[LLVMdev] Simple Loop Vectorize Question

Nadav Rotem nrotem at apple.com
Thu May 9 15:06:27 PDT 2013

Hi Josh, 

Your modules does not have a triple, so the target machine and TargetTransformInfo have no way of knowing if you are running on a machine with vector registers.  Try adding the '-mcpu=XXXX' to opt and see what happens. 


On May 9, 2013, at 1:42 PM, Josh Klontz <josh.klontz at gmail.com> wrote:

> Hi! I am trying to get the loop vectorizer to work on a simple example
> (http://pastebin.com/tGhpc4y0) that doubles every element in a vector.
> I've found that 'opt -loop-vectorize -force-vector-width=4 -S -debug
> double.ll' works as expected. However, removing the -force-vector-width flag
> results in no vectorization. From the debug output I can see that the issue
> boils down to:
> LV: The Widest type: 32 bits.
> LV: The Widest register is:32bits.
> I tried to work back through the source code to figure out why the widest
> register is incorrect, though I get lost following the code logic for how
> TargetTransformInfo gets initialized. Therefore, I have two questions:
> 1) Can -force-vector-width be specified from the C++ API? And if so, how?
> 2) What am I neglecting to do so that TargetTransformInfo is set correctly
> and vectorization happens without forcing a vector width? Ultimately I would
> like use vectorization in conjunction with the JIT ExecutionEngine.
> Thank you to those of you who have answered my questions in the past, the
> answers have helped tremendously and I am extremely grateful!
> Kindly,
> Josh
> --
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