[llvm-dev] Lazily Loaded Modules and Linker::LinkOnlyNeeded

[Mehdi Amini via llvm-dev]

Hi Neil,

> On Apr 20, 2016, at 5:20 AM, Neil Henning via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> TL;DR - when linking from a lazily loaded module and using Linker::LinkOnlyNeeded, bodies of used functions aren't being copied during linking.
> 
> Previously on one of our products, we would lazily load our runtime module (around 9000 functions), and link some user module into this (which is in all practical use cases much smaller).

It sounds reverse to what I would intuitively do (i.e. load the runtime into my module).

> Then, post linking, we have a pass that runs over the module and rips out all the un-materialized functions that weren't being used in the original user module.
> 
> I only just noticed that LinkModules has a flags parameter that can take a LinkOnlyNeeded flag, which made me wonder if I could reverse the link order (EG. link from the lazily loaded runtime module into the user module), set the LinkOnlyNeeded flag, and hey presto, we wouldn't need to have a cleanup pass that ran afterwards ripping out functions that weren't used.
> 
> So I tried it, and it failed. Basically any function that was still to be materialized wasn't getting its body copied over during linking.
> 
> The only line of code that differs when you set LinkOnlyNeeded is in LinkModules.cpp -> ModuleLinker::linkIfNeeded:
> 
> if (shouldLinkOnlyNeeded() && !(DGV && DGV->isDeclaration()))
>     return false;
> 
> The isDeclaration() for functions has a call to isMaterializable().
> 
> Things I've tried:
> If I don't pass LinkOnlyNeeded but still link from the lazily loaded runtime module into the user module, it works (albeit it is orders of magnitude slower like we'd expect).
> If I don't lazily load the runtime module, it works (but again, much slower).
> I tried doing the linking and then materializing the newly linked module, but the runtime functions were still missing their bodies (which implies the information was lost during linking).
> If I hack the LinkModules.cpp code such that it checks if the DGV could be materialized, and if so materialize it, before checking for a declaration again, it works:
> if (shouldLinkOnlyNeeded() && !(DGV && DGV->isDeclaration())) {
>     if (DGV && DGV->isMaterializable())
>         DGV->materialize();
> 
>     if (!(DGV && DGV->isDeclaration()))
>         return false;
> }

DGV is the GlobalValue in the *destination* Module, it is not clear to me how materializing has an effect on the *source* Module.
I am probably missing something here...

> Even with the extra cost of the hack above - this has resulted in a 2x speedup in our total link time.
> So really here I am wondering - is this expected behaviour? A bug? And if so how best to go about fixing the issue would be some grand advice from people more in the know!
> 
The linker was written before Module was lazy loaded I think. Many pieces in LLVM assume things their working on are materialized.
On a side note (a bit off-topic), I wonder if `isDeclaration()` should return false for materializable function?

CC Rafael, who knows this code better.

-- 
Mehdi


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