[src/trunk]: src/common/lib/libc/arch x86: Omit needless store in membar_prod...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/5b71fbbddd34
branches:  trunk
changeset: 364727:5b71fbbddd34
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sat Apr 09 12:07:17 2022 +0000

description:
x86: Omit needless store in membar_producer/exit.

On x86, every store is a store-release, so there is no need for any
barrier.  But this wasn't a barrier anyway; it was just a store,
which was redundant with the store of the return address to the stack
implied by CALL even if issuing a store made a difference.

diffstat:

 common/lib/libc/arch/i386/atomic/atomic.S   |  9 ++++++---
 common/lib/libc/arch/x86_64/atomic/atomic.S |  9 ++++++---
 2 files changed, 12 insertions(+), 6 deletions(-)

diffs (46 lines):

diff -r 1fcb11c0680d -r 5b71fbbddd34 common/lib/libc/arch/i386/atomic/atomic.S
--- a/common/lib/libc/arch/i386/atomic/atomic.S Sat Apr 09 12:07:00 2022 +0000
+++ b/common/lib/libc/arch/i386/atomic/atomic.S Sat Apr 09 12:07:17 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: atomic.S,v 1.31 2022/04/09 12:07:00 riastradh Exp $    */
+/*     $NetBSD: atomic.S,v 1.32 2022/04/09 12:07:17 riastradh Exp $    */
 
 /*-
  * Copyright (c) 2007 The NetBSD Foundation, Inc.
@@ -188,8 +188,11 @@
 END(_membar_consumer)
 
 ENTRY(_membar_producer)
-       /* A store is enough */
-       movl    $0, -4(%esp)
+       /*
+        * Every store to normal memory is a store-release on x86, so
+        * there is never any need for explicit barriers to order
+        * anything-before-store.
+        */
        ret
 END(_membar_producer)
 
diff -r 1fcb11c0680d -r 5b71fbbddd34 common/lib/libc/arch/x86_64/atomic/atomic.S
--- a/common/lib/libc/arch/x86_64/atomic/atomic.S       Sat Apr 09 12:07:00 2022 +0000
+++ b/common/lib/libc/arch/x86_64/atomic/atomic.S       Sat Apr 09 12:07:17 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: atomic.S,v 1.24 2022/04/09 12:07:00 riastradh Exp $    */
+/*     $NetBSD: atomic.S,v 1.25 2022/04/09 12:07:17 riastradh Exp $    */
 
 /*-
  * Copyright (c) 2007 The NetBSD Foundation, Inc.
@@ -263,8 +263,11 @@
 END(_membar_consumer)
 
 ENTRY(_membar_producer)
-       /* A store is enough */
-       movq    $0, -8(%rsp)
+       /*
+        * Every store to normal memory is a store-release on x86, so
+        * there is never any need for explicit barriers to order
+        * anything-before-store.
+        */
        ret
 END(_membar_producer)