go/ssa: simplify make([]T, len, k) to new([k]T)[:len] for constant k

+ test.

Change-Id: I0255e38a0f95af95a32dc1d9cf18b26db85af36f
Reviewed-on: https://go-review.googlesource.com/1519
Reviewed-by: Robert Griesemer <gri@golang.org>

go/ssa/builder.go

@@ -241,6 +241,20 @@ func (b *builder) builtin(fn *Function, obj *types.Builtin, args []ast.Expr, typ
 			if len(args) == 3 {
 				m = b.expr(fn, args[2])
 			}
+			if m, ok := m.(*Const); ok {
+				// treat make([]T, n, m) as new([m]T)[:n]
+				cap, _ := exact.Int64Val(m.Value)
+				at := types.NewArray(typ.Underlying().(*types.Slice).Elem(), cap)
+				alloc := emitNew(fn, at, pos)
+				alloc.Comment = "makeslice"
+				v := &Slice{
+					X:    alloc,
+					High: n,
+				}
+				v.setPos(pos)
+				v.setType(typ)
+				return fn.emit(v)
+			}
 			v := &MakeSlice{
 				Len: n,
 				Cap: m,

go/ssa/interp/testdata/coverage.go

@@ -478,7 +478,28 @@ func init() {
 	}
 }
 
-// Test that a nice error is issue by indirection wrappers.
+var one = 1 // not a constant
+
+// Test makeslice.
+func init() {
+	check := func(s []string, wantLen, wantCap int) {
+		if len(s) != wantLen {
+			panic(len(s))
+		}
+		if cap(s) != wantCap {
+			panic(cap(s))
+		}
+	}
+	//                                       SSA form:
+	check(make([]string, 10), 10, 10)     // new([10]string)[:10]
+	check(make([]string, one), 1, 1)      // make([]string, one, one)
+	check(make([]string, 0, 10), 0, 10)   // new([10]string)[:0]
+	check(make([]string, 0, one), 0, 1)   // make([]string, 0, one)
+	check(make([]string, one, 10), 1, 10) // new([10]string)[:one]
+	check(make([]string, one, one), 1, 1) // make([]string, one, one)
+}
+
+// Test that a nice error is issued by indirection wrappers.
 func init() {
 	var ptr *T
 	var i I = ptr

go/ssa/testdata/objlookup.go

@@ -90,9 +90,9 @@ func main() {
 	_ = v8ptr[0]  // v8ptr::Alloc
 	_ = *v8ptr    // v8ptr::Alloc
 
-	v8a := make([]int, 1) // v8a::MakeSlice
+	v8a := make([]int, 1) // v8a::Slice
-	v8a[0] = 0            // v8a::MakeSlice
+	v8a[0] = 0            // v8a::Slice
-	print(v8a[:])         // v8a::MakeSlice
+	print(v8a[:])         // v8a::Slice
 
 	v9 := S{} // &v9::Alloc
 

go/ssa/testdata/valueforexpr.go

@@ -33,7 +33,7 @@ func f(spilled, unspilled int) {
 	_ = /*@Phi*/ (y)
 	map1 := /*@MakeMap*/ (make(map[string]string))
 	_ = map1
-	_ = /*@MakeSlice*/ (make([]int, 0))
+	_ = /*@Slice*/ (make([]int, 0))
 	_ = /*@MakeClosure*/ (func() { print(spilled) })
 
 	sl := []int{}