[llvm-dev] [GSoC] Supporting Efficiently the Shift-vector Instructions of the Connex Vector Processor
Roman Lebedev via llvm-dev
llvm-dev at lists.llvm.org
Fri May 3 12:46:57 PDT 2019
On Fri, May 3, 2019 at 10:27 PM Alex Susu <alex.e.susu at gmail.com> wrote:
> Hello, Roman, llvm-dev,
> There is one aspect I would like to discuss with you, the people from the llvm-dev
> mailing list. There is one thing a bit more special with this back end, namely our back
> end handles symbolic immediate operands (C/C++ expressions written as strings in INLINEASM
> MachineInstrs). This means the back end can output a vector assembly code like:
> VLOAD RegVectorial0, N * 10 + 5 // where N is a variable in the original C program
> Therefore, in order to support retrieving from LLVM IR back to the original source C
I'm not sure how that is supposed to work. What if IR does not originate from C?
How do you verify that the "C string" is in the form that will be understood
by whatever will handle it later on? Is there a clang part of the patch?
Regardless, that was already answered in https://reviews.llvm.org/D60052#1476119
> In general, IR instructions should be lowered to SelectionDAG nodes in a way that
> doesn't require referring back to the original Instruction afterwards.
> I've committed a new version of the source code at https://reviews.llvm.org/D60052 .
> I hope it will get accepted soon - I'm discussing with the reviewers.
As it has already been stated multiple times in the review, that is a
huge patch. It needs to be split into *many* *small*, standalone patches,
each one with proper test coverage and no dead code, before any review
can happen. You won't get an an alternative opinion from any other reviewer.
> Best regards,
> On 4/29/2019 10:14 AM, Roman Lebedev wrote:
> > On Mon, Apr 29, 2019 at 1:56 AM Alex Susu via llvm-dev
> > <llvm-dev at lists.llvm.org> wrote:
> >> Hello, Anton,
> >> I'd like to add a small reply regarding this GSoC project that I would like to mentor
> >> and I discussed also with Andrei.
> >> A good part of our GSoC project is indeed related to this Connex back end that it's
> >> not yet part of the LLVM source repository - an important thing proposed in the project is
> >> that we plan to perform efficient realignment for this Connex vector processor.
> >> I looked a bit in LLVM and I see that support for realignment of misaligned vector
> >> memory accesses is not implemented in the LoopVectorize pass (see
> >> lib/Transforms/Vectorize/LoadStoreVectorizer.cpp) nor in any back end (folder lib/Target).
> >> Please correct me if I'm wrong.
> >> But realignment is an interesting technique useful for many SIMD and (wide) vector
> >> processors - there are still SIMD processors today that either have performance issues or
> >> simply can't perform misaligned accesses, and for wide vector processors with many lanes
> >> this problem is equally important and even more complex. People have already addressed in
> >> a platform-independent way realignment of misaligned vector memory accesses - see, for
> >> example for GCC a paper of Nuzman and Henderson ("Multi-platform Auto-vectorization", CGO
> >> 2006, https://www.researchgate.net/publication/4231612_Multi-platform_auto-vectorization).
> >> It would be interesting to address this issue of realignment of misaligned vector
> >> memory accesses in LLVM - this seems to be already well supported in GCC.
> >> Please note also our Connex vector processor back end has been reviewed and we should
> >> be accepted as experimental (also because it has a few "exotic" features) - see, if
> >> interested, https://reviews.llvm.org/D60052 .
> > I don't believe that statement to be true/faithful to the truth.
> > It was not reviewed as in "ready to land".
> > It's not quite anywhere near that state, yet anyway.
> >> Best regards,
> >> Alex
> > Roman.
> >> On 4/9/2019 1:38 PM, Anton Korobeynikov via llvm-dev wrote:
> >>> Hello Andrei,
> >>> You proposal seems to be centered on improvements of out-of-tree
> >>> backend entirely and therefore is not entirely clear what are the
> >>> benefits of this project to the LLVM project making the project
> >>> slightly outside the scope of LLVM GSoC.
> >>> Probably it would make sense to restructure the proposal in such way
> >>> that it would be clear how the work that is supposed to be done over
> >>> the summer will benefit e.g. other backends that we already having in
> >>> the repository. After all, the vector instructions do exist in many
> >>> architectures.
> >>> On Mon, Apr 8, 2019 at 7:53 PM Andrei Popa via llvm-dev
> >>> <llvm-dev at lists.llvm.org> wrote:
> >>>> Hello,
> >>>> I am applying for Google Summer of Code with a project related to LLVM and Connex SIMD processor and I would appreciate some feedback on the proposal.
> >>>> The proposal can be found here:
> >>>> https://docs.google.com/document/d/1pBRbW8pU9GV8zWCJQrILhynNEBpGXJKtev1j7ekXfqs/edit?usp=sharing
> >>>> Thank you,
> >>>> Andrei Popa
> >>>> _______________________________________________
> >>>> LLVM Developers mailing list
> >>>> llvm-dev at lists.llvm.org
> >>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >> _______________________________________________
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