Re: [patch #6448] [MSVC 7/7] Add MSVC Support

[Ralf Wildenhues]

* Peter Rosin wrote on Wed, Aug 13, 2008 at 10:12:23PM CEST:
>>   24: link-order.at:26   Link order test.
>> Exporting int c variable.
>
> With MSVC, you can declare any variable with __decspec(dllimport), even
> if you are not actually importing it. The only thing that happens is
> that you get an extra indirection in the generated code. This is a
> small price to pay,

The relative cost of extra indirection quite depends on the way it is
used.

To give you an extreme, but real-world example, using Autoconf with the
master branch of GNU m4 is 15% slower with PIC code than without (for an
example package; this amounts to 4 seconds), on GNU/Linux.  That's
basically the overhead of having to go through the GOT indirection.
(Disclaimer: I haven't tried to use PIC objects in a static link.)
I don't know whether the dllimport indirection is as costly though.

Of course for our examples in the test suite it's not the overhead that
matters.

> when the gain is that you don't need to compile
> things differently depending on how you are going to link with the
> library. The situation is quite similar to pic/non-pic, but the trouble
> is that the difference is in the library consumer and not when
> producing the library.

Yep.

> So, ignoring the runtime performance penalty, patch things up like
> this:

Thanks, please apply.

Cheers,
Ralf

> 2008-08-13  Peter Rosin  <address@hidden>
>
>       * tests/link-order.at [MSVC]: Always dllimport exported
>       variables.