[LLVMdev] SIMD trigonometry/logarithms?

[Michael Gottesman]

First let me say that I really like the notion of being able to plug in .bc libraries into the compiler and I think that there are many potential uses (i.e. vector saturation operations and the like). But even so it is important to realize the limitations of this approach.

Generally implementations of transcendental functions require platform specific optimizations to get the best performance and accuracy. Additionally usually if you implement such operations with SIMD the use cases do do not require as much accuracy as a general math library routine implying that if you just perform a blind vectorization of a math library function you will be giving up a lot of potential performance. If speed/accuracy is not an issue to you and you just want *SOMETHING* I suppose it could work ok.

Something else to consider as well is the possibility of creating an interface for plugging in system SIMD libraries. Then one could use structural analysis or the like to recognize (lets say) an FFT or a Matrix Multiplication and just patch in the relevant routine. On OS X you would use Accelerate, on linux you could use MKL or the like/etc.

Just some thoughts.

Michael

On Jan 27, 2013, at 10:56 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
>> From: "Dmitry Mikushin" <dmitry at kernelgen.org>
>> To: "Justin Holewinski" <justin.holewinski at gmail.com>
>> Cc: "Hal Finkel" <hfinkel at anl.gov>, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
>> Sent: Sunday, January 27, 2013 10:19:42 AM
>> Subject: Re: [LLVMdev] SIMD trigonometry/logarithms?
>> 
>> Hi Justin,
>> 
>> I think having .bc math libraries for different backends makes
>> perfect sense! For example, in case of NVPTX backend we have the
>> following problem: many math functions that are only available as
>> CUDA C++ headers could not be easily used in, for instance, GPU
>> program written in Fortran. On our end we are currently doing
>> exactly what you proposed: generating math.bc module and then link
>> it at IR-level with the target application. There is no need for
>> SIMD, but having .bc math library would still be very important!



> 
> I agree. I think that, essentially, all we need is some infrastructure for finding standard bc/ll include files (much like clang can add its own internal include directory).
> 
> -Hal
> 
>> 
>> - D.
>> 
>> 
>> 2013/1/27 Justin Holewinski < justin.holewinski at gmail.com >
>> 
>> 
>> 
>> I'm wondering if it makes sense to instead supply a bc math library.
>> I would think it would be easier to maintain and debug, and should
>> still give you all of the benefits. You could just link with it
>> early in the optimization pipeline to ensure inlining. This may also
>> make it easier to maintain SIMD functions for multiple backends.
>> 
>> 
>> 
>> 
>> 
>> On Sun, Jan 27, 2013 at 8:49 AM, Hal Finkel < hfinkel at anl.gov >
>> wrote:
>> 
>> 
>> 
>> 
>> ----- Original Message -----
>>> From: "Dimitri Tcaciuc" < dtcaciuc at gmail.com >
>>> To: llvmdev at cs.uiuc.edu
>>> Sent: Sunday, January 27, 2013 3:42:42 AM
>>> Subject: [LLVMdev] SIMD trigonometry/logarithms?
>>> 
>>> 
>>> 
>>> Hi everyone,
>>> 
>>> 
>>> I was looking at loop vectorizer code and wondered if there was any
>>> current or planned effort to introduce SIMD implementations of
>>> sin/cos/exp/log intrinsics (in particular for x86-64 backend)?
>> 
>> Ralf Karrenberg had implemented some of these as part of his
>> whole-function vectorization project:
>> https://github.com/karrenberg/wfv/blob/master/src/utils/nativeSSEMathFunctions.hpp
>> https://github.com/karrenberg/wfv/blob/master/src/utils/nativeAVXMathFunctions.hpp
>> 
>> Opinions on pulling these into the X86 backend?
>> 
>> -Hal
>> 
>>> 
>>> 
>>> Cheers,
>>> 
>>> 
>>> 
>>> 
>>> Dimitri.
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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>> 
>> 
>> 
>> --
>> 
>> 
>> Thanks,
>> 
>> 
>> Justin Holewinski
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>> 
>> 
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