Re: MIPS test-and-set

On Mon, Mar 26, 2001 at 07:09:38PM -0500, Tom Lane wrote:
> Thomas Lockhart <lockhart(at)alumni(dot)caltech(dot)edu> writes:
> > That is not already available from the Irix support code?
>
> What we have for IRIX is
> ...
> Doesn't look to me like it's likely to work on anything but IRIX ...

I have attached linuxthreads/sysdeps/mips/pt-machine.h from glibc-2.2.2
below. (Glibc linuxthreads has alpha, arm, hppa, i386, ia64, m68k, mips,
powerpc, s390, SH, and SPARC support, at least in some degree.)

Since the actual instruction sequence is probably lifted from the
MIPS manual, it's probably much freer than GPL. For the paranoid,
the actual instructions, extracted, are just

1:
ll %0,%3
bnez %0,2f
li %1,1
sc %1,%2
beqz %1,1b
2:

Nathan Myers
ncm(at)zembu(dot)com

-----------------------------------
/* Machine-dependent pthreads configuration and inline functions.

Copyright (C) 1996, 1997, 1998, 2000 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ralf Baechle <ralf(at)gnu(dot)org>.
Based on the Alpha version by Richard Henderson <rth(at)tamu(dot)edu>.

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 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
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB. If
not, write to the Free Software Foundation, Inc.,
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */

#include <sgidefs.h>
#include <sys/tas.h>

#ifndef PT_EI
# define PT_EI extern inline
#endif

/* Memory barrier. */
#define MEMORY_BARRIER() __asm__ ("" : : : "memory")

/* Spinlock implementation; required. */

#if (_MIPS_ISA >= _MIPS_ISA_MIPS2)

PT_EI long int
testandset (int *spinlock)
{
long int ret, temp;

__asm__ __volatile__
("/* Inline spinlock test & set */\n\t"
"1:\n\t"
"ll %0,%3\n\t"
".set push\n\t"
".set noreorder\n\t"
"bnez %0,2f\n\t"
" li %1,1\n\t"
".set pop\n\t"
"sc %1,%2\n\t"
"beqz %1,1b\n"
"2:\n\t"
"/* End spinlock test & set */"
: "=&r" (ret), "=&r" (temp), "=m" (*spinlock)
: "m" (*spinlock)
: "memory");

return ret;
}

#else /* !(_MIPS_ISA >= _MIPS_ISA_MIPS2) */

PT_EI long int
testandset (int *spinlock)
{
return _test_and_set (spinlock, 1);
}
#endif /* !(_MIPS_ISA >= _MIPS_ISA_MIPS2) */

/* Get some notion of the current stack. Need not be exactly the top
of the stack, just something somewhere in the current frame. */
#define CURRENT_STACK_FRAME stack_pointer
register char * stack_pointer __asm__ ("$29");

/* Compare-and-swap for semaphores. */

#if (_MIPS_ISA >= _MIPS_ISA_MIPS2)

#define HAS_COMPARE_AND_SWAP
PT_EI int
__compare_and_swap (long int *p, long int oldval, long int newval)
{
long int ret;

__asm__ __volatile__
("/* Inline compare & swap */\n\t"
"1:\n\t"
"ll %0,%4\n\t"
".set push\n"
".set noreorder\n\t"
"bne %0,%2,2f\n\t"
" move %0,%3\n\t"
".set pop\n\t"
"sc %0,%1\n\t"
"beqz %0,1b\n"
"2:\n\t"
"/* End compare & swap */"
: "=&r" (ret), "=m" (*p)
: "r" (oldval), "r" (newval), "m" (*p)
: "memory");

return ret;
}

#endif /* (_MIPS_ISA >= _MIPS_ISA_MIPS2) */