cmd/compile/internal/gc: add unit test for cmpstackvar

A followup CL will rewrite listsort to use the new cmpstackvarlt and
change cmpstackvar to avoid stringsCompare.

Change-Id: Idf0857a3bd67f9e2243ba82aa0bff510612927c3
Reviewed-on: https://go-review.googlesource.com/14611Reviewed-by: Dave Cheney <dave@cheney.net>

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

@@ -164,6 +164,11 @@ func emitptrargsmap() {
 	ggloblsym(sym, int32(off), obj.RODATA|obj.LOCAL)
 }
 
+// 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.

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

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gc
+
+import "testing"
+
+// Test all code paths for cmpstackvarlt.
+func TestCmpstackvar(t *testing.T) {
+	testdata := []struct {
+		a, b Node
+		lt   bool
+	}{
+		{
+			Node{Class: PAUTO},
+			Node{Class: PFUNC},
+			false,
+		},
+		{
+			Node{Class: PFUNC},
+			Node{Class: PAUTO},
+			true,
+		},
+		{
+			Node{Class: PFUNC, Xoffset: 0},
+			Node{Class: PFUNC, Xoffset: 10},
+			true,
+		},
+		{
+			Node{Class: PFUNC, Xoffset: 20},
+			Node{Class: PFUNC, Xoffset: 10},
+			false,
+		},
+		{
+			Node{Class: PFUNC, Xoffset: 10},
+			Node{Class: PFUNC, Xoffset: 10},
+			false,
+		},
+		{
+			Node{Class: PAUTO, Used: true},
+			Node{Class: PAUTO, Used: false},
+			true,
+		},
+		{
+			Node{Class: PAUTO, Used: false},
+			Node{Class: PAUTO, Used: true},
+			false,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{Haspointers: 1}}, // haspointers -> false
+			Node{Class: PAUTO, Type: &Type{Haspointers: 2}}, // haspointers -> true
+			false,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{Haspointers: 2}}, // haspointers -> true
+			Node{Class: PAUTO, Type: &Type{Haspointers: 1}}, // haspointers -> false
+			true,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{Needzero: true}},
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{Needzero: false}},
+			true,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{Needzero: false}},
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{Needzero: true}},
+			false,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{Width: 1}, Name: &Name{}},
+			Node{Class: PAUTO, Type: &Type{Width: 2}, Name: &Name{}},
+			false,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{Width: 2}, Name: &Name{}},
+			Node{Class: PAUTO, Type: &Type{Width: 1}, Name: &Name{}},
+			true,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "abc"}},
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "xyz"}},
+			true,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "abc"}},
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "abc"}},
+			false,
+		},
+		{
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "xyz"}},
+			Node{Class: PAUTO, Type: &Type{}, Name: &Name{}, Sym: &Sym{Name: "abc"}},
+			false,
+		},
+	}
+	for _, d := range testdata {
+		got := cmpstackvarlt(&d.a, &d.b)
+		if got != d.lt {
+			t.Errorf("want %#v < %#v", d.a, d.b)
+		}
+	}
+}