Re: [Boost-users] [weak_ptr] thread safe?

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2006-04-19 13:23:06

Hi!

In Visual Studio 2003's MSDN is written:

"#pragma intrinsic( function1 [, function2, ...] )
Specifies that calls to functions specified in the pragma's argument list are intrinsic. The
compiler generates intrinsic functions as inline code, not as function calls. The library
functions with intrinsic forms are listed below. Once an intrinsic pragma is seen, it takes effect
at the first function definition containing a specified intrinsic function. The effect continues
to the end of the source file or to the appearance of a function pragma specifying the same
intrinsic function. The intrinsic pragma can be used only outside of a function definition — at
the global level."

Online MSDN states (which is for VC 2005):

"Some intrinsics are available only as intrinsics, and some are available both in function and
intrinsic implementations. You can instruct the compiler to use the intrinsic implementation in
one of two ways, depending on whether you want to enable only specific functions or you want to
enable all intrinsics. The first way is to use #pragma intrinsic(intrinsic-function-name-list).
The pragma can be used to specify a single intrinsic or multiple intrinsics separated by commas.
The second is to use the /Oi (Generate Intrinsic Functions) compiler option, which makes all
intrinsics on a given platform available. Under /Oi, use #pragma
function(intrinsic-function-name-list) to force a function call to be used instead of an
intrinsic. If the documentation for a specific intrinsic notes that the routine is only available
as an intrinsic, then the intrinsic implementation is used regardless of whether /Oi or #pragma
intrinsic is specified. In all cases, /Oi or #pragma intrinsic allows, _but does not force_, the
optimizer to use the intrinsic. The optimizer can still call the function."

May be VC6.0 will force intrinsic and newer compilers not always...

With Kind Regards,

Ovanes Markarian

On Wed, April 19, 2006 18:34, John Wilkinson said:
> There is a pragma in VC6 that will activate or deactivate optimization.
> It can be used to wrap optimization-sensitive blocks of code.  I do not
> remember what it is, though, and I do not have VC6 handy.  I have used
> it, and it did alleviate an optimization-related bug.
>
> John
>
> -----Original Message-----
> From: boost-users-bounces_at_[hidden]
> [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Peter Dimov
> Sent: Wednesday, April 19, 2006 10:45 AM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] [weak_ptr] thread safe?
>
> Alain Cormier wrote:
>
>> So, can I conclude that it is a bad behavior in boost::weak_ptr?
>
> No, it is a bug in VC6.
>
>> For now, I have simply changed add_ref_lock to :
>>
>> long tmp2 = tmp + 1;
>> if(InterlockedCompareExchange ( &use_count_, tmp2, tmp ) == tmp2 - 1 )
>> return true;
>>
>> This way, VC6 doesn't optimise the return value of
>> InterlockedCompareExchange and the disassembly looks ok.
>
> I don't think that there is any guarantee that this modification will
> reliably produce correct code, even if it happens to work in the
> examples we
> try. But if we can't think of a better way to work around the issue,
> this
> may be our best bet.
>
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