[libcxx-commits] [PATCH] D134625: Summary:bug fix! Rotation direction on `__countl_zero()`probably unnoticed because only affects rare cases(does not affect 128 bit ints because rotation is effectively swap)(does not affect integers of sizes less or equal to 64bits)

[Armando Martín via Phabricator via libcxx-commits]

Delta-dev-99 added a comment.

Possible test case: apply the operation to 256-bit integer 0x8000000000000000000000000000000000000000000000000000000000000000 (first bit from the left is set, all other bits cleared)
Correct answer: 0. The first bit from the left is set.
Current implementation's answer (probably, not tested yet): 128. It counts the bits cleared in the 2 blocks (64 bits each) in the middle of the number (bits 64 to 191) before finding the bit set in the left-most block (block 4).

Another possible test case: 256-bit integer 0x0000000000000000000000000000000080000000000000000000000000000000 (bit 127 is set, which is the left-most bit of block 2, all other bits cleared)
Correct answer: 128
Current implementation's answer (probably, not tested yet): 0


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D134625/new/

https://reviews.llvm.org/D134625