cmd/compile: propagate len([]T{}) to make builtin to allow stack allocation

Updates #75620

Change-Id: I6a6a6964af4512e30eb4806e1dc7b0fd0835744f
Reviewed-on: https://go-review.googlesource.com/c/go/+/707255
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: mateusz834

src/cmd/compile/internal/escape/escape.go

@@ -563,7 +563,10 @@ func (b *batch) rewriteWithLiterals(n ir.Node, fn *ir.Func) {
 			if ro == nil {
 				base.Fatalf("no ReassignOracle for function %v with closure parent %v", fn, fn.ClosureParent)
 			}
-			if s := ro.StaticValue(*r); s.Op() == ir.OLITERAL {
+
+			s := ro.StaticValue(*r)
+			switch s.Op() {
+			case ir.OLITERAL:
 				lit, ok := s.(*ir.BasicLit)
 				if !ok || lit.Val().Kind() != constant.Int {
 					base.Fatalf("unexpected BasicLit Kind")
@@ -577,6 +580,14 @@ func (b *batch) rewriteWithLiterals(n ir.Node, fn *ir.Func) {
 					assignTemp(n.Pos(), *r, n.PtrInit())
 					*r = ir.NewBasicLit(n.Pos(), (*r).Type(), lit.Val())
 				}
+			case ir.OLEN:
+				x := ro.StaticValue(s.(*ir.UnaryExpr).X)
+				if x.Op() == ir.OSLICELIT {
+					x := x.(*ir.CompLitExpr)
+					// Preserve any side effects of the original expression, then update the value.
+					assignTemp(n.Pos(), *r, n.PtrInit())
+					*r = ir.NewBasicLit(n.Pos(), types.Types[types.TINT], constant.MakeInt64(x.Len))
+				}
 			}
 		}
 	case ir.OCONVIFACE:

test/escape_make_non_const.go

@@ -106,3 +106,18 @@ type m struct {
 func newM(l int) m { // ERROR "can inline"
 	return m{make(map[string]int, l)} // ERROR "make.*escapes to heap"
 }
+
+//go:noinline
+func testLenOfSliceLit() {
+	ints := []int{0, 1, 2, 3, 4, 5} // ERROR "\[\]int\{\.\.\.\} does not escape"'
+	_ = make([]int, len(ints))      // ERROR "make\(\[\]int, 6\) does not escape"
+	_ = allocLenOf(ints)            // ERROR "inlining call", "make\(\[\]int, 6\) does not escape"
+
+	_ = make([]int, 2, len(ints))  // ERROR "make\(\[\]int, 2, 6\) does not escape"
+	_ = make([]int, len(ints), 2)  // ERROR "make\(\[\]int, len\(ints\), 2\) does not escape"
+	_ = make([]int, 10, len(ints)) // ERROR "make\(\[\]int, 10, 6\) does not escape"
+}
+
+func allocLenOf(s []int) []int { // ERROR "can inline" "s does not escape"
+	return make([]int, len(s)) // ERROR "escapes to heap"
+}