[LLVMdev] [RFC] LCSSA vs. SSAUpdater ... FIGHT!

[Andrew Trick]

On Feb 7, 2014, at 3:39 AM, Chandler Carruth <chandlerc at gmail.com> wrote:

> (1) We're forcing a significant analysis to run between passes to make it easier to update SSA after transformation. But we don't even know if any transformations are needed. I would prefer to use SSAUpdater to handle the transformations as they occur. We could save 1-2% of opt time [1].
> 
> Do you have any measurement of this? I have no evidence that LCSSA is a remotely significant optimization time cost, but I've not gone looking.

1-2% is the time I measured when I wrote that email. But that doesn’t account for any speedups we might get by being in LCSSA for (I’m speculating that calls to SSAUpdater might be faster).

I don’t think it’s a problem now, or will ever be for the standard -O2 pipeline. Just saying that an on-demand SSA updater is better if all else is equal.

-Andy
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