[PATCH] D12618: Reserve a vendor reserved block ID for bitcode

[Rafael Espíndola via llvm-commits]

> Our current issue is not reading old bitcode but what to do with new
bitcode.
>
> For instance you install a beta version of Xcode, build a project with
LTO, and then revert Xcode to the previous version.
> At this point the bitcode on your disk is too new for your installed
toolchain and a non-clean build may crash libLTO.

That is a bug in llvm. It should never crash on any input. A newer bitcode
is just a special case.

I am confident we can get there by fuzz testing.

> Another case is when some people distribute static archive that contains
bitcode. Right now we end-up with terrible crash reports while we want to
improve user experience with great diagnostics.

With what we have in trunk we should be able to implement errors like:

Error: unknown linkage number XX. Input is likely from a newer llvm
version.

Would that be sufficient?

Cheers, Rafael
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