cmd/compile: use more range fors in gc

Slightly simplifies the code. Made sure to exclude the cases that would
change behavior, such as when the iterated value is a string, when the
index is modified within the body, or when the slice is modified.

Also checked that all the elements are of pointer type, to avoid the
corner case where non-pointer types could be copied by mistake.

Change-Id: Iea64feb2a9a6a4c94ada9ff3ace40ee173505849
Reviewed-on: https://go-review.googlesource.com/100557
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -100,8 +100,8 @@ func assembleInlines(fnsym *obj.LSym, dwVars []*dwarf.Var) dwarf.InlCalls {
 		var m map[varPos]int
 		if ii == 0 {
 			if !fnsym.WasInlined() {
-				for j := 0; j < len(sl); j++ {
-					sl[j].ChildIndex = int32(j)
+				for j, v := range sl {
+					v.ChildIndex = int32(j)
 				}
 				continue
 			}
@@ -121,19 +121,19 @@ func assembleInlines(fnsym *obj.LSym, dwVars []*dwarf.Var) dwarf.InlCalls {
 		// parented by the inlined routine and not the top-level
 		// caller.
 		synthCount := len(m)
-		for j := 0; j < len(sl); j++ {
-			canonName := unversion(sl[j].Name)
+		for _, v := range sl {
+			canonName := unversion(v.Name)
 			vp := varPos{
 				DeclName: canonName,
-				DeclFile: sl[j].DeclFile,
-				DeclLine: sl[j].DeclLine,
-				DeclCol:  sl[j].DeclCol,
+				DeclFile: v.DeclFile,
+				DeclLine: v.DeclLine,
+				DeclCol:  v.DeclCol,
 			}
-			synthesized := strings.HasPrefix(sl[j].Name, "~r") || canonName == "_"
+			synthesized := strings.HasPrefix(v.Name, "~r") || canonName == "_"
 			if idx, found := m[vp]; found {
-				sl[j].ChildIndex = int32(idx)
-				sl[j].IsInAbstract = !synthesized
-				sl[j].Name = canonName
+				v.ChildIndex = int32(idx)
+				v.IsInAbstract = !synthesized
+				v.Name = canonName
 			} else {
 				// Variable can't be found in the pre-inline dcl list.
 				// In the top-level case (ii=0) this can happen
@@ -141,7 +141,7 @@ func assembleInlines(fnsym *obj.LSym, dwVars []*dwarf.Var) dwarf.InlCalls {
 				// and we're looking at a piece. We can also see
 				// return temps (~r%d) that were created during
 				// lowering, or unnamed params ("_").
-				sl[j].ChildIndex = int32(synthCount)
+				v.ChildIndex = int32(synthCount)
 				synthCount += 1
 			}
 		}
@@ -215,8 +215,7 @@ func unversion(name string) string {
 func makePreinlineDclMap(fnsym *obj.LSym) map[varPos]int {
 	dcl := preInliningDcls(fnsym)
 	m := make(map[varPos]int)
-	for i := 0; i < len(dcl); i++ {
-		n := dcl[i]
+	for i, n := range dcl {
 		pos := Ctxt.InnermostPos(n.Pos)
 		vp := varPos{
 			DeclName: unversion(n.Sym.Name),
@@ -338,7 +337,7 @@ func (s byClassThenName) Less(i, j int) bool { return cmpDwarfVar(s[i], s[j]) }
 func (s byClassThenName) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 
 func dumpInlCall(inlcalls dwarf.InlCalls, idx, ilevel int) {
-	for i := 0; i < ilevel; i += 1 {
+	for i := 0; i < ilevel; i++ {
 		Ctxt.Logf("  ")
 	}
 	ic := inlcalls.Calls[idx]
@@ -363,9 +362,8 @@ func dumpInlCall(inlcalls dwarf.InlCalls, idx, ilevel int) {
 }
 
 func dumpInlCalls(inlcalls dwarf.InlCalls) {
-	n := len(inlcalls.Calls)
-	for k := 0; k < n; k += 1 {
-		if inlcalls.Calls[k].Root {
+	for k, c := range inlcalls.Calls {
+		if c.Root {
 			dumpInlCall(inlcalls, k, 0)
 		}
 	}

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

@@ -241,7 +241,7 @@ func (s *phiState) insertVarPhis(n int, var_ *Node, defs []*ssa.Block, typ *type
 				v := c.NewValue0I(currentRoot.Pos, ssa.OpPhi, typ, int64(n)) // TODO: line number right?
 				// Note: we store the variable number in the phi's AuxInt field. Used temporarily by phi building.
 				s.s.addNamedValue(var_, v)
-				for i := 0; i < len(c.Preds); i++ {
+				for range c.Preds {
 					v.AddArg(s.placeholder) // Actual args will be filled in by resolveFwdRefs.
 				}
 				if debugPhi {

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

@@ -487,8 +487,8 @@ func (lv *Liveness) prologue() {
 
 		// Walk the block instructions forward to update avarinit bits.
 		// avarinit describes the effect at the end of the block, not the beginning.
-		for j := 0; j < len(b.Values); j++ {
-			pos, e := lv.valueEffects(b.Values[j])
+		for _, val := range b.Values {
+			pos, e := lv.valueEffects(val)
 			if e&varkill != 0 {
 				be.avarinit.Unset(pos)
 			}

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

@@ -73,9 +73,9 @@ func flusherrors() {
 		return
 	}
 	sort.Stable(byPos(errors))
-	for i := 0; i < len(errors); i++ {
-		if i == 0 || errors[i].msg != errors[i-1].msg {
-			fmt.Printf("%s", errors[i].msg)
+	for i, err := range errors {
+		if i == 0 || err.msg != errors[i-1].msg {
+			fmt.Printf("%s", err.msg)
 		}
 	}
 	errors = errors[:0]