[llvm-dev] RFC: Enabling Module passes post-ISel

Matthias Braun via llvm-dev llvm-dev at lists.llvm.org
Sun Jul 17 10:57:00 PDT 2016

> On Jul 17, 2016, at 9:34 AM, James Molloy via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hi,
> [Apologies to those receiving this mail twice - used the old list address by accident]
> In LLVM it is currently not possible to write a Module-level pass (a pass that modifies or analyzes multiple MachineFunctions) after DAG formation. This inhibits some optimizations[1] and is something I'd like to see changed.
> The problem is that in the backend, we emit a function at a time, from DAG formation to object emission. So no two MachineFunctions ever exist at any one time. Changing this necessarily means increasing memory usage.
> I've prototyped this change and have measured peak memory usage in the worst case scenario - LTO'ing llc and clang. Without further ado:
>   llvm-lto llc:   before: 1.44GB maximum resident set size
>                   after:  1.68GB (+17%)
>   llvm-lto clang: before: 2.48GB maximum resident set size
>                   after:  3.42GB (+33%)
> The increases are very large. This is worst-case (non-LTO builds would see the peak usage of the backend masked by the peak of the midend) but still - pretty big. Thoughts? Is this completely no-go? is this something that we *just need* to do? Is crippling the backend architecture to keep memory down justified? Is this something we could enable under an option?
I also recently did a prototype that enables MachineModulePasses (https://github.com/MatzeB/llvm/tree/MachineModulePass <https://github.com/MatzeB/llvm/tree/MachineModulePass>). I assume your patch looks similar (I did not do any measurements with mine).

I believe this could be done in a way so the memory stays at current levels if no machine module pass issued (simply remove the memory used by the MachineFunction after we AsmPrinted it). So that would at least make the infrastructure available for people to experiment with. So maybe that is a good first step?

So far all the uses I have seen did not convince me that the increased memory consumption is worth it (by default). But IMO if we can provide by infrastructure for MachineModulePasses with nearly zero cost in case no machinemodulepass is used, then I’d say we should go for it.

- Matthias

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