[llvm-dev] ThinLTO promotion is ending up with "invalid" IR before IR-Linking

[Mehdi Amini via llvm-dev]

> On Dec 23, 2016, at 9:34 AM, Teresa Johnson <tejohnson at google.com> wrote:
> 
> 
> 
> On Thu, Dec 22, 2016 at 8:55 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> Hey,
> 
> As I’m playing with Metadata lazy-loading, I added a verifier right before running the IRLinker in FunctionImport.cpp, and it does not pass (on current trunk) in a few cases. One that I looked at ended up with aliases pointing to available_externally functions for instance.
> 
> How do these look after IRLinking (in the dest module)? I looked at the logic in renameModuleForThinLTO and all the conversions to available_externally are predicated on it being in the provided GlobalsToImport set. But in FunctionImport.cpp selectCallee() we specifically prevent importing of aliases that would result in the aliasee becoming available_externally. Presumably the resulting IRLinked dest module looks legit, otherwise we would have later verifier failures.

So the source module is:

@weakalias = weak alias void (...), bitcast (void ()* @globalfunc1 to void (...)*)
define void @globalfunc1() #0 {
entry:
  ret void
}

But we turn globalfunc1 into available_externally in renameModuleForThinLTO(), which make the alias invalid.

We don’t import the IR so the destination module is OK.
36


> 
> 
> I’m hesitant about breaking the IR verifier like that before calling the IRLinker. The alternative I can see now would be:
> - to perform any handling that requires breaking the verifier as part of the IRLinker process,
> - or to perform the verifier-breaking changes that we do as a pre-process step to the IRLinker as a post-process step on the linked module.
> 
> Wouldn't this second option result in verification errors on the resulting dest module?

Not sure why? Since currently the destination module is valid, the post-process should be OK as well?

— 
Mehdi
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