[PATCH] D77009: Optimisations and bug-fixes for BitVector and SmallBitVector

[Brad Moody via Phabricator via llvm-commits]

bmoody created this revision.
bmoody added reviewers: zturner, llvm-commits.
Herald added subscribers: dexonsmith, hiraditya.
Herald added a project: LLVM.

The main purpose of this is to address the TODO in SmallBitVector to store
the large mode in a single allocation. I made some other optimisations and
fixed a few bugs while I was at it.

Behaviour changes:

- operator== now returns false when sizes are not equal
- Hashing
  - Fixed bug where BitVectors with equal contents but different capacities would get different hashes
  - Fixed bug where SmallBitVector would hash differently in small and large modes
  - Allow empty BitVectors and SmallBitVectors to be used as DenseMap keys
- BitVector::getData now returns only in-use words

Internal changes:

- SmallBitVector Large-mode now lives in a single allocation
- sizeof(BitVector) is now 16, previously 24
- BitVector(unsigned, bool) no longer allocates when s==0
- Most uses of Bits now call getActiveBits, returning only the BitWords currently in use.
- set_unused_bits now only affects bits in the top used word. Unused BitWords are now uninitialised, which should make many operations faster on very over-allocated vectors. This change should be safe because of the above change - accessing the uninitialised data should trip a bounds check. See commit f93a82f08f29163c342f320b88710b8eb0beb804 which introduced this behaviour
- grow no longer initialises data
- Re-implemented the two-vector operations in SmallBitVector to improve performance in the mixed small/large case
- SmallBitVector::getSmallBits now masks out size bits only. setSmallSize already ensures that unused data bits are zero.

Also included a bug-fix to RegScavenger, which started tripping an assert that I added. This
change should probably go in on its own but I don't know how to write a test exercising that
code, or who might be able to help me.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D77009

Files:
  llvm/include/llvm/ADT/BitVector.h
  llvm/include/llvm/ADT/SmallBitVector.h
  llvm/lib/CodeGen/RegisterScavenging.cpp
  llvm/unittests/ADT/BitVectorTest.cpp

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