LLVM-pipe: most powerful instructions set ;)

[Jose Fonseca]

Trying to combine two OpenGL implementations it's technically difficult.  Furthermore one might thing combining two implementations gives the best of both, but easily gives the worst of both worlds, because there's overhead moving data between them.

Image the worst case scenario: draw on driver A, copy data to driver B, draw on driver B, copy data to driver A.  One might devise a heuristic to avoid switching so often, but the end result would end being the bulk of the rendering beind done on the most capable drvier (llvmpipe in this case), so not much better than always using llvmpipe.


This is why technologies like NVIDIA SLI which divide the work across multiple identical GPUs tend to split the work in a granular fashion, like whole frames (GPU 1 draws frame 2*N, GPU 2 draws frame 2*N + 1).

Furthermore, mixing different implementations easily leads to artifacts, like depth fighting, overlapped geometry or gaps between, because 3D APIs specification have somewhat  lax rules about rasterization, and float conversion.

Honestly, it's more effective to buy a more modern device.

Jose


________________________________
From: mesa-dev <mesa-dev-bounces at lists.freedesktop.org> on behalf of test at profit-grand.ru <test at profit-grand.ru>
Sent: Tuesday, November 15, 2022 12:48
To: mesa-dev at lists.freedesktop.org <mesa-dev at lists.freedesktop.org>
Subject: LLVM-pipe: most powerful instructions set ;)

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First, thanks for saving my day. Software renderer works way faster than
no renderer at all :)

But there are an "instruction set" which is way faster than the MMX or
SSE sets: it's an existing OpenGL 2.0 driver which is not sufficient in
most cases, because at least 3.0 is required nowadays.

Of course, it's not literally an instruction set, but a source of
additional computing power for basic operations (assuming advanced
operations are not supported by the GPU, making LLVM-pipe necessary for
such systems).

So, the actual question is:

Can LLVM-pipe somehow support (today or in future versions) a "bridge
mode", relying on a different OpenGL driver for basic operations
(supported by some ancient GPU) and closing a gap between that driver
and the required OpenGL version it's usual way, by the state-of-art
software renderer?

It's also somewhat related to my OGLOED driver idea (see my Github), the
difference is "bridge mode" is local, OGLOED is not. But there are some
diagrams which can help understanding my question :)


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