Re: [Dri-devel] Re: OpenGL and the LinuxThreads pthread_descr structure

[Keith Whitwell <keith at tungstengraphics dot com>]

Jakub Jelinek wrote:

> On Fri, May 17, 2002 at 01:17:22PM +0100, Keith Whitwell wrote:
> 
>>The __indirect stuff is rarely used.
>>
> 
> Ok, fine. But if it is rarely used, why isn't it -fpic?



The dispatch table in libGL.so is used all the time - this thread was started 
by Gareth with discussion of an optimization to that code (which also applies 
to another thread-local variable used by the 'driver.so' backend).

The __indirect stuff is just along for the ride.  It's a fallback for 
network-transparent rendering, broken configs, and a couple of actual useful 
cases.

I don't think it can be split out completely however as we still have to send 
a few packets off to the X server.

The use of sections you described earlier sounds like we can get that code to 
be PIC and still have a fast dispatch, which is nice.

It sounds like you haven't looked at the driver backends - this is where most 
of the code is.  Our most up-to-date driver illustrates some of Gareth's 
concerns and is on the tcl-0-0-branch of DRI CVS in the 
lib/GL/mesa/src/drv/radeon directory.  This does some of the runtime 
code-generation and dispatch-flipping that he is talking about.


> Few more random things which might help the drivers:
> 1) using fprintf (stderr, ...) all around is a bad idea,
>    that means 2 runtime relocations for each when not -fpic, much better
>    take it offline into a separate .hidden function which will call
>    vfprintf


That's reasonable.  In a lot of cases these aren't compiled into release or 
non-debug drivers, but in the tcl version, I've found it very helpful to keep 
them in there.


> 2) use __builtin_expect if you know better than the compiler what's likely
>    and what is unlikely executed


I'll have to look this one up too.

Keith