[PATCH] D55153: [ThinLTO] Implement index-based WPD

[Sylvain Audi via Phabricator via llvm-commits]

saudi added a comment.

When switching from clang 9 to clang 10, we encountered a freeze in one of our games built with thin-LTO.
git bisect pointed to this commit.
Unfortunately, it is a pretty large project, and I couldn't repro that behavior in other -smaller- contexts.

Behavior:
In a call to a virtual function, a direct call is made to an address containing a single instruction : a short jump to that same instruction, causing an infinite loop:

  00007FF72C7DA80C 5B                   pop         rbx  
  00007FF72C7DA80D 5F                   pop         rdi  
  00007FF72C7DA80E 5E                   pop         rsi  
  00007FF72C7DA80F C3                   ret  
  --- [path_to_file.h]
  00007FF72C7DA810 EB FE                jmp         00007FF72C7DA810      # <- Infinite loop here
  --- No source file -------------------------------------------------------------
  00007FF72C7DA812 CC                   int         3  
  00007FF72C7DA813 CC                   int         3  
  00007FF72C7DA814 CC                   int         3  

I noticed, looking into the output map file, that several other methods of that same class had that same address.

Our project is built clang-cl, with options `-flto=thin` and `-Xclang -fwhole-program-vtables`, andl links with pre-compiled libs that are built with MSVC. 
Without `-fwhole-program-vtables` the bug disappears.

Notes:

- We tried applying the patch from D77421 <https://reviews.llvm.org/D77421>, without any luck.
- Our project links with pre-compiled libs (that were built without LTO). Would `-fwhole-program-vtables` be invalid to use in our case?
- The bug also occurs on the master branch.
- I tried with an assert-enabled build, no asserts were raised
- The class hierarchy has several virtual methods inlined; the others are built without LTO in a separate library.

At this point, I would like to create a simple repro, but I'm running out of ideas, as I'm not very familiar with LTO detail. Our repro involves a pretty big compilation (resulting .exe file is about 100MB, link time ~8 min).

I would be interested in suggestions about how to extract helpful information, for example:

- specific command line arguments to `clang-cl.exe` or `lld-link.exe` that would give details about devirtualization;
- a significant place to put a breakpoint in lld's code;
- command lines to run on the thin-lto generated object files;
- ... ?

Thanks!


Repository:
  rL LLVM

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  https://reviews.llvm.org/D55153/new/

https://reviews.llvm.org/D55153