[llvm] r190936 - Prevent LoopVectorizer and SLPVectorizer running if the target has no vector registers.

[Tobias Grosser]

On 09/18/2013 02:43 PM, Robert Lytton wrote:
> Author: rlytton
> Date: Wed Sep 18 07:43:35 2013
> New Revision: 190936
>
> URL: http://llvm.org/viewvc/llvm-project?rev=190936&view=rev
> Log:
> Prevent LoopVectorizer and SLPVectorizer running if the target has no vector registers.
>
> XCore target: Add XCoreTargetTransformInfo
> This is where getNumberOfRegisters() resides, which in turn returns the
> number of vector registers (=0).

Hi Robert,

your patch may prevent partial loop unrolling as performed by the loop 
vectorizer.

Some history:

In commit 'r189281 - LoopVectorize: Implement partial loop unrolling 
when vectorization is not profitable.' Nadav implemented partial loop 
unrolling as part of the Loop Vectorizer. I am unsure if partial loop 
unrolling might still be useful/beneficial for targets without vector 
registers. However, in case it is, your current patch prevents it on 
such targets.

At the time Nadav's patch was committed there was some discussion if 
doing unrolling within the vectorizer may cause misunderstandings due to 
the fact that a vectorizer now does more than just vectorization. Is 
your commit based on such a misunderstanding or does e.g. the XCore 
target not benefit from partial loop unrolling?

Cheers,
Tobias