switch from assembly stubs to go:linkname comment

The go:linkname way works with Go 1.17 (with regabi), too.

Upstream did this in 2015 in
0b8d583320

See also https://sitano.github.io/2016/04/28/golang-private/ for more background
about both techniques.

internal/buffer/out_message.go

@@ -70,7 +70,7 @@ func (m *OutMessage) OutHeader() *fusekernel.OutHeader {
 func (m *OutMessage) Grow(n int) unsafe.Pointer {
 	p := m.GrowNoZero(n)
 	if p != nil {
-		memclr(p, uintptr(n))
+		jacobsa_fuse_memclr(p, uintptr(n))
 	}
 
 	return p
@@ -113,7 +113,7 @@ func (m *OutMessage) Append(src []byte) {
 	}
 
 	sh := (*reflect.SliceHeader)(unsafe.Pointer(&src))
-	memmove(p, unsafe.Pointer(sh.Data), uintptr(sh.Len))
+	jacobsa_fuse_memmove(p, unsafe.Pointer(sh.Data), uintptr(sh.Len))
 
 	return
 }
@@ -126,7 +126,7 @@ func (m *OutMessage) AppendString(src string) {
 	}
 
 	sh := (*reflect.StringHeader)(unsafe.Pointer(&src))
-	memmove(p, unsafe.Pointer(sh.Data), uintptr(sh.Len))
+	jacobsa_fuse_memmove(p, unsafe.Pointer(sh.Data), uintptr(sh.Len))
 
 	return
 }

internal/buffer/out_message_test.go

@@ -80,7 +80,7 @@ func TestMemclr(t *testing.T) {
 				p = unsafe.Pointer(&b[0])
 			}
 
-			memclr(p, uintptr(len(b)))
+			jacobsa_fuse_memclr(p, uintptr(len(b)))
 
 			// Check
 			if i := findNonZero(p, len(b)); i != len(b) {

internal/buffer/runtime.go

@@ -21,9 +21,11 @@ import "unsafe"
 // Zero the n bytes starting at p.
 //
 // REQUIRES: the region does not contain any Go pointers.
-func memclr(p unsafe.Pointer, n uintptr)
+//go:linkname jacobsa_fuse_memclr runtime.memclrNoHeapPointers
+func jacobsa_fuse_memclr(p unsafe.Pointer, n uintptr)
 
 //go:noescape
 
 // Copy from src to dst, allowing overlap.
-func memmove(dst unsafe.Pointer, src unsafe.Pointer, n uintptr)
+//go:linkname jacobsa_fuse_memmove runtime.memmove
+func jacobsa_fuse_memmove(dst unsafe.Pointer, src unsafe.Pointer, n uintptr)

internal/buffer/runtime_go1.8.s

@@ -1,40 +0,0 @@
-// Copyright 2015 Google Inc. All Rights Reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// +build amd64 arm64 ppc64 ppc64le arm
-// +build go1.8
-
-// Assembly code isn't subject to visibility restrictions, so we can jump
-// directly into package runtime.
-//
-// Technique copied from here:
-//     https://github.com/golang/go/blob/d8c6dac/src/os/signal/sig.s
-
-#include "textflag.h"
-
-#ifdef GOARCH_arm
-#define JMP B
-#endif
-#ifdef GOARCH_ppc64
-#define JMP BR
-#endif
-#ifdef GOARCH_ppc64le
-#define JMP BR
-#endif
-
-TEXT ·memclr(SB),NOSPLIT,$0-16
-	JMP runtime·memclrNoHeapPointers(SB)
-
-TEXT ·memmove(SB),NOSPLIT,$0-24
-	JMP runtime·memmove(SB)

internal/buffer/runtimestub.s

@@ -12,29 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build amd64 arm64 ppc64 ppc64le arm
-// +build !go1.8
-
-// Assembly code isn't subject to visibility restrictions, so we can jump
-// directly into package runtime.
-//
-// Technique copied from here:
-//     https://github.com/golang/go/blob/d8c6dac/src/os/signal/sig.s
-
-#include "textflag.h"
-
-#ifdef GOARCH_arm
-#define JMP B
-#endif
-#ifdef GOARCH_ppc64
-#define JMP BR
-#endif
-#ifdef GOARCH_ppc64le
-#define JMP BR
-#endif
-
-TEXT ·memclr(SB),NOSPLIT,$0-16
-	JMP runtime·memclr(SB)
-
-TEXT ·memmove(SB),NOSPLIT,$0-24
-	JMP runtime·memmove(SB)
+// The buffer package uses //go:linkname to push a few functions into this
+// package but we still need a .s file so the Go tool does not pass -complete
+// to the go tool compile so the latter does not complain about Go functions
+// with no bodies.