cmd/compile: prettify loop iterations

This commit replaces some of

for i := len(x) - 1; i >= 0; i-- {...}

style loops, which do not rely on reverse iteration order.

Change-Id: I5542834286562da058200c06e7a173b13760e54d
Reviewed-on: https://go-review.googlesource.com/21044
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -455,15 +455,15 @@ func escAnalyze(all []*Node, recursive bool) {
 	e.nodeEscState(&e.theSink).Escloopdepth = -1
 	e.recursive = recursive
 
-	for i := len(all) - 1; i >= 0; i-- {
-		if n := all[i]; n.Op == ODCLFUNC {
+	for _, n := range all {
+		if n.Op == ODCLFUNC {
 			n.Esc = EscFuncPlanned
 		}
 	}
 
 	// flow-analyze functions
-	for i := len(all) - 1; i >= 0; i-- {
-		if n := all[i]; n.Op == ODCLFUNC {
+	for _, n := range all {
+		if n.Op == ODCLFUNC {
 			escfunc(e, n)
 		}
 	}
@@ -477,8 +477,8 @@ func escAnalyze(all []*Node, recursive bool) {
 	}
 
 	// for all top level functions, tag the typenodes corresponding to the param nodes
-	for i := len(all) - 1; i >= 0; i-- {
-		if n := all[i]; n.Op == ODCLFUNC {
+	for _, n := range all {
+		if n.Op == ODCLFUNC {
 			esctag(e, n)
 		}
 	}

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

@@ -436,9 +436,7 @@ func Main() {
 	if Debug['l'] != 0 {
 		// Find functions that can be inlined and clone them before walk expands them.
 		visitBottomUp(xtop, func(list []*Node, recursive bool) {
-			// TODO: use a range statement here if the order does not matter
-			for i := len(list) - 1; i >= 0; i-- {
-				n := list[i]
+			for _, n := range list {
 				if n.Op == ODCLFUNC {
 					caninl(n)
 					inlcalls(n)

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

@@ -2801,9 +2801,8 @@ func keydup(n *Node, hash map[uint32][]*Node) {
 	case CTSTR:
 		h = 0
 		s := n.Val().U.(string)
-		for i := len(n.Val().U.(string)); i > 0; i-- {
-			h = h*PRIME1 + uint32(s[0])
-			s = s[1:]
+		for i := 0; i < len(s); i++ {
+			h = h*PRIME1 + uint32(s[i])
 		}
 	}
 

src/cmd/compile/internal/ssa/dom.go

@@ -163,11 +163,10 @@ func postDominators(f *Func) []*Block {
 
 	// find the exit blocks
 	var exits []*Block
-	for i := len(f.Blocks) - 1; i >= 0; i-- {
-		switch f.Blocks[i].Kind {
+	for _, b := range f.Blocks {
+		switch b.Kind {
 		case BlockExit, BlockRet, BlockRetJmp, BlockCall, BlockCheck:
-			exits = append(exits, f.Blocks[i])
-			break
+			exits = append(exits, b)
 		}
 	}
 

src/cmd/compile/internal/ssa/stackalloc.go

@@ -155,7 +155,7 @@ func (s *stackAllocState) stackalloc() {
 		slots = make([]int, n)
 		s.slots = slots
 	}
-	for i := f.NumValues() - 1; i >= 0; i-- {
+	for i := range slots {
 		slots[i] = -1
 	}