[libcxx-commits] [PATCH] D133661: [libc++] Improve binary size when using __transaction

[David Blaikie via Phabricator via libcxx-commits]

dblaikie added a comment.

> the definition of the `__exception_guard` class being different is somehow more problematic.

That's correct - Clang/LLVM debug info under LTO does require structures with linkage to have consistent size at least. 
The sort of errors are:

  fragment is larger than or outside of variable
    call void @llvm.dbg.declare(metadata ptr undef, metadata !4274, metadata !DIExpression(DW_OP_LLVM_fragment, 200, 56)), !dbg !4288
  !4274 = !DILocalVariable(name: "__guard", scope: !4275, file: !2826, line: 550, type: !2906)

Because LLVM LTO is validating that the debug info describing a variable makes sense given the type - but the type is deduplicated based on the linkage name of the type - so some debug info was emitted in some +exceptions code that is larger than the type definition taken from some -exceptions code.

I think some possible fixes would include

- make the structure the same size regardless of +/-exceptions
- use a macro to choose between two differently-linkage-named types (yeah, this is still an ODR violaiton for the function definitions that use these different entities - but within the realms of things we do for +/-exception compatibility, and nothing validates that ODR function definitions are identical)

I'll see if I can repro this with godbolt, but I think this ^ should be enough to justify a timely revert and/or fix. This patch is broken for mixed exceptions LTO builds.


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D133661