[LLVMdev] Simple Loop Vectorize Question

Joshua Klontz josh.klontz at gmail.com
Thu May 9 17:53:29 PDT 2013


Please forgive my ignorance, but 'opt -mcpu=corei7 -loop-vectorize -S
-debug double.ll' doesn't appear to make a difference. In fact it seems to
be ignored as garbage values for -mcpu don't raise an error. Am I
overlooking something else also?

Many Thanks,

On Thu, May 9, 2013 at 6:06 PM, Nadav Rotem <nrotem at apple.com> wrote:

> 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.
> Thanks,
> Nadav
> 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
> --
> View this message in context:
> http://llvm.1065342.n5.nabble.com/Simple-Loop-Vectorize-Question-tp57584.html
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