[llvm-dev] Questions about annotations

[Kaylor, Andrew via llvm-dev]

Hi everyone,

I've recently discovered a problem with reading LLVM bitcode that was produced by older versions of LLVM. Specifically, if the older bitcode file contains calls to llvm.var.annotations or llvm.ptr.annotations, I get an error because the number of arguments to these intrinsics was changed. I've filed a bug for this (https://bugs.llvm.org/show_bug.cgi?id=49006) and I'm working on a patch to auto-upgrade the older IR.

The extra argument was introduced in this patch: https://reviews.llvm.org/D88645

In looking closer at that patch, I see that an extra member was also added to the initializer structures for the llvm.global.annotations value. Here I have a problem that I'm not entirely sure how to handle. Specifically, the Annotation2Metadata pass, which was introduced here (https://reviews.llvm.org/D91195) expects the llvm.global.annotations values to have four members, but after the patch above clang creates initializers with five members.

The top-of-trunk LLVM code will happily read a module containing four member initializers, and having read that module the Annotation2Metadata pass will create annotation metadata for it. Unfortunately, it appears that Annotation2Metadata will not create annotation metadata for the llvm.global.annotations with five-member initializers that are generated by the latest version of clang. So, I don't know what the correct behavior here is.

Should the BitcodeReader auto-upgrade the four-member initializers to five-member initializers with a null pointer in the fifth field? Or should Annotation2Metadata handle either type of initializer?

I don't know enough about how these things are being used to confidently answer.

Thanks for any help anyone can offer.

-Andy
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