[PATCH] D58248: [EarlyCSE & MSSA] Cap the clobbering calls in EarlyCSE.

Alina Sbirlea via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Fri Feb 15 14:12:38 PST 2019

asbirlea marked an inline comment as done.
asbirlea added inline comments.

Comment at: lib/Transforms/Scalar/EarlyCSE.cpp:1206
   // Reset the current generation.
   CurrentGeneration = LiveOutGeneration;
george.burgess.iv wrote:
> asbirlea wrote:
> > george.burgess.iv wrote:
> > > Should we be resetting `ClobberCounter` here, as well?
> > AFAICT `run()` is only invoked once for an EarlyCSE instance and it's not recursive.
> > 
> > I'm also missing the purpose of `CurrentGeneration = LiveOutGeneration;` and
> > the above
> > ```
> >   // Save the current generation.
> >   unsigned LiveOutGeneration = CurrentGeneration;
> > ```
> > Could you please clarify?
> > AFAICT run() is only invoked once for an EarlyCSE instance and it's not recursive.
> That's what I thought, too, but I saw this, so I assumed I was missing something.
> Looks like that was added circa 2012; I assume this pass has changed quite a lot since then. I'm hoping davide (or one of the subscribers) may have context on EarlyCSE's usage patterns?
> If no one is sure, I vote that we replace it with an early `assert(!CurrentGeneration && "Create a new EarlyCSE instance to run it again")` (which can optionally be removed if no one complains within a reasonable amount of time). 
I agree with cleaning up existing code, but I will argue against doing it in this patch. Happy to send a follow up one adding the assert.




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