[PATCH] D82058: [ADT] Add Bitfield utilities - alternative design

[Guillaume Chatelet via Phabricator via llvm-commits]

gchatelet created this revision.
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gchatelet updated this revision to Diff 271503.
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- update API documentation


Alternative design for D81580 <https://reviews.llvm.org/D81580>

Context: There are places in LLVM where we need to pack typed fields into opaque values.
For instance, the `XXXInst` classes in `llvm/include/llvm/IR/Instructions.h` that extract informations from `Value::SubclassData` via `getSubclassDataFromInstruction()`.
The bit twiddling is done manually: this impairs readability and prevent consistent handling of out of range values (e.g. Instructions.h <https://github.com/llvm/llvm-project/blob/435b458ad0a4630e6126246a6865748104ccad06/llvm/include/llvm/IR/Instructions.h#L564>)
More importantly, the bit pattern is scattered throughout the implementation making it hard to pack additionnal fields or check for overlapping bits.

Design: The Bitfield structs are to be declared together so it is clear which bits are used or not.
The code is designed with simplicity in mind, hence a few limitations:

- Storage is limited to a single integer that cannot be bigger than `uint64_t`,
- Values and storage have to be `unsigned` or `enum` (`enum class` works), **(edit: the new design allows signed integer but not signed enums)**
- Is is not possible to store a full `uint64_t` at once i.e. Bitfield<uint64_t, 0, 64> will fail, **(edit: the new design allows this)**
- There are no automatic detection of overlapping fields (packed bitfield declaration should help though),
- The interface is C like so `storage` needs to be passed in every time (code is simpler and lifetime considerations more obvious)


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D82058

Files:
  llvm/include/llvm/ADT/Bitfields.h
  llvm/unittests/ADT/BitFieldsTest.cpp
  llvm/unittests/ADT/CMakeLists.txt

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