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// Special softMax shader for matrices with rows of 1500 elements.
// Uses group shared buffer of that length to save global memory bandwidth, more than 2x faster than the original.
// Dispatch [ nr, 1, 1 ] thread groups of this shader
RWBuffer<float> result: register( u0 );
cbuffer Constants: register( b0 )
{
uint4 elements: packoffset( c0 );
uint4 strides: packoffset( c1 );
uint nr: packoffset( c2.x );
float inputScale: packoffset( c2.y );
}
#include "miscUtils.hlsli"
#include "groupReduce64.hlsli"
static const uint THREADS = 64;
static const uint ROW_LENGTH = 1500;
groupshared float rowBuffer[ ROW_LENGTH ];
static const float negativeInfinity = asfloat( 0xff800000 );
[ numthreads( THREADS, 1, 1 ) ]
void main( uint3 group: SV_GroupID, uint thread : SV_GroupIndex )
{
const uint p = group.x * strides[ 1 ];
const uint nc = ROW_LENGTH;
uint i;
float m = negativeInfinity;
// First pass: compute maximum, and copy the row into the group shared buffer
for( i = thread; i < nc; i += THREADS )
{
float f = result[ p + i ];
m = max( m, f );
rowBuffer[ i ] = f;
}
horizontalMaxBroadcast( thread, m );
// Second pass: apply initial scale, compute the exponent, and compute total sum over the row
float sum = 0;
for( i = thread; i < nc; i += THREADS )
{
float f = rowBuffer[ i ];
[branch]
if( f != negativeInfinity )
{
f = ( f - m ) * inputScale;
#if 1
// At least on Radeon Graphics GPU inside Ryzen 7 5700G, computing exponent instead of loading from the buffer improves the performance
f = exp( f );
#else
uint s = fp16Rounded( f );
s = lookupTable[ s ];
f = f16tof32( s );
#endif
sum += f;
}
else
f = 0;
rowBuffer[ i ] = f;
}
horizontalSum( thread, sum );
if( 0 == thread )
sharedAccumulators[ 0 ] = 1.0 / sum;
GroupMemoryBarrierWithGroupSync();
const float scale = sharedAccumulators[ 0 ];
// Final pass: apply the final scale, and copy the row from the group shared buffer back into the global memory
for( i = thread; i < nc; i += THREADS )
{
float f = rowBuffer[ i ];
f *= scale;
result[ p + i ] = f;
}
}
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