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Optimized sample generation in Sobol sequence code #593

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Optimized sample generation in Sobol sequence code #593

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6 changes: 6 additions & 0 deletions src/luxrays/accelerators/embreeaccel.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,12 @@ namespace luxrays {
EmbreeAccel::EmbreeAccel(const Context *context) : ctx(context),
uniqueRTCSceneByMesh(MeshPtrCompare), uniqueGeomByMesh(MeshPtrCompare),
uniqueInstMatrixByMesh(MeshPtrCompare) {
#if defined (_MSC_VER) || defined (__INTEL_COMPILER)
// Embree recommends settings these flags to improve performance
_MM_SET_FLUSH_ZERO_MODE(_MM_FLUSH_ZERO_ON);
_MM_SET_DENORMALS_ZERO_MODE(_MM_DENORMALS_ZERO_ON);
#endif

embreeDevice = rtcNewDevice(NULL);
embreeScene = NULL;
}
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6 changes: 6 additions & 0 deletions src/slg/engines/pathcpu/pathcputhread.cpp
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Expand Up @@ -37,6 +37,12 @@ PathCPURenderThread::PathCPURenderThread(PathCPURenderEngine *engine,
}

void PathCPURenderThread::RenderFunc() {
#if defined (_MSC_VER) || defined (__INTEL_COMPILER)
// Embree recommends settings these flags in each thread to improve performance
_MM_SET_FLUSH_ZERO_MODE(_MM_FLUSH_ZERO_ON);
_MM_SET_DENORMALS_ZERO_MODE(_MM_DENORMALS_ZERO_ON);
#endif

//SLG_LOG("[PathCPURenderEngine::" << threadIndex << "] Rendering thread started");

//--------------------------------------------------------------------------
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14 changes: 10 additions & 4 deletions src/slg/samplers/sobolsequence.cpp
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Expand Up @@ -44,13 +44,19 @@ void SobolSequence::RequestSamples(const u_int size) {
}

u_int SobolSequence::SobolDimension(const u_int index, const u_int dimension) const {
const u_int offset = dimension * SOBOL_BITS;
u_int offset = dimension * SOBOL_BITS;
u_int result = 0;
u_int i = index;

for (u_int j = 0; i; i >>= 1, j++) {
if (i & 1)
result ^= directions[offset + j];
while (i) {
result ^= ((i & 1) * directions[offset]);
i >>= 1; offset++;
result ^= ((i & 1) * directions[offset]);
i >>= 1; offset++;
result ^= ((i & 1) * directions[offset]);
i >>= 1; offset++;
result ^= ((i & 1) * directions[offset]);
i >>= 1; offset++;
Comment on lines +52 to +59
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There is no longer a bounds check on i for 3 of the 4 indexes into directions before the loop repeats, how do you ensure the offset is always a valid index?

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@sshudler sshudler Oct 18, 2022 •
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Before the change and after the change i wasn't used as an index into directions. Whenever the loop was hitting a 1 bit in i it would index into directions by that bit's offset. The way directions defined, starting from the offset it has SOBOL_BITS range for access, so in the new code the access to directions is valid and when i becomes 0 none of these directions values will be used in the result because of & with (0 & 1). Also, SOBOL_BITS == sizeof(i).

}

return result;
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