[llvm-dev] Benchmark GlobalsModRef in non-LTO pass pipeline

[Chandler Carruth via llvm-dev]

Cool, I was going to suggest exactly that variation of the change. =] Now
that there is no update API, it should be straight forward for *every* pass
to preserve AA. I'll see if I can simulate this in the current pass manager
in some way.

On Mon, Aug 10, 2015 at 8:06 AM James Molloy via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> To amend what I said earlier, after a lot more investigation it turns out
> the problem is that a bunch of optimization passes don't preserve
> AliasAnalysis. If I add AU.setPreserved<AliasAnalysis> to a bunch of them,
> passes such as LICM end up seeing GMR in its alias analysis chain. That's
> when I start to see improvements.
>
> James
>
> On Mon, 10 Aug 2015 at 10:06 James Molloy <james at jamesmolloy.co.uk> wrote:
>
>> Hi Chandler,
>>
>> Sorry it took so long for me to get to this. I'm very keen to get GMR
>> enabled in the non-LTO pipeline. I see nontrivial perf improvements in a
>> bunch of workloads I care about with it enabled, but with a patch applied
>> to run GMR again after the LoopPasses but before the FunctionPasses.
>>
>> I've found this is critical - LICM needs GMR available, and the
>> passmanager kills it by the time the early LICM is run.
>>
>> I'd be really keen to get this enabled by default, with that extra run
>> added. I assume the need for this extra run will evaporate with the new
>> pass manager?
>>
>> Cheers,
>>
>> James
>>
>> On Thu, 6 Aug 2015 at 03:45 Chandler Carruth <chandlerc at gmail.com> wrote:
>>
>>> Greetings folks!
>>>
>>> I would like to enable globalsmodref-aa in the non-LTO pass pipeline so
>>> that it gets tested more and there are fewer differences between the two.
>>> For all of my benchmarks, this is performance neutral, but I'd appreciate
>>> others benchmarking this combination to see if they see any benefits or
>>> regressions.
>>>
>>> You can demo this mode easily: -mllvm -enable-non-lto-gmr
>>>
>>> Please let me know!
>>> -Chandler
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