[PATCH] Improve DAG combine pass on certain IR vector patterns

[Chandler Carruth]

On Fri, Jan 16, 2015 at 2:40 PM, Quentin Colombet <qcolombet at apple.com>
wrote:

> Well, that may be the conclusion: The performance impact may be within the
> noise.
> Since this kind of patterns are very specific, this is not surprising.
> For the record, I tend to ignore the tests that run for less than 1 second
> (too noisy). Then, the noise level is usually around 1% on a quiet computer
> with fixed frequency, which is not too bad.
>

Numbers would mostly be nice because I don't know if other targets have the
thing that makes this such a huge win on x86 -- implicit concat with undef
to form 2x-wide vectors.

This may be an x86-specific win, in which case it should just be added as a
target-specific combine.
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