[release-branch.go1.15] runtime: implement StorepNoWB for wasm in assembly

The second argument of StorepNoWB must be forced to escape.
The current Go code does not explicitly enforce that property.
By implementing in assembly, and not using go:noescape, we
force the issue.

Test is in CL 249761. Issue #40975.

This CL is needed for CL 249917, which changes how go:notinheap
works and breaks the previous StorepNoWB wasm code.

I checked for other possible errors like this. This is the only
go:notinheap that isn't in the runtime itself.

Included test from CL 249761.

Update #41432

Change-Id: I43400a806662655727c4a3baa8902b63bdc9fa57
Reviewed-on: https://go-review.googlesource.com/c/go/+/249962
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
(cherry picked from commit c0602603b2)
Reviewed-on: https://go-review.googlesource.com/c/go/+/260878
Trust: Keith Randall <khr@golang.org>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/runtime/internal/atomic/asm_wasm.s

@@ -0,0 +1,10 @@
+// Copyright 2020 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.
+
+#include "textflag.h"
+
+TEXT runtime∕internal∕atomic·StorepNoWB(SB), NOSPLIT, $0-16
+	MOVD ptr+0(FP), R0
+	MOVD val+8(FP), 0(R0)
+	RET

src/runtime/internal/atomic/atomic_test.go

@@ -220,3 +220,13 @@ func TestBitwiseContended(t *testing.T) {
 		}
 	}
 }
+
+func TestStorepNoWB(t *testing.T) {
+	var p [2]*int
+	for i := range p {
+		atomic.StorepNoWB(unsafe.Pointer(&p[i]), unsafe.Pointer(new(int)))
+	}
+	if p[0] == p[1] {
+		t.Error("Bad escape analysis of StorepNoWB")
+	}
+}

src/runtime/internal/atomic/atomic_wasm.go

@@ -153,14 +153,11 @@ func Store64(ptr *uint64, val uint64) {
 	*ptr = val
 }
 
-//go:notinheap
-type noWB struct{}
-
-//go:noinline
-//go:nosplit
-func StorepNoWB(ptr unsafe.Pointer, val unsafe.Pointer) {
-	*(**noWB)(ptr) = (*noWB)(val)
-}
+// StorepNoWB performs *ptr = val atomically and without a write
+// barrier.
+//
+// NO go:noescape annotation; see atomic_pointer.go.
+func StorepNoWB(ptr unsafe.Pointer, val unsafe.Pointer)
 
 //go:nosplit
 //go:noinline