[PATCH] Fold selects when the non-selected elements are undef from shufflevectors

[Nadav Rotem]

Hi Filipe, 

Thank you for working on this. I appreciate the fact that you are eager to improve LLVM and that you went through the trouble of testing your code on multiple targets and on considering different alternatives to the problem.  I think that the patch below is unrelated to the blend->select lowering so I will only address this patch. We discussed on the possibility of creating new shuffles on the llvm mailing lists more than once. We decided not to create new shuffles because we may create new shuffle patterns that we can’t lower efficiently. I ran into this problem a few times myself in different LLVM-based projects, mainly in the domain of graphics where people write swizzles themselves. Please do not commit this patch because it creates new shuffles. It is not the correct approach. You may get lucky in you selection of testcase, but I am sure that there are lots of other shuffle inputs on X86 and other targets that this patch makes worse. 

Thanks,
Nadav  


> Hi Nadav,
> 
> I'd really like this patch to get in, since it improves code generation on many targets (at the very least, I've tried arm and arm64 (mcpu=cyclone), ppc32 and ppc64 (mcpu=g5), and x86 and x86-64 (mcpu=nehalem) and they all improve (they end up with no loads vs. 1 or 2 loads, and code size is about half of what it was).
> 
> Even if we'd like to make the select instruction be the proper canonicalization of this operation, it seems that, for now, a shufflevector is better when it comes to lowering it.
> 
> Several targets have the action for the vselect as Expand, which will generate a bunch of code which would then have to be pattern-matched, instead of lowering the vselect as custom and falling-through to Expand if there's no special-case instruction/set of instructions that we can emit.
> 
> I ran opt+llc in this way, for the variants I mentioned above (./bin/opt is opt with this optimization):
> ./bin/opt -O3 select-from-shuffles.ll | ./bin/llc -march=arm64 -mcpu=cyclone > arm64-cylone-opt
> opt -O3 select-from-shuffles.ll | ./bin/llc -march=arm64 -mcpu=cyclone > arm64-cylone-opt
> 
> I can understand if we eventually drop this after we make the backend treat the select as the canonical form of this operation and have llvm always use that form, but for now it seems that the shufflevector is better lowered in some important targets.
> 
> 
>   Filipe
> 
> 
> 
> 
> On Fri, May 2, 2014 at 11:55 PM, Nadav Rotem <nrotem at apple.com> wrote:
> Hi Filipe,
> 
> We don’t generate new shufflevector instructions during optimizations because the lowering of shuffle instructions is really complicated and we don’t want to generate a shuffle that we don’t lower by accident.
> 
> Thanks,
> Nadav
> 
> On May 2, 2014, at 4:34 PM, Filipe Cabecinhas <filcab+llvm.phabricator at gmail.com> wrote:
> 
> > Remove unused argument
> >
> > http://reviews.llvm.org/D3561
> >
> > Files:
> >  lib/Transforms/InstCombine/InstCombineSelect.cpp
> >  test/Transforms/InstCombine/select.ll
> > <D3561.9050.patch>_______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> 
> 

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