[llvm-dev] GlobalsAA from GVN

Vaivaswatha Nagaraj via llvm-dev llvm-dev at lists.llvm.org
Thu Dec 3 03:20:34 PST 2015

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?

As a side note, I didn't have this problem on release_37.

Thanks a lot.


  - Vaivaswatha

On Thu, Dec 3, 2015 at 3:02 PM, James Molloy <james at jamesmolloy.co.uk>

> 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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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