gallivm: add early cse pass

Graphics / Mesa 3D Graphics Library / Mesa - Roland Scheidegger [] - 24 April 2018 02:49 EDT

This pass is quite cheap, and can simplify the IR quite a bit for our generated IR. In particular on a variety of shaders I've found the time saved by other passes due to the simplified IR more than makes up for the cost of this pass, and on top of that the end result is actually better. The only downside I've found is this enables the LICM pass to move some things out of the main shader loop (in the case I've seen, instanced
vertex fetch (which is constant within the jit shader) plus the derived instructions in the shader) which it couldn't do before for some reason. This would actually be desirable but can increase compile time considerably (licm seems to have considerable cost when it actually can move things out of loops, due to alias analysis). But blaming early cse for this seems inappropriate. (Note that the first two sroa / earlycse passes are similar to what a standard llvm opt -O1/-O2 pipeline would do, albeit this has some more passes even before but I don't think they'd do much for us.) It also in particular helps some crazy shader used for driver
verification (don't ask...) a lot (about factor of 6 faster in compile time) (due to simplfiying the ir before LICM is run). While here, also move licm behind simplifycfg. For some shaders there seems to be very significant compile time gains (we've seen a factor of 10000 albeit that was a really crazy shader you'd certainly never see in a real app), beause LICM is quite expensive and there's cases where running simplifycfg (along with sroa and early-cse) before licm reduces IR complexity significantly. (I'm not entirely sure if it would make sense to also run it afterwards.)

8b9ab674b9 gallivm: add early cse pass
src/gallium/auxiliary/gallivm/lp_bld_init.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)


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