[InstCombine] canonicalize "extract lowest set bit" away from cttz intrinsic

1 << (cttz X) --> -X & X
https://alive2.llvm.org/ce/z/qv3E9e

This creates an extra use of the input value, so that's generally
not preferred, but there are advantages to this direction:
1. 'negate' and 'and' allow for better analysis than 'cttz'.
2. This is more likely to induce follow-on transforms (in the
   example from issue #60801, we'll get the decrement pattern).
3. The more basic ALU ops are more likely to result in better
   codegen across a variety of targets.

This won't solve the motivating bugs (see issue #60799) because
we do not recognize the redundant icmp+sel, and the x86 backend
may not have the pattern-matching to produce the optimal BMI
instructions.

Differential Revision: https://reviews.llvm.org/D144329

0 files changed, 0 insertions, 0 deletions