x86: Remove low-level lock optimization

The current approach is to do this optimizations at a higher level,
in generic code, so that single-threaded cases can be specifically
targeted.

Furthermore, using IS_IN (libc) as a compile-time indicator that
all locks are private is no longer correct once process-shared lock
implementations are moved into libc.

The generic <lowlevellock.h> is not compatible with assembler code
(obviously), so it's necessary to remove two long-unused #includes.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/i386/nptl/pthread_spin_lock.S

@@ -16,7 +16,6 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <sysdep.h>
-#include <lowlevellock.h>
 
 ENTRY (pthread_spin_lock)
 	mov	4(%esp), %eax

sysdeps/unix/sysv/linux/x86/lowlevellock.h

@@ -1,87 +0,0 @@
-/* Low-level lock implementation, x86 version.
-   Copyright (C) 2019-2021 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <https://www.gnu.org/licenses/>.  */
-
-#ifndef _X86_64_LOWLEVELLOCK_H
-#define _X86_64_LOWLEVELLOCK_H	1
-
-#ifndef __ASSEMBLER__
-#include <sysdeps/nptl/lowlevellock.h>
-#include <single-thread.h>
-
-/* The lll_trylock, lll_lock, and lll_unlock implements a single-thread
-   optimization using the cmpxchgl instruction.  It checks if the process
-   is single thread and avoid a more expensive atomic instruction.  */
-
-/* The single-thread optimization only works for libc itself, we need
-   atomicity for libpthread in case of shared futexes.  */
-#if !IS_IN(libc)
-# define is_single_thread 0
-#else
-# define is_single_thread SINGLE_THREAD_P
-#endif
-
-/* In the __lllc_as we simply return the value in %eax after the cmpxchg
-   instruction.  In case the operation succeeded this value is zero.  In case
-   the operation failed, the cmpxchg instruction has loaded the current value
-   of the memory work which is guaranteed to be nonzero.  */
-static inline int
-__attribute__ ((always_inline))
-__lll_cas_lock (int *futex)
-{
-  int ret;
-  asm volatile ("cmpxchgl %2, %1"
-		: "=a" (ret), "=m" (*futex)
-		: "r" (1), "m" (*futex), "0" (0)
-		: "memory");
-  return ret;
-}
-
-#undef lll_trylock
-#define lll_trylock(lock)						     \
-  ({									     \
-    int __ret;								     \
-    if (is_single_thread)						     \
-      __ret = __lll_cas_lock (&(lock));					     \
-    else								     \
-      __ret = __lll_trylock (&(lock));					     \
-    __ret;								     \
-  })
-
-#undef lll_lock
-#define lll_lock(lock, private)						     \
-  ((void)								     \
-   ({									     \
-     if (is_single_thread)						     \
-       __lll_cas_lock (&(lock));					     \
-     else								     \
-       __lll_lock (&(lock), private);					     \
-   }))
-
-#undef lll_unlock
-#define lll_unlock(lock, private)					     \
-  ((void)								     \
-   ({									     \
-     if (is_single_thread)						     \
-       (lock)--;							     \
-     else								     \
-       __lll_unlock (&(lock), private);					     \
-   }))
-
-#endif  /* !__ASSEMBLER__ */
-
-#endif	/* lowlevellock.h */

sysdeps/x86_64/nptl/pthread_spin_lock.S

@@ -15,7 +15,6 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <lowlevellock.h>
 #include <sysdep.h>
 
 ENTRY(pthread_spin_lock)