examples/static_net: WaveOf binds its source to a local for right shifts (wave4 44 B → 42 B)

[neu-rah]

Requested by Roy · project thread

Before: on avr-gcc 7.3 the Banknote wave4 cell was one instruction longer than the same cell written in plain C (28 vs 27 instructions, #9). The extra instruction was a register copy. static_net reported it as 44 B and 25 cycles.

After: the HAPI cell is byte-identical to the plain C one. static_net's wave4 is 42 B, 24 cycles, and every other program of check/build.sh keeps its size.

gcc 7.3 at -Os does a right shift followed by a mask in 8 bits only when the shifted byte is a variable and nothing narrows it before the mask. WaveOf read the byte as Src::get(in) inside the expression and cast the shift to u8, so the shift stayed in int. AVR gcc 16.1 folds the old spelling as well, so this is a 7.3 workaround in the example's cell, not a change to HAPI.

How:

• examples/static_net/include/waveCell.h: WaveOf::proc reads the source into a local first.
  • Right shifts do shift, add and mask with no narrowing in between.
  • Left shifts keep the early u8: without it gcc 7.3 does them in 16 bits, and mixed_avr_size grows by 30 B.
  • The result is the same as before for every n, every s in −7..7, and every p, m and input byte (3.0e9 cases, host).
• check/build.sh passes 65 ok. The only expectation that changed is banknote_avr_check: 2344 B, was 2346.
• compare_emlearn/run.sh: wave4 is 42 B, 24 cycles, 274/274 correct, was 44 B, 25 cycles. results.md is regenerated and only that row changed.
• measure/'s bn_wave is unchanged at 23 cycles.
• tests/single_header/run.sh passes 36 ok.
• These files quote the figures and are updated:
  • the root README
  • examples/static_net/README.md, including its ratios against emlearn
  • .RnD/openDerivation/godbolt/README.md
• measure/silicon_results.md now notes that its ELFs predate this change. The new wave4 ELF has not been run on a chip.

Not changed: .RnD/openDerivation/round2 has failed on main since #6 (5 ok, 32 FAIL): its : sources still use snet::Slot. Its copy of waveCell.h and the FINDINGS.md figures are left as that round's record.

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