[LLVMdev] loop metdata instruction

[Dmitry Babokin]

Hi Pekka,

On Tue, Feb 26, 2013 at 11:08 AM, Pekka Jääskeläinen <
pekka.jaaskelainen at tut.fi> wrote:
>
>
> Isn't it possible that multiple nested loops share the header and
> the pre-header in normalized loops? Thus, then adding metadata to the
> preheader's branch would make the MD ambiguous for nested loops.
>
> The header can't be shared, otherwise it's the same loop. Though strictly
speaking it depends on definition of the loop, but in my experience, the
most practical definition is "loop == header".

Another observation is that pre-header is highly important for
optimizations as a placeholder for hoisting the code out of the loop. So
even if the pre-header doesn't exist, it makes sense to create it during
optimization. But pre-header of the inner loop may make it more difficult
to detect that loop-nest is perfect nest (i.e. the only real code of
the outer loop is the inner loop).

-Dmitry.
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