AES chunkers: unroll the x86-64 aes-ni and vaes scan paths, branch-free lane test

[ThomasWaldmann]

Two bit-identical speedups for the x86-64 hardware paths of the toeplitz-aes / rabin-aes / goldilocks-aes chunkers, one commit each, from the chunker kernel audit (perf counters on a Zen 4, gcc 14, Debian's -O2 python flags):

1. aes-ni path (every x86-64 CPU without VAES): the AES rounds and the 8-way lane test were loops. gcc -O2 keeps them rolled (12 instructions per round, two branches per tested position), clang serialises the early-exit test loop into 8 dependent AES chains. Now the rounds are spelled out by macro and the 8 lanes are tested branch-free in the vector domain (unpcklqdq + mask + pcmpeqq + or-reduce; SSE4.1 is now part of the availability check, every AES-NI CPU has it). borg benchmark cpu with BORG_AES_CHUNKER_KERNEL=aes-ni: toeplitz-aes 735 -> 960 MB/s, rabin-aes 720 -> 870, goldilocks-aes 472 -> 505.
2. vaes path (the x86-64 default where available): the 15 stride-2 roll steps and the 9 AES rounds over 8 zmm vectors were loops too (gcc -O2: an index chain plus 72 zmm register copies per group). Unrolled by macro: toeplitz-aes 871 -> 1000 MB/s, rabin-aes 879 -> ~1010, goldilocks-aes 489 -> ~555.

The aarch64 path is untouched. Verified with the chunker test suite (which checks that evp, aes-ni and vaes produce identical cut points) on the Zen 4 box and on macOS, and with the audit's C harness (bit-identity on PRNG and adversarial data, perf-cycle measurements).

Not included: the software-pipelined vaes variant from the audit (a further -10% for toeplitz/goldilocks but codegen-sensitive) and the arm64 instruction diet (+7-8%, needs inline asm).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.70%. Comparing base (26fcb6d) to head (9d831da).
⚠️ Report is 18 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10309   +/-   ##
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  Coverage   87.69%   87.70%           
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  Files         103      103           
  Lines       18712    18717    +5     
  Branches     2880     2881    +1     
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+ Hits        16410    16415    +5     
  Misses       1600     1600           
  Partials      702      702           

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