[Bug 105256] Slow performance using glDrawElementsBaseVertex

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=105256

--- Comment #1 from Roland Scheidegger <sroland at vmware.com> ---
At a quick glance, I'd suspect the problem isn't really drawElementsBaseVertex
per se, but the use of GL_UNSIGNED_BYTE indices as the code seemed to do.
The hw doesn't support ubyte indices, and this has to be emulated by converting
the elements to ushort (and if you don't start from zero, IIRC this can get
huge buffers allocated especially if start is negative, but I don't remember
the details).
I don't know if the driver could do better, but in general you're advised to
not use ubyte indices (this is a gl only feature, d3d doesn't support it, so
it's unsurprising some hw doesn't support it natively).
(Note that even early GCN does not support ubyte indices, they are only
supported starting with Vulcanic Islands.)

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