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Fix Access Violation when using lld & ThinLTO on windows-msvc #103353

Merged
merged 2 commits into from
Nov 9, 2022
Merged

Fix Access Violation when using lld & ThinLTO on windows-msvc #103353

merged 2 commits into from
Nov 9, 2022

Commits on Nov 3, 2022

  1. Add test case

    @wesleywiser
    wesleywiser committed Nov 3, 2022
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  2. Fix Access Violation when using lld & ThinLTO on windows-msvc 

    Users report an AV at runtime of the compiled binary when using lld and
ThinLTO on windows-msvc. The AV occurs when accessing a static value
which is defined in one crate but used in another. Based on the
disassembly of the cross-crate use, it appears that the use is not
correctly linked with the definition and is instead assigned a garbage
pointer value.

If we look at the symbol tables for each crates' obj file, we can see
what is happening:

*lib.obj*:

```
COFF SYMBOL TABLE
...
00E 00000000 SECT2  notype       External     | _ZN10reproducer7memrchr2FN17h612b61ca0e168901E
...
```

*bin.obj*:

```
COFF SYMBOL TABLE
...
010 00000000 UNDEF  notype       External     | __imp__ZN10reproducer7memrchr2FN17h612b61ca0e168901E
...
```

The use of the symbol has the "import" style symbol name but the
declaration doesn't generate any symbol with the same name. As a result,
linking the files generates a warning from lld:

> rust-lld: warning: bin.obj: locally defined symbol imported: reproducer::memrchr::FN::h612b61ca0e168901 (defined in lib.obj) [LNK4217]

and the symbol reference remains undefined at runtime leading to the AV.

To fix this, we just need to detect that we are performing ThinLTO (and
thus, static linking) and omit the `dllimport` attribute on the extern
item in LLVM IR.
    @wesleywiser
    wesleywiser committed Nov 3, 2022
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