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Re: [PATCH, rs6000] Fix vec_construct vectorization cost to be somewhat more accurate


Hi Bill,

On Fri, Jul 15, 2016 at 08:55:08AM -0500, Bill Schmidt wrote:
> This patch is a follow-up to Richard's patch of
> https://gcc.gnu.org/ml/gcc-patches/2016-06/msg00584.html.  The cost of a
> vec_construct (initialization of an N-way vector by N scalars) is too low,
> which can cause too-aggressive vectorization in particular for N=8 or
> higher.  Richard changed the default cost to N-1, which is generally
> sensible.  For powerpc I am going with a slightly higher cost of N, which
> will keep us from being less conservative than the previous values when N=2.

> In any case, the purpose of this patch is simply to avoid vectorizing
> things we shouldn't when we've undercounted the cost of a vec_construct.
> Bootstrapped and tested on powerpc64le-unknown-linux-gnu with no
> regressions (hence the vectorization decisions in the test suite have
> not changed).  Is this ok for trunk?

Do you also have a testcase where it does matter?  It would be good to
add that, then.  Or is it fixing a regression?

I know nothing about the cost model, so someone else will have to review,
or I can just say "okay" ;-)


Segher


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