Integrate llvm to llvm/llvm-project@a1984ec5eab09f

[hanhanW]

No description provided.

[pashu123]

LGTM! I'm curious, what was the reason for shifting from undefined to poison semantics?

[hanhanW]

LGTM! I'm curious, what was the reason for shifting from undefined to poison semantics?

I don't work on this area, but the upstream commit has the answer:

LLVM itself is generally moving away from using undef and towards
using poison, to the point of having a lint that caches new uses of
undef in tests.

In order to not trip the lint on new patterns and to conform to the
evolution of LLVM

• Rename valious ::undef() methods on StructBuilder subclasses to
  ::poison()
• Audit the uses of UndefOp in the MLIR libraries and replace almost all
  of them with PoisonOp

[kuhar]

what was the reason for shifting from undefined to poison semantics?

undef is an llvm 3.x relict that's generally broken and difficult to optimize around, poison allows for more self-consistent rewrites: https://llvm.org/docs/UndefinedBehavior.html