[LLVMdev] Vectorizer using Instruction, not opcodes

[Renato Golin]

On 5 February 2013 11:03, David Tweed <david.tweed at arm.com> wrote:

> since the more instructions there
> are the more an out-of-order CPU can put them into otherwise unused slots.
> I
> can't think of a way of figuring out such a mapping other than empirically.
>


Given the amount of uncertainty on these OOO guesses, I don't think we can
get anything worth trying, even empirically. The noise will always outweigh
the signal.

You can normally save a few cycles on a static micro-benchmark and think
you're in control, but you normally don't evaluate the exact impact that
the same guess had on other benchmarks or real code. Since this is
auto-vectorization, it'll be hard to compare, but more so to evaluate the
overall impact on the rest.

--renato
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