[llvm] [SROA][TTI][DirectX] Add support for struct alloca decomposition (PR #161601)

[Nikita Popov via llvm-commits]

https://github.com/nikic requested changes to this pull request.

> It should be noted that struct alloca decomposition is unsafe in languages that allow dynamic indexing across struct members (e.g., treating a pointer to one member as a base for accessing others via computed offsets). Such behavior can violate the assumptions of this decomposition, which expects each member to be accessed independently and explicitly.
> Therefore struct alloca decomposition is disabled by default and must be explicitly enabled via an SROA pass option or TargetTransformInfo via shouldDecomposeStructAllocas.

Notably, these languages also include LLVM IR. Transforms must always operate in terms of LLVM IR semantics, and cannot make use of additional guarantees that are not implied by those semantics.

I believe that to make this kind of transform we first need to implement the proposal at 
https://discourse.llvm.org/t/rfc-adding-instructions-to-to-carry-gep-type-traversal-information/88141, including the additional limitation on notional over-indexing I mention in my first reply. Or some other approach that provides the necessary guarantees at the IR level.

https://github.com/llvm/llvm-project/pull/161601