[llvm-dev] [LLVMdev] LLVM loop vectorizer

Alex Susu via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 3 18:13:46 PDT 2016

     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,

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>> 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>> 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-conversionit is 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
>>>>> 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
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