Re: [boost] lightweight_once

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-08-27 20:57:12

Tobias Schwinger:

> AFAIK (not claiming to be an expert in this field, however) it takes a
> rather atypical processor architecture to pull load instructions out of
> their regular execution path (logically) before a conditional branch, no?

No. It's not atypical at all to execute loads speculatively, many 
instructions in advance. The CPU doesn't know whether it will take the 
branch, it predicts it.

Stores that depend on a conditional branch aren't reordered on a PPC, but 
loads - to the extent of my knowledge - can be. You need an 'isync' 
instruction after the branch to discard the speculatively executed loads.

I'm not a PPC expert either. Feel free to not use barriers on PPC. :-)

>> Hasn't Anthony Williams already implemented a header-only call_once? I'm 
>> not
>> sure I see a reason to reinvent that particular wheel. Once boost::mutex 
>> is
>> made header-only, there'd be no need for lightweight_mutex either and 
>> I'll
>> be able to retire it as well.
>
> Where can I find it?

His latest work is here:

http://www.justsoftwaresolutions.co.uk/threading/index.html

Odd that he's not watching this thread.