[PATCH] Improve spinlock inline assembly for x86. - Mailing list pgsql-hackers

Hi,

I'm currently experimenting with just-in-time compilation using libfirm.
While discussing issues with its developers, it was pointed out to me
that our spinlock inline assembly is less than optimal.  Attached is a
patch that addresses this.

,----
| Remove the LOCK prefix from the XCHG instruction.  Locking is implicit
| with XCHG and the prefix wastes a byte.  Also remove the "cc" register
| from the clobber list as the XCHG instruction does not modify any flags.
|
| Reported by Christoph Mallon.
`----

regards,
Andreas

>From c836b4f3e0b60d070481d4061e6fe0ffbe488495 Mon Sep 17 00:00:00 2001
From: Andreas Seltenreich <seltenreich@gmx.de>
Date: Sun, 17 Jan 2016 11:51:53 +0100
Subject: [PATCH] Improve spinlock inline assembly for x86.

Remove the LOCK prefix from the XCHG instruction.  Locking is implicit
with XCHG and the prefix wastes a byte.  Also remove the "cc" register
from the clobber list as the xchg instruction does not modify any
flags.

Reported by Christoph Mallon.
---
 src/include/storage/s_lock.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h
index 8b240cd..933bb76 100644
--- a/src/include/storage/s_lock.h
+++ b/src/include/storage/s_lock.h
@@ -158,7 +158,6 @@ tas(volatile slock_t *lock)
     __asm__ __volatile__(
         "    cmpb    $0,%1    \n"
         "    jne        1f        \n"
-        "    lock            \n"
         "    xchgb    %0,%1    \n"
         "1: \n"
 :        "+q"(_res), "+m"(*lock)
@@ -226,11 +225,10 @@ tas(volatile slock_t *lock)
     register slock_t _res = 1;

     __asm__ __volatile__(
-        "    lock            \n"
         "    xchgb    %0,%1    \n"
 :        "+q"(_res), "+m"(*lock)
 :        /* no inputs */
-:        "memory", "cc");
+:        "memory");
     return (int) _res;
 }

--
2.1.4