[Bug rtl-optimization/114338] (x & (-1 << y)) should be optimized to ((x >> y) << y) or vice versa

[rearnsha at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114338

--- Comment #1 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
Why would that be better?  On a machine that does not lack registers, there's
more instruction-level parallelism in 
 (set (tmp) (-1))
 (set (tmp) (ashift (tmp) (count)))
 (and (x) (x) (tmp))

What's more, on Arm/AArch64 insns 2 and 3 can be merged into a single
instruction:

  (set (tmp) (-1))
  (set (x) (and (ashift (tmp) (count)) (x)))

which is definitely preferable to two register-controlled shifts.