[llvm-dev] [LLVMdev] LLVM loop vectorizer - start and end locations
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Fri Aug 12 15:52:24 PDT 2016
Hi Alex,
If you want to get both the starting and ending locations, I think your best bet is to enhance Clang to insert into the loop metadata, not just the location of the start of the loop, but also the location of the end of the loop. Then you can grab that in the backend.
What's your use case for this exactly?
-Hal
----- Original Message -----
> From: "Alex Susu" <alex.e.susu at gmail.com>
> To: "llvm-dev" <llvm-dev at lists.llvm.org>
> Cc: "Adam Nemet" <anemet at apple.com>, "Hal Finkel" <hfinkel at anl.gov>
> Sent: Friday, August 12, 2016 4:43:27 AM
> Subject: Re: [llvm-dev] [LLVMdev] LLVM loop vectorizer - start and end locations
>
> Hello.
> Hal, Adam, thank you very much for the fix mentioned. I ran an
> opt built with this
> fix and I got the precise start loop location.
> I am interested in getting both exact start and end locations
> for a loop in order to
> replace the loop with a different content in the source file
> (basically perform a rather
> non-standard source-to-source transformation).
>
> I've tried to compute the end location for the loop by "parsing"
> the file (looking at
> each character) at least from the start location, but this can be
> quite complex for nested
> blocks in the loop, etc.
>
> Also, I've tried to get more information from the LLVM IR
> instructions:
> - Loop::getUniqueExitBlock()::front()::getDebugLoc() returns
> the first statement
> after the loop. But, in the case of a nested loop the first (and
> last) statement after the
> loop is the "increment" statement in the outer enclosing loop. So,
> even if for simple
> loops getUniqueExitBlock etc looks promising, this is still not
> great.
> - I also iterated through all the statements of all
> basic-blocks of the loop (used
> Loop::block_begin() and block_end(), etc). From these I can choose
> the the min and max
> locations. This is not great either because the loops can contain
> comments before the
> final "}" of the loop (if there is one) and this would result in
> imprecise end location -
> most importantly the "}" of the loop basically does not have a
> corresponding LLVM IR
> instruction. Of course "parsing" to the right of the max location
> found above for an
> uncommented "}" is not very difficult.
>
> I could also try to get more information from the AST of Clang
> while being in the opt
> tool, but I don't know how to read it - maybe I could use Libtooling.
> Do you have an idea
> here?
> Can I get the corresponding AST node from an LLVM IR
> instruction? (Here I got an
> interesting pointer:
> http://clang-developers.42468.n3.nabble.com/Matching-Clang-s-AST-nodes-to-the-LLVM-IR-instructions-they-produced-td3665037.html,
> but maybe it is outdated)
>
> Thank you,
> Alex
>
>
> On 6/8/2016 1:29 AM, Adam Nemet wrote:
> > Hi Alex,
> >
> > This has been very recently fixed by Hal. See
> > http://reviews.llvm.org/rL270771
> >
> > Adam
> >
> >> On Jun 4, 2016, at 3:13 AM, Alex Susu via llvm-dev
> >> <llvm-dev at lists.llvm.org
> >> <mailto:llvm-dev at lists.llvm.org>> wrote:
> >>
> >> Hello.
> >> Mikhail, I come back to this older thread.
> >> I need to do a few changes to LoopVectorize.cpp.
> >>
> >> One of them is related to figuring out the exact C source line
> >> and column number of
> >> the loops being vectorized. I've noticed that a recent version of
> >> LoopVectorize.cpp
> >> prints imprecise debug info for vectorized loops such as, for
> >> example, the location of a
> >> character of an assignment statement inside the respective loop.
> >> It would help me a lot in my project to find the exact C source
> >> line and column
> >> number of the first and last character of the loop being
> >> vectorized. (imprecise location
> >> would make my life more complicated).
> >> Is this feasible? Or are there limitations at the level of
> >> clang of retrieving the
> >> exact C source line and column number location of the beginning
> >> and end of a loop (it
> >> can include indent chars before and after the loop)?
> >> (I've seen other examples with imprecise location such as the
> >> "Reading diagnostics"
> >> chapter in the book
> >> https://books.google.ro/books?isbn=1782166939.)
