cmd/compile: improve single blank variable handling in walkrange

Refactor walkrange to treat "for _ = range a" as "for range a".

This avoids generating some later discarded nodes in the compiler.

Passes toolstash -cmp.

Change-Id: Ifb2e1ca3b8519cbb67e8ad5aad514af9d18f1ec4
Reviewed-on: https://go-review.googlesource.com/61017
Run-TryBot: Martin Möhrmann <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

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

@@ -155,27 +155,36 @@ func walkrange(n *Node) *Node {
  lno := setlineno(a)
  n.Right = nil
 
- var v1 *Node
- if n.List.Len() != 0 {
+ var v1, v2 *Node
+ l := n.List.Len()
+ if l > 0 {
  v1 = n.List.First()
  }
- var v2 *Node
- if n.List.Len() > 1 && !isblank(n.List.Second()) {
+
+ if l > 1 {
  v2 = n.List.Second()
  }
 
- if v1 == nil && v2 != nil {
- Fatalf("walkrange: v2 != nil while v1 == nil")
+ if isblank(v2) {
+ v2 = nil
  }
 
- var ifGuard *Node
+ if isblank(v1) && v2 == nil {
+ v1 = nil
+ }
 
- translatedLoopOp := OFOR
+ if v1 == nil && v2 != nil {
+ Fatalf("walkrange: v2 != nil while v1 == nil")
+ }
 
  // n.List has no meaning anymore, clear it
  // to avoid erroneous processing by racewalk.
  n.List.Set(nil)
 
+ var ifGuard *Node
+
+ translatedLoopOp := OFOR
+
  var body []*Node
  var init []*Node
  switch t.Etype {

test/range.go

@@ -23,12 +23,57 @@ func seq(lo, hi int) chan int {
  return c
 }
 
+const alphabet = "abcdefghijklmnopqrstuvwxyz"
+
+func testblankvars() {
+ n := 0
+ for range alphabet {
+ n++
+ }
+ if n != 26 {
+ println("for range: wrong count", n, "want 26")
+ panic("fail")
+ }
+ n = 0
+ for _ = range alphabet {
+ n++
+ }
+ if n != 26 {
+ println("for _ = range: wrong count", n, "want 26")
+ panic("fail")
+ }
+ n = 0
+ for _, _ = range alphabet {
+ n++
+ }
+ if n != 26 {
+ println("for _, _ = range: wrong count", n, "want 26")
+ panic("fail")
+ }
+ s := 0
+ for i, _ := range alphabet {
+ s += i
+ }
+ if s != 325 {
+ println("for i, _ := range: wrong sum", s, "want 325")
+ panic("fail")
+ }
+ r := rune(0)
+ for _, v := range alphabet {
+ r += v
+ }
+ if r != 2847 {
+ println("for _, v := range: wrong sum", r, "want 2847")
+ panic("fail")
+ }
+}
+
 func testchan() {
  s := ""
  for i := range seq('a', 'z') {
  s += string(i)
  }
- if s != "abcdefghijklmnopqrstuvwxyz" {
+ if s != alphabet {
  println("Wanted lowercase alphabet; got", s)
  panic("fail")
  }
@@ -38,6 +83,7 @@ func testchan() {
  }
  if n != 26 {
  println("testchan wrong count", n, "want 26")
+ panic("fail")
  }
 }
 
@@ -426,6 +472,7 @@ func testcalls() {
 }
 
 func main() {
+ testblankvars()
  testchan()
  testarray()
  testarray1()