How HotSpot's JIT Learned to Reason About Bits and Delete Code

The mask that compiles to nothing: how HotSpots JIT learned to reason about bits

I explore how HotSpot's C2 compiler evolved beyond simple numeric ranges to track individual bit states. By introducing 'known bits' alongside value ranges, the JIT can now prove that certain bitwise operations, like masking after a shift, are redundant. This refinement allows the compiler to safely eliminate unnecessary instructions, turning complex expressions into efficient, minimal code.

"A range can say the value is small, but it can't say the value is even, let alone a multiple of four."