openharmony/third_party_meshoptimizer
1
0
Fork 0
mirror of https://github.com/openharmony/third_party_meshoptimizer.git synced 2026-07-20 23:46:19 -04:00
Code Issues Packages Projects Releases Wiki Activity

indexcodec: Make decoder ~10% faster by reducing branch mispredictions

Browse Source 
We use a few branches in the decoder loop that are content-based; many
of them are not triggering often and it's more important to keep the
fast path, well, fast.

One exception however was fec==0 branch; in many meshes, this branch
often alternates between taken/not-taken and it's hard to predict this.

Since fec==0 and fec<15 paths are very similar, convert this to
branchless code. Note that we use branchless pushVertexFifo which is a
bit interesting - we *always* save the vertex, which relies on the fact
that if we don't increment the offset, the vertex effectively replaces
the last vertex in the FIFO - which is never referenced since our FIFO
indices always use 15 as a magic encoding for out-of-FIFO data.
This commit is contained in:
Arseny Kapoulkine
2018-03-20 18:32:53 -07:00
parent 7a28177313
commit b1eb4a0a50
1 changed files with 39 additions and 16 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/indexcodec.cpp
+39 -16
View File
@@ -69,10 +69,10 @@ static int getVertexFifo(VertexFifo fifo, unsigned int v, size_t offset)
return -1;
}
static void pushVertexFifo(VertexFifo fifo, unsigned int v, size_t& offset)
static void pushVertexFifo(VertexFifo fifo, unsigned int v, size_t& offset, int cond = 1)
{
fifo[offset] = v;
offset = (offset + 1) & 15;
offset = (offset + cond) & 15;
}
static void encodeVByte(unsigned char*& data, unsigned int v)
@@ -340,33 +340,56 @@ int meshopt_decodeIndexBuffer(unsigned int* destination, size_t index_count, con
unsigned char codetri = *code++;
int fe = codetri >> 4;
if (fe < 15)
if (codetri < 0xf0)
{
int fe = codetri >> 4;
// fifo reads are wrapped around 16 entry buffer
unsigned int a = edgefifo[(edgefifooffset - 1 - fe) & 15][0];
unsigned int b = edgefifo[(edgefifooffset - 1 - fe) & 15][1];
int fec = codetri & 15;
unsigned int c = (fec == 0) ? next++ : vertexfifo[(vertexfifooffset - 1 - fec) & 15];
// note: this is the most common branch in the entire decoder
// inside this if we try to stay branchless (by using cmov/etc.) since these aren't predictable
if (fec != 15)
{
// fifo reads are wrapped around 16 entry buffer
unsigned int cf = vertexfifo[(vertexfifooffset - 1 - fec) & 15];
unsigned int c = (fec == 0) ? next : cf;
// note that we need to update the last index since free indices are delta-encoded
if (fec == 15)
int fec0 = fec == 0;
next += fec0;
// output triangle
destination[i + 0] = a;
destination[i + 1] = b;
destination[i + 2] = c;
// push vertex/edge fifo must match the encoding step *exactly* otherwise the data will not be decoded correctly
pushVertexFifo(vertexfifo, c, vertexfifooffset, fec0);
pushEdgeFifo(edgefifo, c, b, edgefifooffset);
pushEdgeFifo(edgefifo, a, c, edgefifooffset);
}
else
{
unsigned int c = 0;
// note that we need to update the last index since free indices are delta-encoded
last = c = decodeIndex(data, next, last);
// output triangle
destination[i + 0] = a;
destination[i + 1] = b;
destination[i + 2] = c;
// output triangle
destination[i + 0] = a;
destination[i + 1] = b;
destination[i + 2] = c;
// push vertex/edge fifo must match the encoding step *exactly* otherwise the data will not be decoded correctly
if (fec == 0 || fec == 15)
// push vertex/edge fifo must match the encoding step *exactly* otherwise the data will not be decoded correctly
pushVertexFifo(vertexfifo, c, vertexfifooffset);
pushEdgeFifo(edgefifo, c, b, edgefifooffset);
pushEdgeFifo(edgefifo, a, c, edgefifooffset);
pushEdgeFifo(edgefifo, c, b, edgefifooffset);
pushEdgeFifo(edgefifo, a, c, edgefifooffset);
}
}
else
{