[Mesa-dev] [PATCH 02/10] i965 gs: Reduce information in key to avoid unnecessary recompiles.

[Eric Anholt]

On Mon, 5 Dec 2011 15:14:13 -0800, Paul Berry <stereotype441 at gmail.com> wrote:
> On 5 December 2011 14:53, Eric Anholt <eric at anholt.net> wrote:
> 
> > On Mon,  5 Dec 2011 09:40:45 -0800, Paul Berry <stereotype441 at gmail.com>
> > wrote:
> > > Previously, the geometry shader program key was storing all the
> > > information necessary to compute the exact structure of the VUE map
> > > (attrs and userclip_active).  However, the GS program doesn't depend
> > > on the exact structure of the VUE map; only on the size (in 256-bit
> > > registers) of the VUE.  So we were recompiling the GS program
> > > unnecessarily when the VUE map changed in structure but not in size.
> > >
> > > This patch changes the key to store just the size of the VUE map, not
> > > its structure.
> >
> > Hmm.  I'm betting that brw_compute_vue_map() is way more expensive than
> > the extra couple bytes of key data to compare.
> >
> 
> True, but it's way less expensive than unnecessarily compiling (and
> uploading to the GPU) a brand new GS program that is identical to the one
> that's already there.  That's what this patch is avoiding.

That still happens a limited number of times, while key setup is per
drawing operation.  And if you've got the same program code, you should
even end up associating the same instructions with the identical copies.
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