[llvm] r237978 - MergedLoadStoreMotion preserves MemoryDependenceAnalysis, it does not require it.

[Quentin Colombet]

> On May 21, 2015, at 5:36 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
> 
> On Thu, May 21, 2015 at 5:30 PM, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>> Hi Daniel,
>> 
>> 
>>> On May 21, 2015, at 5:13 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>>> 
>>> Author: dannyb
>>> Date: Thu May 21 19:13:05 2015
>>> New Revision: 237978
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=237978&view=rev
>>> Log:
>>> MergedLoadStoreMotion preserves MemoryDependenceAnalysis, it does not require it.
>>> (It already was coded assuming it can sometimes be null, so no other changes are necessary)
>> 
>> What happens when the pass has the MemoryDependenceAnalysis?
> 
> Nothing.
> It doesn't use it.
> It calls the AA API directly.  MD has no effect on AA's results (if it
> did, we would have  really serious problems).

Indeed!

> The only calls to MD are to the update functions.
> Here is the list of things it does with MD:
> 
>    MD->removeInstruction(Inst);
> 
>      MD->invalidateCachedPointerInfo(LI->getPointerOperand());
> 
>      MD->invalidateCachedPointerInfo(Inst);
> 
>        MD->invalidateCachedPointerInfo(NewPN);
> 
> None of these do anything but update MD.
> 
> It doesn't pass MD to any utilities or anything, either.
> 
> 
>> If so, are we sure this commit does not regress anything?
> 
> Yes.
> It performs exactly the same set of opts before or after.

Thanks for the clarifications.

Cheers,
-Quentin
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