> >>
> >> Note: to be able to retrieve the debug info from the C source
> >> file we require to run
> >> clang with -Rpass* options, as discussed before. Otherwise, if we
> >> run clang first, then
> >> opt on the resulting .ll file which runs LoopVectorize, we lose
> >> the C source file debug
> >> info (DebugLoc class, etc) and obtain the debug info from the .ll
> >> file. An example:
> >> clang -O3 3better.c -arch=mips -ffast-math -Rpass=debug
> >> -Rpass=loop-vectorize
> >> -Rpass-analysis=loop-vectorize -S -emit-llvm -fvectorize -mllvm
> >> -debug -mllvm
> >> -force-vector-width=16 -save-temps
> >>
> >> Thank you,
> >> Alex
> >>
> >>
> >>
> >> On 2/18/2016 2:17 AM, Mikhail Zolotukhin wrote:
> >>> Hi Alex,
> >>>
> >>> I'm not aware of efforts on loop coalescing in LLVM, but probably
> >>> polly can do
> >>> something like this. Also, one related thought: it might be worth
> >>> making it a separate
> >>> pass, not a part of loop vectorizer. LLVM already has several
> >>> 'utility' passes (e.g.
> >>> loop rotation), which primarily aims at enabling other passes.
> >>>
> >>> Thanks, Michael
> >>>
> >>>> On Feb 15, 2016, at 6:44 AM, RCU <alex.e.susu at gmail.com
> >>>> <mailto:alex.e.susu at gmail.com>
> >>>> <mailto:alex.e.susu at gmail.com>> wrote:
> >>>>
> >>>> Hello, Michael. I come back to this older email. Sorry if you
> >>>> receive it again.
> >>>>
> >>>> I am trying to implement coalescing/collapsing of nested loops.
> >>>> This would be
> >>>> clearly beneficial for the loop vectorizer, also. I'm normally
> >>>> planning to start
> >>>> modifying the LLVM loop vectorizer to add loop coalescing of the
> >>>> LLVM language.
> >>>>
> >>>> Are you aware of a similar effort on loop coalescing in LLVM
> >>>> (maybe even a different
> >>>> LLVM pass, not related to the LLVM loop vectorizer)?
> >>>>
> >>>> Thank you, Alex
> >>>>
> >>>> On 7/9/2015 10:38 AM, RCU wrote:
> >>>>>
> >>>>>
> >>>>> With best regards, Alex Susu
> >>>>>
> >>>>> On 7/8/2015 9:17 PM, Michael Zolotukhin wrote:
> >>>>>> Hi Alex,
> >>>>>>
> >>>>>> Example from the link you provided looks like this:
> >>>>>>
> >>>>>> |for (i=0; i<M; i++ ){ z[i]=0; for (ckey=row_ptr[i];
> >>>>>> | ckey<row_ptr[i+1];
> >>>>>> ckey++) { z[i] += data[ckey]*x[colind[ckey]]; } }|
> >>>>>>
> >>>>>> Is it the loop you are trying to vectorize? I don’t see any
> >>>>>> ‘if’ inside the
> >>>>>> innermost loop.
> >>>>> I tried to simplify this code in the hope the loop vectorizer
> >>>>> can take care of it
> >>>>> better: I linearized...
> >>>>>
> >>>>>> But anyway, here vectorizer might have following troubles: 1)
> >>>>>> iteration count of
> >>>>>> the innermost loop is unknown. 2) Gather accesses ( a[b[i]] ).
> >>>>>> With AVX512 set of
> >>>>>> instructions it’s possible to generate efficient code for such
> >>>>>> case, but a) I
> >>>>>> think it’s not supported yet, b) if this ISA isn’t available,
> >>>>>> then vectorized
> >>>>>> code would need to ‘manually’ gather scalar values to vector,
> >>>>>> which might be slow
> >>>>>> (and thus, vectorizer might decide to leave the code scalar).
> >>>>>>
> >>>>>> And here is a list of papers vectorizer is based on: // The
> >>>>>> reduction-variable
> >>>>>> vectorization is based on the paper: // D. Nuzman and R.
> >>>>>> Henderson.
> >>>>>> Multi-platform Auto-vectorization. // // Variable uniformity
> >>>>>> checks are inspired
> >>>>>> by: // Karrenberg, R. and Hack, S. Whole Function
> >>>>>> Vectorization. // // The
> >>>>>> interleaved access vectorization is based on the paper: //
> >>>>>> Dorit Nuzman, Ira
> >>>>>> Rosen and Ayal Zaks. Auto-Vectorization of Interleaved //
> >>>>>> Data for SIMD // //
> >>>>>> Other ideas/concepts are from: // A. Zaks and D. Nuzman.
> >>>>>> Autovectorization in
> >>>>>> GCC-two years later. // // S. Maleki, Y. Gao, M. Garzaran, T.
> >>>>>> Wong and D. Padua.
> >>>>>> An Evaluation of // Vectorizing Compilers. And probably, some
> >>>>>> of the parts are
> >>>>>> written from scratch with no reference to a paper.
