVQ name(SB), AX -> NOP; MOVQ name@GOT(SB), R15; MOVQ (R15), AX
+MOVQ name+10(SB), AX -> NOP; MOVQ name@GOT(SB), R15; MOVQ 10(R15), AX
 
-CMPQ name(SB), $0 -> MOVQ name@GOT(SB), R15; CMPQ (R15), $0
+CMPQ name(SB), $0 -> NOP; MOVQ name@GOT(SB), R15; CMPQ (R15), $0
 
-MOVQ $1, name(SB) -> MOVQ name@GOT(SB), R15; MOVQ $1, (R15)
-MOVQ $1, name+10(SB) -> MOVQ name@GOT(SB), R15; MOVQ $1, 10(R15)
+MOVQ $1, name(SB) -> NOP; MOVQ name@GOT(SB), R15; MOVQ $1, (R15)
+MOVQ $1, name+10(SB) -> NOP; MOVQ name@GOT(SB), R15; MOVQ $1, 10(R15)
 `
 
 type ParsedTestData struct {