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Simplify Square Clipped ReLU code.

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Squared numbers are never negative, so barring any wraparound there
is no need to clamp to 0. From reading the code, there's no obvious
way to get wraparound, so the entire operation can be simplified
away. Updated original truncated code comments to be sensible.

Verified by running ./stockfish bench 128 1 24 and by the following test:

STC: https://tests.stockfishchess.org/tests/view/64da4db95b17f7c21c0eabe7
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 60224 W: 15425 L: 15236 D: 29563
Ptnml(0-2): 195, 6576, 16382, 6763, 196

closes https://github.com/official-stockfish/Stockfish/pull/4751

No functional change
This commit is contained in:
Gian-Carlo Pascutto 2023-08-14 17:30:10 +02:00 • committed by Stéphane Nicolet
parent 4c5919fa95
commit c6f62363a6
1 changed files with 5 additions and 19 deletions
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24
src/nnue/layers/sqr_clipped_relu.h
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@ -65,12 +65,6 @@ namespace Stockfish::Eval::NNUE::Layers {
#if defined(USE_SSE2)
constexpr IndexType NumChunks = InputDimensions / 16;
#ifdef USE_SSE41
const __m128i Zero = _mm_setzero_si128();
#else
const __m128i k0x80s = _mm_set1_epi8(-128);
#endif
static_assert(WeightScaleBits == 6);
const auto in = reinterpret_cast<const __m128i*>(input);
const auto out = reinterpret_cast<__m128i*>(output);
@ -82,21 +76,13 @@ namespace Stockfish::Eval::NNUE::Layers {
_mm_load_si128(&in[i * 4 + 2]),
_mm_load_si128(&in[i * 4 + 3]));
// Not sure if
// We shift by WeightScaleBits * 2 = 12 and divide by 128
// which is an additional shift-right of 7, meaning 19 in total.
// MulHi strips the lower 16 bits so we need to shift out 3 more to match.
words0 = _mm_srli_epi16(_mm_mulhi_epi16(words0, words0), 3);
words1 = _mm_srli_epi16(_mm_mulhi_epi16(words1, words1), 3);
const __m128i packedbytes = _mm_packs_epi16(words0, words1);
_mm_store_si128(&out[i],
#ifdef USE_SSE41
_mm_max_epi8(packedbytes, Zero)
#else
_mm_subs_epi8(_mm_adds_epi8(packedbytes, k0x80s), k0x80s)
#endif
);
_mm_store_si128(&out[i], _mm_packs_epi16(words0, words1));
}
constexpr IndexType Start = NumChunks * 16;
@ -108,7 +94,7 @@ namespace Stockfish::Eval::NNUE::Layers {
output[i] = static_cast<OutputType>(
// really should be /127 but we need to make it fast
// needs to be accounted for in the trainer
std::max(0ll, std::min(127ll, (((long long)input[i] * input[i]) >> (2 * WeightScaleBits)) / 128)));
std::min(127ll, (((long long)input[i] * input[i]) >> (2 * WeightScaleBits)) / 128));
}
}
};