keccak: Rework absorption of the final input bytes - #1620
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Pull request overview
This PR optimizes Keccak input absorption for the final partial (1–7 byte) tail by constructing the last word via shifts instead of a byte-copy loop that GCC may recognize and lower into a small-size memcpy() call (notably affecting some RISC-V targets and zkVM cost models).
Changes:
- Replace the byte-by-byte tail copy into a stack
uint64_t(and subsequentto_le64()swap) with a shift/OR construction of the partial word plus padding byte. - Keep the Keccak padding behavior the same by placing
0x01at byte indexsizeand XORing into the next state lane.
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #1620 +/- ##
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- Coverage 97.46% 97.46% -0.01%
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Files 170 170
Lines 15402 15395 -7
Branches 3604 3604
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- Hits 15012 15005 -7
Misses 282 282
Partials 108 108
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In the Keccak input processing loop, rework the code handling the last 0–7 bytes of input and the padding byte. The new code is not spectacularly better than the old one, but it fixes edge case where a compiler may decide to replace the old code with memcpy call.
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In the Keccak input processing loop, rework the code handling the last
0–7 bytes of input and the padding byte. The new code is not
spectacularly better than the old one, but it fixes edge case where
a compiler may decide to replace the old code with memcpy call.