hash niche optimisation#322
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The pointers saved to the seen state set typically have some unused
bits. For example, on x86-64 they are 8-byte aligned, making the bottom
3 bits known-zero. With x86-64 Linux 4-level paging, the upper 16 bits
are also known-zero.
This change leverages the upper known-zero bits to store a state
discrimination hint in the form of part of the state’s hash:
63 48 47 3 2 0
▼ ▼ ▼ ▼ ▼ ▼
┌────────────────┬─────────────────────────────────────────────┬───┐
└────────────────┴─────────────────────────────────────────────┴───┘
hash[63:48] state pointer 0
The point of this is that state equality comparisons can be optimised by
first checking these upper bits. If two state slot’s upper bits are not
equal, we know the pointed to states differ. This is a slight runtime
performance improvement:
┌─────────────────────────┬────────┬────────┐
│ │ before │ after │
├─────────────────────────┼────────┼────────┤
│ misc/pending-queue.m │ 8m02s │ 7m45s │
│ misc/pending-queue-4k.m │ 8m01s │ 7m59s │
└─────────────────────────┴────────┴────────┘
Note that the discussion above implies we are still leaving some
performance on the table. The low 3 bits are known-zero and could also
be repurposed in future.
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