[PATCH] Unrolling improvements (target indep. and for x86)

Chandler Carruth chandlerc at google.com
Thu Mar 6 11:42:38 PST 2014

On Mon, Mar 3, 2014 at 2:54 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> However, Chandler had made a similar change to the loop vectorizer (to
> prefer power-of-2 unrolling factors), because it helps with X86 addressing
> modes, and as a result, I wonder whether it is worth keeping this part
> regardless. Chandler?

To try and answer this specifc question first:

The power-of-two thing is an interesting and pesky issue. I suspect that it
matters more for "widening" style unrolls than for this version. Consider
that in the widening version, we need to have all N unroll-step pointers
live at the same time (potentially), whereas here we can just lea them
however we like on each unrolled iteration.

So I wouldn't include it until we have benchmarks showing it matters.

Regarding the entire patch series, please at least commit everything but
the gep-cost-metric change at least. I'm more hesitant there and would like
to stare at the patch a bit.
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