> >>>>>>
> >>>>>> The presentations you found are a good starting point, but
> >>>>>> while they’re still
> >>>>>> good from getting basics of the vectorizer, they are a bit
> >>>>>> outdated now in a
> >>>>>> sense that a lot of new features has been added since then
> >>>>>> (and bugs fixed:) ).
> >>>>>> Also, I’d recommend trying a newer LLVM version - I don’t
> >>>>>> think it’ll handle the
> >>>>>> example above, but it would be much more convenient to
> >>>>>> investigate why the loop
> >>>>>> isn’t vectorized and fix vectorizer if we figure out how.
> >>>>>>
> >>>>>> Best regards, Michael
> >>>>>>
> >>>>>
> >>>>> Thanks for the papers - these appear to be written in the
> >>>>> header of the file
> >>>>> implementing the loop vect. tranformation (found at
> >>>>> "where-you-want-llvm-to-live"/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
> >>>>> ).
> >>>>>
> >>>>>>> On Jul 8, 2015, at 10:01 AM, RCU <alex.e.susu at gmail.com
> >>>>>>> <mailto:alex.e.susu at gmail.com>
> >>>>>>> <mailto:alex.e.susu at gmail.com><mailto:alex.e.susu at gmail.com>>
> >>>>>>> wrote:
> >>>>>>>
> >>>>>>> Hello. I am trying to vectorize a CSR SpMV (sparse matrix
> >>>>>>> vector
> >>>>>>> multiplication) procedure but the LLVM loop vectorizer is not
> >>>>>>> able to handle
> >>>>>>> such code. I am using cland and llvm version 3.4 (on Ubuntu
> >>>>>>> 12.10). I use the
> >>>>>>> -fvectorize option with clang and -loop-vectorize with
> >>>>>>> opt-3.4 . The CSR SpMV
> >>>>>>> function is inspired from
> >>>>>>> http://stackoverflow.com/questions/13636464/slow-sparse-matrix-vector-product-csr-using-open-mp
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >> (I can provide the exact code samples used).
> >>>>>>>
> >>>>>>> Basically the problem is the loop vectorizer does NOT work
> >>>>>>> with if inside loop
> >>>>>>> (be it 2 nested loops or a modification of SpMV I did with
> >>>>>>> just 1 loop - I can
> >>>>>>> provide the exact code) changing the value of the accumulator
> >>>>>>> z. I can sort of
> >>>>>>> understand why LLVM isn't able to vectorize the code.
> >>>>>>> However,
> >>>>>>> athttp://llvm.org/docs/Vectorizers.html#if-conversionitis
> >>>>>>> written: <<The Loop
> >>>>>>> Vectorizer is able to "flatten" the IF statement in the code
> >>>>>>> and generate a
> >>>>>>> single stream of instructions. The Loop Vectorizer supports
> >>>>>>> any control flow in
> >>>>>>> the innermost loop. The innermost loop may contain complex
> >>>>>>> nesting of IFs,
> >>>>>>> ELSEs and even GOTOs.>> Could you please tell me what are
> >>>>>>> these lines exactly
> >>>>>>> trying to say.
> >>>>>>>
> >>>>>>> Could you please tell me what algorithm is the LLVM loop
> >>>>>>> vectorizer using
> >>>>>>> (maybe the algorithm is described in a paper) - I currently
> >>>>>>> found only 2
> >>>>>>> presentations on this
> >>>>>>> topic:http://llvm.org/devmtg/2013-11/slides/Rotem-Vectorization.pdfand
> >>>>>>> <http://llvm.org/devmtg/2013-11/slides/Rotem-Vectorization.pdfand>
> >>>>>>> https://archive.fosdem.org/2014/schedule/event/llvmautovec/attachments/audio/321/export/events/attachments/llvmautovec/audio/321/AutoVectorizationLLVM.pdf
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >> .
> >>>>>>>
> >>>>>>> Thank you very much, Alex
> >>>>>>> _______________________________________________ LLVM
> >>>>>>> Developers mailing listLLVMdev at cs.uiuc.edu
> >>>>>>> <mailto:LLVMdev at cs.uiuc.edu>
> >>>>>>> <mailto:LLVMdev at cs.uiuc.edu><mailto:LLVMdev at cs.uiuc.edu>http://llvm.cs.uiuc.edu
> >>>>>>> <http://llvm.cs.uiuc.edu/>
> >>>>>>>
> >>>>>>>
> >> <http://llvm.cs.uiuc.edu/>
> >>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
More information about the llvm-dev
mailing list