[M3devel] Self() locking slots almost unnecessary -- if only we had "MemoryBarrier".

[Jay K]

I don't know. I just look at all code overly critically..including for overly coarse grained locking (which includes some vs. none).

I guess the argument could be that the critical section -- the part of code that executes under the lock -- is very short, so it can't make much of a difference.

 

Writing the global with an "InterlockedExchange" might be good.

 

Maybe we should add this as a portably available interface?

"This" being MemoryBarrier and/or well, er, um, I guess you already did, the IA64 stuff, which is similar to the Win32 stuff.

I should update the NT/x86 backend for that stuff and then we can move on and use them.

 

 - Jay

 


From: hosking at cs.purdue.edu
To: jay.krell at cornell.edu
Date: Sun, 8 Nov 2009 19:39:00 -0500
CC: m3devel at elegosoft.com
Subject: Re: [M3devel] Self() locking slots almost unnecessary -- if only we had "MemoryBarrier".





Not portably.  Different memory models will behave differently.  For safety we need the lock.  But, seriously, how much contention will there be?


On 8 Nov 2009, at 19:35, Jay K wrote:

Self() doesn't have to lock slots AS LONG AS in AssignSlot:
 
          SUBARRAY (new_slots^, 0, n) := slots^;
          slots := new_slots;

occurs in the order written.
That SUBARRAY() := finishes before slots := runs.
Aggressively compilers/processors need not execute these in the order written.
 
Do we have a way to guarantee that?
 
Something like:
          SUBARRAY (new_slots^, 0, n) := slots^;
>        MemoryBarrier();
          slots := new_slots;

?
 
MemoryBarrier on Windows is implemented as one "special" instruction -- for x86, AMD64, and IA64.
Those implementations are portable to any OS running those architectures.
Though they aren't expressed in a portable form (x86 inline assembly and C compiler intrinsincs).
In particular:
 
winnt.h:
amd64:
#define MemoryBarrier __faststorefence

x86:
FORCEINLINE
VOID
MemoryBarrier (
    VOID
    )
{
    LONG Barrier;
    __asm {
        xchg Barrier, eax
    }
}

ia64:
#define MemoryBarrier           __mf

 
 - Jay
 

 		 	   		  
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