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SIMD: faster vint4 load/store with unsigned char conversion #4071

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Dec 8, 2023
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SIMD: faster vint4 load/store with unsigned char conversion #4071

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25 changes: 14 additions & 11 deletions src/include/OpenImageIO/simd.h
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Original file line number Diff line number Diff line change
Expand Up @@ -4293,6 +4293,11 @@ OIIO_FORCEINLINE void vint4::load (const unsigned char *values) {
// Trickery: load one float worth of bits = 4 uchars!
simd_t a = _mm_castps_si128 (_mm_load_ss ((const float *)values));
m_simd = _mm_cvtepu8_epi32 (a);
#elif OIIO_SIMD_SSE >= 2
// Trickery: load one float worth of bits = 4 uchars!
simd_t a = _mm_castps_si128 (_mm_load_ss ((const float *)values));
a = _mm_unpacklo_epi8(a, _mm_setzero_si128());
m_simd = _mm_unpacklo_epi16(a, _mm_setzero_si128());
#else
SIMD_CONSTRUCT (values[i]);
#endif
Expand Down Expand Up @@ -4784,17 +4789,15 @@ OIIO_FORCEINLINE void vint4::store (unsigned char *values) const {
#if OIIO_AVX512VL_ENABLED
_mm_mask_cvtepi32_storeu_epi8 (values, __mmask8(0xf), m_simd);
#elif OIIO_SIMD_SSE
// Expressed as bytes and considering little endianness, we
// currently have AxBxCxDx (the 'x' means don't care).
vint4 clamped = m_simd & vint4(0xff); // A000 B000 C000 D000
vint4 swapped = shuffle_sse<1,0,3,2>(clamped); // B000 A000 D000 C000
vint4 shifted = swapped << 8; // 0B00 0A00 0D00 0C00
vint4 merged = clamped | shifted; // AB00 xxxx CD00 xxxx
vint4 merged2 = shuffle_sse<2,2,2,2>(merged); // CD00 ...
vint4 shifted2 = merged2 << 16; // 00CD ...
vint4 result = merged | shifted2; // ABCD ...
memcpy(values, &result, 4); // memcpy because it may be unaligned
// At this point, values[] should hold A,B,C,D
vint4 clamped = m_simd & vint4(0xff); // A000 B000 C000 D000
simd_t val16 = _mm_packs_epi32(clamped, _mm_setzero_si128()); // A0B0 C0D0 xxxx xxxx
simd_t val8 = _mm_packus_epi16(val16, _mm_setzero_si128()); // ABCD xxxx xxxx xxxx
_mm_store_ss((float*)values, _mm_castsi128_ps(val8));
#elif OIIO_SIMD_NEON
vint4 clamped = m_simd & vint4(0xff);
simd_t val16 = vcombine_s16(vqmovn_s32(clamped), vdup_n_s16(0));
simd_t val8 = vcombine_u8(vqmovun_s16(val16), vdup_n_u8(0));
vst1q_lane_u32((uint32_t*)values, val8, 0);
#else
SIMD_DO (values[i] = m_val[i]);
#endif
Expand Down
8 changes: 8 additions & 0 deletions src/libutil/simd_test.cpp
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Expand Up @@ -506,6 +506,14 @@ void test_conversion_loadstore_int ()
OIIO_CHECK_SIMD_EQUAL (VEC(uc1234), C1234);
OIIO_CHECK_SIMD_EQUAL (VEC( c1234), C1234);

// Check store to integers
VEC CStep = VEC::Iota(-130, 131);
unsigned char ucStepExp[] = {126, 1, 132, 7, 138, 13, 144, 19, 150, 25, 156, 31, 162, 37, 168, 43};
unsigned char ucStepGot[VEC::elements] = {};
CStep.store(ucStepGot);
for (int i = 0; i < VEC::elements; ++i)
OIIO_CHECK_EQUAL ((int)ucStepGot[i], (int)ucStepExp[i]);

benchmark ("load from int[]", [](const int *d){ return VEC(d); }, i1234);
benchmark ("load from unsigned short[]", [](const unsigned short *d){ return VEC(d); }, us1234);
benchmark ("load from short[]", [](const short *d){ return VEC(d); }, s1234);
Expand Down
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