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

[Neil Henning via llvm-dev]

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). 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;
}

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!

Cheers,
-Neil.

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