Atomic test&exchange on alpha - assember code for verification

[Grzegorz Prokopski <gadek@debian.org>]

Hello!

I am porting SableVM (JVM) to alpha platform. I am almost finished, but
as I don't really know alpha assemler too well and google can't give
guarantees that what I created works as I wanted it to - I decided
to ask here.

In the app there's small assembler code, which does the following:

function test&exchange(volatile *pword, old_value, new_value)
 if (*pword == old_value)
     {
       *pword = new_value;
       return 1;
     }
   else
     {
       return 0;
     }
}

But it cannot be written in C. That part of operations:

if (*pword == old_value)
     {
       *pword = new_value;

must:
- be atomic / non-preempable "in the middle" by
 exceptions/interrupts/etc
- make sure, that other processors see exactly the same data
 if they also try to do that operation (assure coherency between
 main memory, processor cache and it seems also register - when
 the operation is being done)

I spent all evening and night digging ito many sources, from gcc
assembler documentation, alpha-assembler-guides, to bsd-alpha and
postgresql mailing list archives.

And finally - here's what I wrote:

static inline jboolean
_svmh_compare_and_swap (volatile _svmt_word *pword, _svmt_word
		 old_value, _svmt_word new_value) {
  register int result, tmp;

  __asm__ __volatile__ (
"1:  mb\n\t"                    // make sure (may be unneded?)
"    ldq_l      %1,%4\n\t"      // load *pword into tmp (reg,<= mem)
// does above make sure main memory->cache->register are coherent?
"    cmpeq      %1,%5,%0\n\t"   // result = (*pword == tmp)
"    beq        %0,2f\n\t"      // nothing to do if they differ we
// get 0 just jump away (what happens to processor lock then?)
"    stq_c      %3,%4\n\t"      // *pword = new_value (reg,=> mem)
"    mb\n\t"                    // make sure everything was put back to
// main mem
"2:  nop"
         : "=&r"(result), "=&r"(tmp), "=m"(*pword)
         : "r" (new_value), "m" (*pword), "r" (old_value));

  return result ? JNI_TRUE : JNI_FALSE;
}

_svmt_word is 64 bit (on alpha)

Please verify, comment and elaborate if you can.
Cc: me on replies. Thanks.

Regards

					Grzegorz B. Prokopski

PS: I never wrote in alpha assember, but I did for i386, i8051,
motorola HC11
PSS: If nobody is able to verify that code (can happen?) - please point
me to where I should ask for this.

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