[llvm-dev] [LLVMdev] LLVM loop vectorizer - changing vectorized code

Mikhail Zolotukhin via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 13 16:42:21 PDT 2016 

Hi Alex,

> On Jun 13, 2016, at 12:22 PM, Alex Susu <alex.e.susu at gmail.com> wrote:
> 
>  Hello, Mikhail.
>    I'm planning to do source-to-source transformation for loop vectorization.
Could you please share your reasoning on why you need to do it source-to-source? While I recognize that there might be external reasons to do it, I do think that working on IR is much easier.

>    Basically I want to generate C (C++) code from C (C++) source code:
>      - the code that is not vectorized remains the same - this would be simple to achieve if we can obtain precisely the source location of each statement;
If you work completely in front-end, without generating IR, then yes, it's probably true. But the most complicated part thought would be to check if vectorization is legal. Even in IR it's not a trivial task - if you want the same level of error-proofness as we have now, I'm afraid you'll end with just another IR internal to your transformation. For one, think about how would you handle memory aliasing.

If you do lower to IR first, then there is no "the code that is not vectorized remains the same" - it's already mutated by previous passes anyway. E.g. what if the loop was distributed/unrolled before vectorization?

>      - the code that gets vectorized I want to translate in C code the parts that are sequential and generate SIMD intrinsics for my SIMD processor where normally it would generate vector instructions.
>     I started looking at InnerLoopVectorizer::vectorize() and InnerLoopVectorizer::createEmptyLoop(). Not generating LLVM code but C/C++ code (with the help of LLVM intrinsics) is not trivial, but it should be reasonably simple to achieve.
What you suggest here is like writing a C backend and teach it to generate intrinsics for vector code (such backend existed some time ago btw). It should be doable, but I wouldn't call it simple:)

Thanks,
Michael
> 
>    Would you advise for such an operation as the one described above?  I guess doing this as a Clang phase (working on the source code) is not really a bad idea either, since I would have better control on source code, but I would need to reimplement the loop vectorizer algorithm that is currently implemented on LLVM code.
> 
>  Thank you,
>    Alex
> 
> On 6/4/2016 4:28 AM, Mikhail Zolotukhin wrote:
>> Hi Alex,
>> 
>> I think the changes you want are actually not vectorizer related. Vectorizer just uses
>> data provided by other passes.
>> 
>> What you probably might want is to look into routine Loop::getStartLoc() (see
>> lib/Analysis/LoopInfo.cpp). If you find a way to improve it, patches are welcome:)
>> 
>> Thanks, Michael
>> 
>>> On Jun 3, 2016, at 6:13 PM, Alex Susu <alex.e.susu at gmail.com> 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>> 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 Developers mailing list 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://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>> 
>> 
>>

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