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

[John Criswell via llvm-dev]

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160320/03c807b2/attachment.html>