cmd/compile: don't generate newobject call for 0-sized types

Emit &runtime.zerobase instead of a call to newobject for
allocations of zero sized objects in walk.go.

Fixes #29446

Change-Id: I11b67981d55009726a17c2e582c12ce0c258682e
Reviewed-on: https://go-review.googlesource.com/c/155840
Run-TryBot: Iskander Sharipov <quasilyte@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 34 insertions, 0 deletions

diff --git a/test/codegen/alloc.go b/test/codegen/alloc.go
new file mode 100644
index 0000000000..31455fdabf
--- /dev/null
+++ b/test/codegen/alloc.go
@@ -0,0 +1,34 @@
+// asmcheck
+
+// Copyright 2018 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.
+
+// These tests check that allocating a 0-size object does not
+// introduce a call to runtime.newobject.
+
+package codegen
+
+func zeroAllocNew1() *struct{} {
+	// 386:-`CALL\truntime\.newobject`
+	// amd64:-`CALL\truntime\.newobject`
+	// arm:-`CALL\truntime\.newobject`
+	// arm64:-`CALL\truntime\.newobject`
+	return new(struct{})
+}
+
+func zeroAllocNew2() *[0]int {
+	// 386:-`CALL\truntime\.newobject`
+	// amd64:-`CALL\truntime\.newobject`
+	// arm:-`CALL\truntime\.newobject`
+	// arm64:-`CALL\truntime\.newobject`
+	return new([0]int)
+}
+
+func zeroAllocSliceLit() []int {
+	// 386:-`CALL\truntime\.newobject`
+	// amd64:-`CALL\truntime\.newobject`
+	// arm:-`CALL\truntime\.newobject`
+	// arm64:-`CALL\truntime\.newobject`
+	return []int{}
+}