[LLVMdev] Load MachineFunctionPass plugin from library in llc?

[Ethan J. Johnson]

Good to know – thanks! It has indeed been working for me. Anecdotally, I can’t say it’s helped too much with build times – now it just takes a long time building “libX”, instead of X; but it does feel a little bit faster. 

 

Maybe my observations are a bit misleading since building shared libraries consolidates a lot of code into a single linker product, rendering “make –jX” multithreading less effective. It spends a lot of time “bottlenecked” on linking big things like libclang: it launches a wave of linker tasks, and most of them finish quickly, leaving a single “big” task holding things up at the end. “make” has to wait for it to complete before it can launch a new wave of parallel build tasks (whether compiling, linking, etc.). I’m not sure if this is because of dependencies between components, or perhaps just because “make” isn’t very smart about multithreading. Maybe CMake would be smarter here? Or perhaps ninja?

 

I should be able to improve build times more by disabling the unit tests – I noticed those are responsible for a good portion of the time spent on linking (they don’t seem to use the shared libraries), and since I’m just writing an analysis pass which doesn’t actually modify anything, checking for regressions in the rest of LLVM isn’t so important (not at this stage anyway).

 

Thanks again,

Ethan Johnson

 

From: Jim Grosbach [mailto:grosbach at apple.com] 
Sent: Monday, July 6, 2015 1:18 PM
To: Ethan J. Johnson
Cc: Matthias Braun; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Load MachineFunctionPass plugin from library in llc?

 

 

On Jul 2, 2015, at 8:15 PM, Ethan J. Johnson <ejohns48 at cs.rochester.edu <mailto:ejohns48 at cs.rochester.edu> > wrote:

 

Thanks for the tips! I will definitely try building just llc, since that is all I need for testing/debugging the pass.

 

Out of curiosity, what exactly does BUILD_SHARED_LIBS do? Does it move most of the shared code to .so files and create binaries that load them at run time? If I go that route, is there anything special I need to do to make sure the binaries can “find” the shared libraries at run time? (I do not have root access on my development machine, so “make install” to put the shared libs in a standard location isn’t a really good option.)

 

Yep, that’s exactly what it does. The load paths are relative to the main executable, so everything should “just work” without having to do any path munging or installation tricks.

 

-Jim

 

 

Thanks,

Ethan Johnson

 

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