[LLVMdev] loop metdata instruction

[Redmond, Paul]

Sorry, first reply had bad formatting..

On 2013-02-26, at 8:14 AM, Dmitry Babokin wrote:

> 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".

I think Pekka is correct and in general you can't assume the header belongs to a single loop. However, for reducible CFGs I believe this is true.

> 
> 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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