[LLVMdev] 'loop invariant code motion' and 'Reassociate Expression'

[Jeroen Dobbelaere]

Hi Andrew, Shuxin,

Thanks for the explanation about 'Reassociate'.

The problem is indeed that one of the variables, becomes loop invariant after
some of the loop transformations.


> -----Original Message-----
> From: Andrew Trick [mailto:atrick at apple.com]
[...]
> Right. Reassociate ranks expressions by their order in the IR, so shouldn't
> make matters worse, but it doesn't know about loops so can't expose more
> of these opportunities AFAIK.
> 
> Jeroen, it may be worth filing a bug. I'm not sure why LSR isn't handling it
> later.
> 
> For cases that LSR doesn't handle, we've been wanting an MI reassociate
> pass in the backend anyway to expose ILP. That could clean up cases like this
> when it's actually on the critical path and we don't mind the regpressure.
> 

I inserted a small optimization pass to recognize and fix this exact sequence.
This did indeed improve the performance (good ;) ), but not to the amount that I hoped..
I'll try to come up with a reduced file that shows this problem and file a bugreport for it.

Greetings,

Jeroen

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