[llvm-dev] [GSoC 2016] Need more info on Add a MachineModulePass

John Criswell via llvm-dev
Sun, 20 Mar 2016 19:53:16 -0000

On 3/18/16 12:33 PM, Quentin Colombet via llvm-dev
wrote:
> Hi Vivek,
>
>> On Mar 16, 2016, at 1:00 PM, vivek pandya via llvm-dev
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>>
>> Hello,
>>
>> Probably this may be too late to start thinking about this project
>> but I think this is particularly useful feature for LLVM.
>
> +1. I’d like to use something this feature in GlobalISel. The idea is
> that has as soon as we lowered the LLVM IR of the whole module to
> MachineInstr, the LLVM IR should be deallocable.
> In other words, the MachineModule/MachineFunctions should contain
> enough information such that we do not have to keep the LLVM IR around.

I think this is a separate issue from having a MachineModulePass. My
goal in having a MachineModulePass is to be able to do inter-procedural
analysis and transformation on a program after its code has been
generated.  Examples of such applications include inter-procedural
register allocation, inter-procedural instruction selection,
inter-procedural code layout optimization (I have a colleague working on
this using reference affinity theory), and inter-procedural analysis of
machine code for measuring the efficacy of compiler transformations for
security.

Whether one keeps the LLVM IR around or not is orthogonal.  What is
needed is a way of being able to examine the whole program without
having to worry about another transformation running in parallel (if you
use a MachineFunctionPass, as I understand it, the pass should not be
analyzing/transforming other MachineFunction's).

>
> One of the main challenge is about alias analysis information that are
> tight with LLVM IR, but may be used in MachineFunctionPass.

One of the other projects I proposed is to encode this information
within the MachineInstr IR.  This would be very useful for one of my
projects, and it might solve this problem as well (once the information
is encoded in the MachineInstr IR, you don't need the LLVM IR around
anymore).

Regards,

John Criswell

--
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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