cmd/compile/internal/gc: eliminate stringsCompare for stackvar sorting

Passes go build -a -toolexec 'toolstash -cmp' std cmd.

Change-Id: I2a87d31da74affdf3d0f358d0efdb3f1c646d917
Reviewed-on: https://go-review.googlesource.com/14759Reviewed-by: Dave Cheney <dave@cheney.net>

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

@@ -165,10 +165,7 @@ func emitptrargsmap() {
 }
 
 // cmpstackvarlt reports whether the stack variable a sorts before b.
-func cmpstackvarlt(a, b *Node) bool {
-	return cmpstackvar(a, b) < 0
-}
-
+//
 // Sort the list of stack variables. Autos after anything else,
 // within autos, unused after used, within used, things with
 // pointers first, zeroed things first, and then decreasing size.
@@ -177,48 +174,48 @@ func cmpstackvarlt(a, b *Node) bool {
 // really means, in memory, things with pointers needing zeroing at
 // the top of the stack and increasing in size.
 // Non-autos sort on offset.
-func cmpstackvar(a *Node, b *Node) int {
+func cmpstackvarlt(a, b *Node) bool {
 	if a.Class != b.Class {
 		if a.Class == PAUTO {
-			return +1
+			return false
 		}
-		return -1
+		return true
 	}
 
 	if a.Class != PAUTO {
 		if a.Xoffset < b.Xoffset {
-			return -1
+			return true
 		}
 		if a.Xoffset > b.Xoffset {
-			return +1
+			return false
 		}
-		return 0
+		return false
 	}
 
 	if a.Used != b.Used {
-		return obj.Bool2int(b.Used) - obj.Bool2int(a.Used)
+		return a.Used
 	}
 
-	ap := obj.Bool2int(haspointers(a.Type))
-	bp := obj.Bool2int(haspointers(b.Type))
+	ap := haspointers(a.Type)
+	bp := haspointers(b.Type)
 	if ap != bp {
-		return bp - ap
+		return ap
 	}
 
-	ap = obj.Bool2int(a.Name.Needzero)
-	bp = obj.Bool2int(b.Name.Needzero)
+	ap = a.Name.Needzero
+	bp = b.Name.Needzero
 	if ap != bp {
-		return bp - ap
+		return ap
 	}
 
 	if a.Type.Width < b.Type.Width {
-		return +1
+		return false
 	}
 	if a.Type.Width > b.Type.Width {
-		return -1
+		return true
 	}
 
-	return stringsCompare(a.Sym.Name, b.Sym.Name)
+	return a.Sym.Name < b.Sym.Name
 }
 
 // stkdelta records the stack offset delta for a node
@@ -244,7 +241,7 @@ func allocauto(ptxt *obj.Prog) {
 
 	markautoused(ptxt)
 
-	listsort(&Curfn.Func.Dcl, cmpstackvar)
+	listsort(&Curfn.Func.Dcl, cmpstackvarlt)
 
 	// Unused autos are at the end, chop 'em off.
 	ll := Curfn.Func.Dcl

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

@@ -4,7 +4,10 @@
 
 package gc
 
-import "testing"
+import (
+	"reflect"
+	"testing"
+)
 
 // Test all code paths for cmpstackvarlt.
 func TestCmpstackvar(t *testing.T) {
@@ -100,3 +103,74 @@ func TestCmpstackvar(t *testing.T) {
 		}
 	}
 }
+
+func slice2nodelist(s []*Node) *NodeList {
+	var nl *NodeList
+	for _, n := range s {
+		nl = list(nl, n)
+	}
+	return nl
+}
+
+func nodelist2slice(nl *NodeList) []*Node {
+	var s []*Node
+	for l := nl; l != nil; l = l.Next {
+		s = append(s, l.N)
+	}
+	return s
+}
+
+func TestListsort(t *testing.T) {
+	inp := []*Node{
+		{Class: PFUNC, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PFUNC, Xoffset: 0, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PFUNC, Xoffset: 10, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PFUNC, Xoffset: 20, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Used: true, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{Haspointers: 1}, Name: &Name{}, Sym: &Sym{}}, // haspointers -> false
+		{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{Needzero: true}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{Width: 1}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{Width: 2}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "abc"}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "xyz"}},
+	}
+	want := []*Node{
+		{Class: PFUNC, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PFUNC, Xoffset: 0, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PFUNC, Xoffset: 10, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PFUNC, Xoffset: 20, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Used: true, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{Needzero: true}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{Width: 2}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{Width: 1}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "abc"}},
+		{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "xyz"}},
+		{Class: PAUTO, Type: &Type{Haspointers: 1}, Name: &Name{}, Sym: &Sym{}}, // haspointers -> false
+	}
+	// haspointers updates Type.Haspointers as a side effect, so
+	// exercise this function on all inputs so that reflect.DeepEqual
+	// doesn't produce false positives.
+	for i := range want {
+		haspointers(want[i].Type)
+		haspointers(inp[i].Type)
+	}
+
+	nl := slice2nodelist(inp)
+	listsort(&nl, cmpstackvarlt)
+	got := nodelist2slice(nl)
+	if !reflect.DeepEqual(want, got) {
+		t.Error("listsort failed")
+		for i := range got {
+			g := got[i]
+			w := want[i]
+			eq := reflect.DeepEqual(w, g)
+			if !eq {
+				t.Log(i, w, g)
+			}
+		}
+	}
+}

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

@@ -409,9 +409,10 @@ func list(l *NodeList, n *Node) *NodeList {
 	return concat(l, list1(n))
 }
 
-// listsort sorts *l in place according to the 3-way comparison function f.
+// listsort sorts *l in place according to the comparison function lt.
+// The algorithm expects lt(a, b) to be equivalent to a < b.
 // The algorithm is mergesort, so it is guaranteed to be O(n log n).
-func listsort(l **NodeList, f func(*Node, *Node) int) {
+func listsort(l **NodeList, lt func(*Node, *Node) bool) {
 	if *l == nil || (*l).Next == nil {
 		return
 	}
@@ -436,10 +437,10 @@ func listsort(l **NodeList, f func(*Node, *Node) int) {
 	(*l).End = l1
 
 	l1 = *l
-	listsort(&l1, f)
-	listsort(&l2, f)
+	listsort(&l1, lt)
+	listsort(&l2, lt)
 
-	if f(l1.N, l2.N) < 0 {
+	if lt(l1.N, l2.N) {
 		*l = l1
 	} else {
 		*l = l2
@@ -451,7 +452,7 @@ func listsort(l **NodeList, f func(*Node, *Node) int) {
 
 	var le *NodeList
 	for (l1 != nil) && (l2 != nil) {
-		for (l1.Next != nil) && f(l1.Next.N, l2.N) < 0 {
+		for (l1.Next != nil) && lt(l1.Next.N, l2.N) {
 			l1 = l1.Next
 		}