[LLVMdev] LTO and intrinsics

[Tom Roeder]

I've recently run across a problem with intrinsics interacting with LTO
mode. The problem is that -m* compiler flags can specify architectural
details (like aeskeygenassist, which depends on HasAVX and HasAES in the
subtarget) but these flags are not always known at link time. This isn't a
problem with normal object code, since it simply encodes the instructions,
and linking succeeds without knowing anything about these details.

However, some object code is built with flags that are not passed at link
time, and, LTO fails, e.g., with

LLVM ERROR: Cannot select: intrinsic %llvm.x86.aesni.aeskeygenassist

since LLVM doesn't know that that the subtarget supports this intrinsic.

One solution might be to encode this information in the metadata of the
bitcode files and check compatibility when adding modules at link time; I'd
like to have the LTO code take the union of the architecture options.

Is there a facility for encoding such information already? Or is there
another way to make this work better?

Thanks,

Tom
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