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

5 hours ago

HotSpot's C2 compiler tracks "known bits" alongside ranges to optimize bitwise operations.
The optimization eliminates redundant bitwise ANDs, like removing (x << 2) & -4, by proving bits are unchanged.
The system uses two 32-bit masks per integer: zeros and ones to indicate definite bit states.
Range and known bits mutually refine each other through a process called canonicalization.
Rules for bit propagation are defined for operations like AND, OR, and shift, enabling precise analysis.
The feature was implemented in JDK 26 and improved in JDK 27, with test methods ensuring correctness.

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