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>

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

@@ -1940,6 +1940,16 @@ func callnew(t *types.Type) *Node {
 		yyerror("%v is go:notinheap; heap allocation disallowed", t)
 	}
 	dowidth(t)
+
+	if t.Size() == 0 {
+		// Return &runtime.zerobase if we know that the requested size is 0.
+		// This is what runtime.mallocgc would return.
+		z := newname(Runtimepkg.Lookup("zerobase"))
+		z.SetClass(PEXTERN)
+		z.Type = t
+		return typecheck(nod(OADDR, z, nil), ctxExpr)
+	}
+
 	fn := syslook("newobject")
 	fn = substArgTypes(fn, t)
 	v := mkcall1(fn, types.NewPtr(t), nil, typename(t))

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{}
+}