sha256: specialize rotr_wk and shr_wk per rotation amount

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The generic `rotr_wk(dst, n)` does its byte/bit decomposition
with two runtime `while` loops — necessary in the abstract, but
wasteful in SHA-256 where every rotation amount is one of ten
fixed compile-time constants (2, 6, 7, 11, 13, 17, 18, 19, 22,
25). The loop overhead alone is ~80 cycles of bookkeeping per
call, on top of the actual rotation work.

Replace each `rotr_wk(SIG, 6)` call inside the sigma helpers
with a dedicated `rotr_wk_6(SIG)` (and similarly for the other
amounts and for `shr_wk_3` / `shr_wk_10`). Each new helper just
chains the right number of `byte_rotr_wk` and `rotr1_wk` calls
inline — no `rem` variable, no `>= 8` / `> 0` checks.

The original `rotr_wk` / `shr_wk` wrappers stay defined for
runtime-amount callers; nothing else in the example uses them
today, but keeping them documents the general-purpose form.

Per SHA-256 block: ~45K cycles saved across 384 sigma rotations.
Combined with commit 0b5470b's leaf-function fix, the per-block
compression cost drops by ~3.5 frames at NTSC.

The hash output is unchanged (still
AE9145DB5CABC41FE34B54E34AF8881F462362EA20FD8F861B26532FFBB84E0D
for "NES"), no other examples are affected, and the emulator
harness stays at 34/34.

https://claude.ai/code/session_01FRmSBruVWCufm3LsUVMs8v

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This commit is contained in:

Claude 2026-04-16 16:49:35 +00:00

parent 0b5470b054

commit 726faef538

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2 changed files with 132 additions and 13 deletions

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