[LLVMdev] Converting a i32 pointer to a vector of i32 ( C array to LLVM vector)

[Nadav Rotem]

Both the SLP vectorizer and the Loop vectorizer support vectorizing pointers.  The attached code looks like a candidate for the SLP-vectorizer. Can you run the SLP-vectorizer with the flag -mllvm -debug-only=SLP and attach the log ?  I think that we are missing the pattern for the roots of the tree. 

Thanks,
Nadav

On Oct 16, 2013, at 5:28 PM, Tom Stellard <tom at stellard.net> wrote:

> On Wed, Oct 16, 2013 at 11:14:06AM -0400, Matthieu Dubet wrote:
>> Hi,
>> 
>> Thank you for the information,
>> 
>> So I'm now keeping the array as a pointer (i32*) but the vectorizer doesn't
>> vectorize it .
>> 
>> I've pasted the function code before and after optimization (and the list
>> of optimization that I have activated) in this Gist :
>> https://gist.github.com/maattd/7008683
>> 
>> Some "weird" fact of my LLVM code :
>> 
>> * all variables (even the one used for the loop condition) are pointers to
>> memory allocated from the C world and passed to the LLVM functions as an
>> argument
>> * even with "opt->add(new llvm::DataLayout(*ee->getDataLayout())) ;" in the
>> code, the module->dump() doesn't output neither data layout, nor triple
>> target
>> 
>> Both those points might confuse the vectorizer ?
>> 
>> 
>> On Fri, Oct 11, 2013 at 1:40 PM, Renato Golin <renato.golin at linaro.org>wrote:
>> 
>>> On 11 October 2013 18:27, Matthieu Dubet <maattdd at gmail.com> wrote:
>>> 
>>>> How can I tell LLVM to consider this i32* as an <10 x i32> (and thus get
>>>> the performance improvements thanks to SIMD ..etc..) ?
>>>> 
>>> 
>>> Hi Matthieu,
>>> 
>>> You shouldn't need to do anything, the vectorizer should spot that for
>>> you, if the machine you're compiling to has support for vector
>>> instructions. Any kind of vector operations that you may want to hard-code
>>> will make it not work on anything other than the intrinsics/inline asm
>>> you're using, which is not a good idea.
>>> 
> 
> Which part of the vectorizer is responsible for doing pointer->vector transformations?
> 
> -Tom
> 
>>> If your code didn't get vectorized, it's possible that it is not clear
>>> enough that that pointer is being iterated in a way that it's easy for the
>>> vectorizer to spot, so maybe you need to make it clearer, and that depends
>>> on the code in question. If you could share the code (or a similar example)
>>> with the list, people could help you spot the pattern and make it vectorize.
>>> 
>>> cheers,
>>> --renato
> 
> 
>>> 
> 
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