[cfe-dev] Bug with vectorization of transcendental functions

[Ian Bell]

Perhaps the docs should be updated to remove exp?  Are there architectures
with a SIMD machine instruction for exp()?  Or a link to which
architectures support which SIMD instructions would also be swell.  It's
hard to know a priori what will or will not get vectorized.  Though clang
does at least tell you when it won't be able to vectorize.

That's a nice idea with vecmathlib, but for my application I need something
that is portable, compiles in gcc/clang/msvc for a wide range of compilers
and doesn't require too much hacking.  That said, I did look at it before,
but the docs are *quite* sparse.  The example shows what *not* to do, would
be great if you could come up with an example of best practices.  If you
could, I well might use your library.  What's the expected speedup for AVX
for instance?  How about this as an example:  Take a std::vector<double> x,
calculate exp(x).  I think that would be a seductive application for not
just me.


On Tue, Aug 19, 2014 at 9:28 PM, Erik Schnetter <schnetter at cct.lsu.edu>
wrote:

> On Sun, Aug 17, 2014 at 9:43 AM, Ian Bell <ian.h.bell at gmail.com> wrote:
>
>> Hello all,
>>
>> I am trying to optimize an inner loop in C++ in which nearly 100% of my
>> runtime occurs.  In this loop I have to do some exponential calls, and
>> according to the docs at
>> http://llvm.org/docs/Vectorizers.html#vectorization-of-function-calls ,
>> it seems that the exp function should be able to be vectorized using SIMD
>> instructions.  I'm using the tippy SVN clang.
>>
>
> To my knowledge, Clang does not vectorize arbitrary math functions; it
> only vectorizes those functions for which a SIMD machine instruction
> exists, such as floor.
>
> There are libraries that provide vectorized exp() functions; you could try
> using one of these. However, these libraries require that you rewrite your
> code to use vectors instead of scalars. I maintain such a library
> "vecmathlib" <https://bitbucket.org/eschnett/vecmathlib/wiki/Home> that
> is used in pocl <http://pocl.sourceforge.net/>, an OpenCL implementation
> based on LLVM.
>
> -erik
>
>  The example here shows that the floor function is getting vectorized,
>> but the exp function isn't.  Is this is a bug or known behavior?  I see the
>> same problem if I use floats for what its worth.
>>
>> By the way, there are some problems with the auto-vectorization docs,
>> vectorization is only enabled at -O3, it took me an hour to try to figure
>> out why my -Rpass=loop-vectorize wasn't giving ANY output.
>>
>> My code (loop_test2.cpp):
>>
>> #include <vector>
>> #include <cmath>
>>
>> typedef double numtype;
>>
>> void foo(numtype *f) {
>>   for (int i = 0; i != 1024; ++i)
>>     f[i] = floor(f[i]);
>> }
>>
>> void foo2(numtype *f) {
>>   for (int i = 0; i != 1024; ++i)
>>     f[i] = exp(f[i]);
>> }
>>
>> int main()
>> {
>>   std::vector<numtype> f(1024, 1.3);
>>   foo(&f[0]);
>>   return 0;
>> }
>>
>> Compilation:
>> ../../../build/Debug+Asserts/bin/clang++ -c -Rpass=loop-vectorize
>> -Rpass-analysis=loop-vectorize -O3 -gline-tables-only -gcolumn-info
>> loop_test2.cpp
>>
>> which yields
>>
>> loop_test2.cpp:7:8: remark: unrolled with interleaving factor 2
>> (vectorization
>>       not beneficial) [-Rpass=loop-vectorize]
>>   for (int i = 0; i != 1024; ++i)
>>        ^
>> loop_test2.cpp:13:12: remark: loop not vectorized: call instruction
>> cannot be
>>       vectorized [-Rpass-analysis=loop-vectorize]
>>     f[i] = exp(f[i]);
>>            ^
>> loop_test2.cpp:18:3: remark: vectorized loop (vectorization factor: 2,
>> unrolling
>>       interleave factor: 2) [-Rpass=loop-vectorize]
>>   std::vector<numtype> f(1024, 1.3);
>>   ^
>> loop_test2.cpp:8:12: remark: unrolled with interleaving factor 2
>> (vectorization
>>       not beneficial) [-Rpass=loop-vectorize]
>>     f[i] = floor(f[i]);
>>            ^
>>
>> Thanks,
>> Ian
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>
>>
>
>
> --
> Erik Schnetter <schnetter at cct.lsu.edu>
> http://www.perimeterinstitute.ca/personal/eschnetter/
>
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