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neu-rah merged 1 commit into from
Sep 30, 2026
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RnD openDerivation/godbolt README: side-by-side listings without an HTML table #11

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97 changes: 30 additions & 67 deletions .RnD/openDerivation/godbolt/README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -15,76 +15,39 @@ it is not a comparison with anything outside HAPI.

Both compiled as C++ with AVR gcc 7.3.0, `-std=c++17 -Os -mmcu=atmega328p`. `verify.sh` produced these listings (`verify.log`).

<table>
<tr><th>HAPI (<code>wave4_apiof.cpp</code>): 28 instructions, 56 B</th><th>plain C (<code>wave4_c.c</code>): 27 instructions, 54 B</th></tr>
<tr><td>

```asm
movw r30, r24
ldd r25, Z+3 ; x[3]
mov r24, r25 ; <- the extra instruction
adc r24, r24
eor r24, r24
adc r24, r24 ; x[3] >> 7
ldd r25, Z+2 ; x[2]
lsr r25
```text
HAPI (wave4_apiof.cpp): 28 instructions, 56 B plain C (wave4_c.c): 27 instructions, 54 B
--------------------------------------------- ------------------------------------------
movw r30, r24 movw r30, r24
ldd r25, Z+3 ; x[3] ldd r25, Z+2 ; x[2]
mov r24, r25 ; <- the extra instruction lsr r25
adc r24, r24 lsr r25 ; x[2] >> 2
eor r24, r24 ldd r24, Z+3 ; x[3]
adc r24, r24 ; x[3] >> 7 adc r24, r24
ldd r25, Z+2 ; x[2] eor r24, r24
lsr r25 adc r24, r24 ; x[3] >> 7
lsr r25 ; x[2] >> 2
add r25, r24
subi r25, 0x51 ; + 175
ld r18, Z ; x[0]
lsr r18
lsr r18
subi r18, 0x92 ; + 110
andi r18, 0xBA
add r25, r18
ldd r24, Z+1 ; x[1]
lsr r24
lsr r24
subi r24, 0xB0 ; + 80
andi r24, 0xBF
add r24, r25
com r24
adc r24, r24
eor r24, r24
adc r24, r24 ; sum < 128
ret
add r25, r24 add r25, r24
subi r25, 0x51 ; + 175 subi r25, 0x51 ; + 175
ld r18, Z ; x[0] ld r18, Z ; x[0]
lsr r18 lsr r18
lsr r18 lsr r18
subi r18, 0x92 ; + 110 subi r18, 0x92 ; + 110
andi r18, 0xBA andi r18, 0xBA
add r25, r18 add r25, r18
ldd r24, Z+1 ; x[1] ldd r24, Z+1 ; x[1]
lsr r24 lsr r24
lsr r24 lsr r24
subi r24, 0xB0 ; + 80 subi r24, 0xB0 ; + 80
andi r24, 0xBF andi r24, 0xBF
add r24, r25 add r24, r25
com r24 com r24
adc r24, r24 adc r24, r24
eor r24, r24 eor r24, r24
adc r24, r24 ; sum < 128 adc r24, r24 ; sum < 128
ret ret
```

</td><td>

```asm
movw r30, r24
ldd r25, Z+2 ; x[2]
lsr r25
lsr r25 ; x[2] >> 2
ldd r24, Z+3 ; x[3]
adc r24, r24
eor r24, r24
adc r24, r24 ; x[3] >> 7
add r25, r24
subi r25, 0x51 ; + 175
ld r18, Z ; x[0]
lsr r18
lsr r18
subi r18, 0x92 ; + 110
andi r18, 0xBA
add r25, r18
ldd r24, Z+1 ; x[1]
lsr r24
lsr r24
subi r24, 0xB0 ; + 80
andi r24, 0xBF
add r24, r25
com r24
adc r24, r24
eor r24, r24
adc r24, r24 ; sum < 128
ret
```

</td></tr>
</table>

What this shows, and what it does not:

- **The same computation.** Both do the same loads, shifts, adds and masks, with the same constants. Neither has a call, a
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