[llvm-dev] GlobalsAA from GVN

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Vaivaswatha Nagaraj via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "James Molloy" <james at jamesmolloy.co.uk>
> Cc: "LLVM Dev" <llvm-dev at lists.llvm.org>
> Sent: Thursday, December 3, 2015 5:20:34 AM
> Subject: Re: [llvm-dev] GlobalsAA from GVN
> 
> Hi James,
> 
> Thanks for the help. From the log, I could infer that SLP vectorizer
> is not preserving alias analysis, preventing GVN from getting the
> info. Although the first function to get compiled has GlobalsAA
> available during GVN, rest of them do not as SLP vectorizer run on
> that function invalidates GlobalsAA which is a module pass. Is there
> a way to force re-computation of a particular analysis?

You can specifically insert it into the pass pipeline, but in this case, we should just fix SLP to preserve the analysis.

 -Hal

> 
> As a side note, I didn't have this problem on release_37.
> 
> Thanks a lot.
> 
> Regards,
> 
> 
> 
> 
> 
> - Vaivaswatha
> 
> 
> On Thu, Dec 3, 2015 at 3:02 PM, James Molloy <
> james at jamesmolloy.co.uk > wrote:
> 
> 
> 
> Hi Vaivaswatha,
> 
> 
> GlobalsAA is not an immutable pass because it needs to cache queries
> to avoid them being unusably slow. It therefore relies on passes
> explicitly preserving it. Most of the passes in the scalar pipeline
> have been modified to setPreserved<GlobalsAA>() and I know the pass
> gets preserved at least until LICM.
> 
> 
> You can use -debug-pass=Executions to determine at what point
> GlobalsAA is not preserved (which pass clobbers it) - you'd then
> need to add GlobalsAA to its preserved pass list.
> 
> 
> Cheers,
> 
> 
> James
> 
> 
> 
> 
> On Wed, 2 Dec 2015 at 09:35 Vaivaswatha Nagaraj via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
> 
> 
> 
> 
> 
> 
> 
> 
> Hi,
> 
> I've noticed that alias analysis queries arising from GVN do not use
> the results from GlobalsAA.
> The last call to AAResultsWrapperPass::runOnFunction() before GVN
> does not add GlobalsAAWrapperPass due to unavailability. This leads
> to the alias queries from GVN not having any globals mod-ref info.
> 
> Is this a known issue? and is there any way to have globals mod-ref
> info available for GVN?
> 
> Thanks,
> 
> 
> 
> 
> 
> 
> 
> 
> - Vaivaswatha
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory