[cfe-dev] [llvm-dev] generate vectorized code

[Mehdi Amini via cfe-dev]

> On Mar 18, 2016, at 2:17 PM, Rail Shafigulin <rail at esenciatech.com> wrote:
> 
> 
> 
> On Fri, Mar 18, 2016 at 2:03 PM, Rail Shafigulin <rail at esenciatech.com <mailto:rail at esenciatech.com>> wrote:
> On Fri, Mar 18, 2016 at 1:53 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> 
>> On Mar 18, 2016, at 1:47 PM, Rail Shafigulin <rail at esenciatech.com <mailto:rail at esenciatech.com>> wrote:
>> 
>> Yes this IR does not build or shuffle any vector. Try to write a function that takes 8 ints and a pointer to a <4xi32>, builds two vectors with the 8 ints,
>> 
>> This might sound like a dumb question, but how does one build a vector of ints out of regular ints in IR? 
> 
> See: http://llvm.org/docs/LangRef.html#vector-operations <http://llvm.org/docs/LangRef.html#vector-operations>
> 
> In short, the IR has "insertelement", which maps to "INSERT_VECTOR_ELT" in SDAG and "extractelement", which maps to "EXTRACT_VECTOR_ELT" in SDAG.
> 
> I usually find good example by grepping in the lit tests. Another way is to write the function in clang, and run it with -O3 -emit-llvm -S to get a good starting point.
> I tried using clang test.c -O3 -emit-llvm -S, but the only I didn't see any of the insertvectorelt or extractvectorelt. I'm wondering how does one trigger vector operations? 
> 
> Below is the test.c file. It seemed to me like a good candidate for vectorization, however nothing happened. I would really appreciate  if you could point me in the right
> direction with respect to vector generation.

I see vectorization happening on this example (see below).


> 
> Any help is appreciated.
>  
> 
> -- 
> Mehdi
> 
> 
> 
> -- 
> Rail Shafigulin
> Software Engineer 
> Esencia Technologies
> 
> Forgot to attach a C file. Here it is:
> 
> #define N 32
> 
> int main () {
> 
>   int  a[N], b[N];
>   int c[N];
> 
>   for (int i = 0; i < N; ++i)
>        c[i] = a[i] + b[i];
> 
>   int sum=0;
>   for (int i = 0; i < N; ++i)
>        sum += c[i];
> 
>   return sum;
> }
> 

This will be vectorized without any insertelement, here is a few lines extracted from the output of clang on this code:

  %wide.load8.6 = load <4 x i32>* %48, align 16, !tbaa !2
  %49 = add nsw <4 x i32> %wide.load8.6, %wide.load.6
  %50 = getelementptr inbounds [32 x i32]* %c, i64 0, i64 24
  %51 = bitcast i32* %50 to <4 x i32>*
  store <4 x i32> %49, <4 x i32>* %51, align 16, !tbaa !2

Because you didn't write the example as I described it, i.e. taking integer, doing a few arithmetic and writing result to contiguous memory, the vectorizer will be able to load directly vectors from memory, operates on them, and store the results. For example try with the following C code:

void foo (int a1, int a2, int a3, int a4, int b1, int b2, int b3, int b4, int *res) {
  res[0] = a1 + b1 * 2;
  res[1] = a2 + b2 * 2;
  res[2] = a3 + b3 * 2;
  res[3] = a4 + b4 * 2;
}


That's for the clang part, you can look at the vectorizer lit test to have examples of IR before/after vectorization.

-- 
Mehdi
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