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 12:30:56AM +0100, Keith Whitwell wrote:
> > > BTW: Last time I looked at libGL (in March), these were things which I
> > > came over:
> > > 1) libGL should IMHO use a version script (at least an anonymous one
> > >    if you want to avoid assigning a specific GL_x.y symbol version to it),
> > >    that way you get rid of thousands of expensive run-time relocations
> >
> > Where can I get info on this?
> 
> info ld, study glibc sources.
> 
> Just checked the latest libGL.so, it has:
>  0x00000016 (TEXTREL)                    0x0
>  0x6ffffffa (RELCOUNT)                   3615
>  0x00000000 (NULL)                       0x0
> 
> Relocation section '.rel.dyn' at offset 0x14074 contains 12341 entries:
> 
> This effectively means libGL.so is not a shared library at all and takes
> awfully lot of time to load during program startup.

Yes, we've been aware of this for a little while.  One thing that we've got a
bit of in there is assembly for the non-threaded dispatch case (the opensource
libGL.so doesn't really handle the threaded case in a performant way, but
we've made some effort on the non-threaded case), that looks a bit like this:

ALIGNTEXT16
GLOBL_FN(GL_PREFIX(NewList))
GL_PREFIX(NewList):
	MOV_L(GLNAME(_glapi_Dispatch), EAX)
	JMP(GL_OFFSET(_gloffset_NewList))

This generates the library entrypoint 'glNewList', which just grabs the active
dispatch table and jumps to the real function.  I had some emails with HJ Lu
about this, but didn't really get what he was saying.  Are these a problem for
building with -fPIC?  I'm not really interested in giving this up as I believe
any benefits from -fPIC will be quickly outweighed by any loss at the dispatch
layer.


> What you can and should do:
> 
> create either linker script like:
> { global: gl*; DRI*; XF86DRI*; local: *; }
> or
> GL_1.0 { global: gl*; DRI*; XF86DRI*; local: *; }
> then pass this file to linker at libGL.so link time, like
> gcc -shared ... -Wl,--version-script,libGL.map ...

I'm playing with this now.

Keith