cmd/compile: add clear(x) builtin

To clear map, and zero content of slice.

Updates #56351

Change-Id: I5f81dfbc465500f5acadaf2c6beb9b5f0d2c4045
Reviewed-on: https://go-review.googlesource.com/c/go/+/453395
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/test/clear.go b/test/clear.go
new file mode 100644
index 0000000000..60ee4ecf85
--- /dev/null
+++ b/test/clear.go
@@ -0,0 +1,47 @@
+// run
+
+// Copyright 2022 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 main
+
+import "math"
+
+func checkClearSlice() {
+	s := []int{1, 2, 3}
+	clear(s)
+	for i := range s {
+		if s[i] != 0 {
+			panic("clear not zeroing slice elem")
+		}
+	}
+
+	clear([]int{})
+}
+
+func checkClearMap() {
+	m1 := make(map[int]int)
+	m1[0] = 0
+	m1[1] = 1
+	clear(m1)
+	if len(m1) != 0 {
+		panic("m1 is not cleared")
+	}
+
+	// map contains NaN keys is also cleared.
+	m2 := make(map[float64]int)
+	m2[math.NaN()] = 1
+	m2[math.NaN()] = 1
+	clear(m2)
+	if len(m2) != 0 {
+		panic("m2 is not cleared")
+	}
+
+	clear(map[int]int{})
+}
+
+func main() {
+	checkClearSlice()
+	checkClearMap()
+}