[llvm-dev] [RFC] llvm-mca: a static performance analysis tool

[Quentin Colombet via llvm-dev]

> On Mar 5, 2018, at 6:28 PM, Matthias Braun <mbraun at apple.com> wrote:
> 
> 
> 
>> On Mar 5, 2018, at 6:14 PM, Andrew Trick via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> 
>> 
>>> On Mar 5, 2018, at 3:38 PM, Quentin Colombet <qcolombet at apple.com <mailto:qcolombet at apple.com>> wrote:
>>> 
>>> When Ahmed and I worked on the decompiler, we first targeted MC. Going to MI was more difficult and really wouldn’t have gotten us a lot of benefits. Instead, Ahmed pushed for directly decompiling to IR (look for dagger).
>> 
>> Thanks for the pointer Quentin.
>> 
>>> I would actually be in favor for more infrastructure in MC for this kind of things. For instance, dealing with the code layout is not something MI is good at whereas MC is perfect.
>> 
>> Most of LLVM’s target-specific information is in TargetInstrInfo. TargetSchedModel depends on that. I don’t understand how you would build a performance analysis tool based on LLVM without either decompiling to MI, or rewriting those interfaces to be based on MC. I think you have to pick one direction or the other.
> You are right those are the two choices. Given that the actual data is in MCSchedModel and TargetSchedModel mostly being interested in the MCInstrDesc you can quert from an MI it seemed to me like the easier/cleaner route to not build up MI datastructures but stay with MC specific things.

That’s exactly how I approached things when I was printing some stats in assembly :). Things were readily accessible through MC.

> You are right though that there is a whole bunch of callbacks in TargetInstrInfo that would need alternatives if we want to go this route. Indeed building up MI makes things fit in better with the APIs today, it just feels less clean to me with all the extra codegen info around those datastructures. Either way I think this choice has to be made by whoever is working on decompiling tools.
> 
> - Matthias

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