Commits on Sep 13, 2021

1. [mono] Use SROA-friendly struct type declarations …

   LLVM's SROA can decompose loads and stores of aggregate type into a
   sequence of aggregate-element-typed loads and stores. Before this
   change, Mono translates .NET-level value types into LLVM IR-level
   structs containing nothing but `i8` elements.
   
   When a value type field has reference type, and a value of this value
   type is copied using a `memcpy` intrinsic or an LLVM IR load followed by
   a store, LLVM will emit code that loads managed references in multiple
   byte-sized fragments before reconstructing the original pointer using a
   sequence of ALU ops. This causes sgen to fail to pin the referent.
   
   This change works around this by translating value types to LLVM IR
   structs with pointer-sized fields. Packed value types with non-standard
   alignment will be translated into LLVM IR structs containing
   alignment-sized fields.
   
   Note that this does not completely guarantee that the code we generate
   will respect sgen's requirements. No specific guarantees are provided
   about the translation of non-atomic LLVM IR loads and stores to machine
   code. And we'll need some alternative means (perhaps a special
   `gc_copy_unaligned` runtime call or similar) to copy packed or
   misaligned value types that contain managed references. For stronger
   LLVM IR-level guarantees, we'll want to make use of unordered atomic
   loads and stores and unordered atomic memcpy, but that work is out of
   scope for this change.
   
   See:
   - https://github.com/dotnet/llvm-project/blob/release/11.x/llvm/lib/Transforms/Scalar/SROA.cpp#L3371-L3388
   - https://github.com/dotnet/llvm-project/blob/release/11.x/llvm/lib/Transforms/Scalar/SROA.cpp#L3327-L3340

   

   imhameed committed Sep 13, 2021
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