lammps/lib/gpu/lj_gpu_kernel.h

/* ----------------------------------------------------------------------
   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
   http://lammps.sandia.gov, Sandia National Laboratories
   Steve Plimpton, sjplimp@sandia.gov

   Copyright (2003) Sandia Corporation.  Under the terms of Contract
   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
   certain rights in this software.  This software is distributed under 
   the GNU General Public License.

   See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */

/* ----------------------------------------------------------------------
   Contributing authors: Mike Brown (SNL), wmbrown@sandia.gov
                         Peng Wang (Nvidia), penwang@nvidia.com
                         Paul Crozier (SNL), pscrozi@sandia.gov
------------------------------------------------------------------------- */

#ifndef LJ_GPU_KERNEL
#define LJ_GPU_KERNEL

/* Cell list version of LJ kernel */
template<bool eflag, bool vflag, int blockSize>
__global__ void kernel_lj_cell(float3 *force3,
			       float *energy, float3 *virial, 
			       float3 *cell_list, unsigned int *cell_idx, 
			       int *cell_type, int *cell_atom,
			       const int inum, const int nall, const int ncell, 
			       const int ncellx, const int ncelly, const int ncellz)
{
	
  
	
  // calculate 3D block idx from 2d block
  int bx = blockIdx.x;
  int by = blockIdx.y % ncelly;
  int bz = blockIdx.y / ncelly;

  int tid = threadIdx.x;
  
  // compute cell idx from 3D block idx
  int cid = bx + INT_MUL(by, ncellx) + INT_MUL(bz, INT_MUL(ncellx,ncelly));
  
  __shared__ int typeSh[blockSize];
  __shared__ float posSh[blockSize*3];
  __shared__ float cutsqSh[MAX_SHARED_TYPES*MAX_SHARED_TYPES];
  __shared__ float lj1Sh[MAX_SHARED_TYPES*MAX_SHARED_TYPES];
  __shared__ float lj2Sh[MAX_SHARED_TYPES*MAX_SHARED_TYPES];

  extern __shared__ float smem[];

  __shared__ float *lj3Sh;
  __shared__ float *lj4Sh;
  __shared__ float *offsetSh;

  // load force parameters into shared memory
  for (int i = tid; i < MAX_SHARED_TYPES*MAX_SHARED_TYPES; i += blockSize) {
    int itype = i/MAX_SHARED_TYPES;
    int jtype = i%MAX_SHARED_TYPES;
    cutsqSh[i] = _cutsq_<float>(itype,jtype);
    lj1Sh[i]   = _lj1_<float>(itype,jtype).x;
    lj2Sh[i]   = _lj1_<float>(itype,jtype).y;
  }

  // Only allocate shared memory when needed, 
  // this reduces shared memory limitation on occupancy
  if (eflag || vflag) {
    lj3Sh = smem;
    lj4Sh = lj3Sh + MAX_SHARED_TYPES*MAX_SHARED_TYPES;
    offsetSh = lj4Sh + MAX_SHARED_TYPES*MAX_SHARED_TYPES;
    for (int i = tid; i < MAX_SHARED_TYPES*MAX_SHARED_TYPES; i += blockSize) {
      int itype = i/MAX_SHARED_TYPES;
      int jtype = i%MAX_SHARED_TYPES;
      lj3Sh[i]   = _lj3_<float>(itype,jtype).x+0.01;
      lj4Sh[i]   = _lj3_<float>(itype,jtype).y;
      offsetSh[i]= _offset_<float>(itype,jtype);
    }
  }

  __syncthreads();

  int nborz0 = max(bz-1,0), nborz1 = min(bz+1, ncellz-1),
      nbory0 = max(by-1,0), nbory1 = min(by+1, ncelly-1),
      nborx0 = max(bx-1,0), nborx1 = min(bx+1, ncellx-1);

  for (int ii = 0; ii < ceil((float)(cell_atom[cid])/blockSize); ii++) {
    float3 f = {0.0f, 0.0f, 0.0f};
    float ener = 0.0f;
    float3 v0 = {0.0f, 0.0f, 0.0f}, v1 = {0.0f, 0.0f, 0.0f};
    int itype;
    float ix, iy, iz;
    int i = tid + ii*blockSize;
    unsigned int answer_pos = cell_idx[cid*blockSize+i];

    // load current cell atom position and type into sMem
    for (int j = tid; j < cell_atom[cid]; j += blockSize) {
      int pid = cid*blockSize + j;
      float3 pos = cell_list[pid];
      posSh[j            ] = pos.x;
      posSh[j+  blockSize] = pos.y;
      posSh[j+2*blockSize] = pos.z;
      typeSh[j]            = cell_type[pid];
    }
    __syncthreads();
    if (answer_pos < inum) {
      itype = typeSh[i];
      ix = posSh[i            ];
      iy = posSh[i+  blockSize];
      iz = posSh[i+2*blockSize];

      // compute force from current cell
      for (int j = 0; j < cell_atom[cid]; j++) {
	if (j == i) continue;
	float delx = ix - posSh[j            ];
	float dely = iy - posSh[j+  blockSize];
	float delz = iz - posSh[j+2*blockSize];
	int jtype = typeSh[j];
	int mtype = itype + jtype*MAX_SHARED_TYPES;
	float r2inv = delx*delx + dely*dely + delz*delz;
	
	if (r2inv < cutsqSh[mtype]) {
	  r2inv = 1.0f/r2inv;
	  float r6inv = r2inv * r2inv * r2inv;
	  float force = r2inv*r6inv*(lj1Sh[mtype]*r6inv - lj2Sh[mtype]);
	  f.x += delx * force;
	  f.y += dely * force;
	  f.z += delz * force;

	  if (eflag) {
	    float e = r6inv*(lj3Sh[mtype]*r6inv - lj4Sh[mtype]);
	    ener += (e - offsetSh[mtype]); 
	  }
	  
	  if (vflag) {
	    v0.x += delx*delx*force;
	    v0.y += dely*dely*force;
	    v0.z += delz*delz*force;
	    v1.x += delx*dely*force;
	    v1.y += delx*delz*force;
	    v1.z += dely*delz*force;
	  }

	} 
      }
    }
    __syncthreads();

    // compute force from neigboring cells
    for (int nborz = nborz0; nborz <= nborz1; nborz++) {
      for (int nbory = nbory0; nbory <= nbory1; nbory++) {
	for (int nborx = nborx0; nborx <= nborx1; nborx++) {
	  if (nborz == bz && nbory == by && nborx == bx) continue;
	  
	  // compute cell id
	  int cid_nbor = nborx + INT_MUL(nbory,ncellx) + 
	    INT_MUL(nborz,INT_MUL(ncellx,ncelly));
	
	  // load neighbor cell position and type into smem
	  for (int j = tid; j < cell_atom[cid_nbor]; j += blockSize) {
	    int pid = INT_MUL(cid_nbor,blockSize) + j;
	    float3 pos = cell_list[pid];
	    posSh[j            ] = pos.x;
	    posSh[j+  blockSize] = pos.y;
	    posSh[j+2*blockSize] = pos.z;
	    typeSh[j]           = cell_type[pid];
	  }
	  __syncthreads();
	  // compute force
	  if (answer_pos < inum) {
	    for (int j = 0; j < cell_atom[cid_nbor]; j++) {
	      float delx = ix - posSh[j           ];
	      float dely = iy - posSh[j+  blockSize];
	      float delz = iz - posSh[j+2*blockSize];
	      int jtype = typeSh[j];
	      int mtype = itype + jtype*MAX_SHARED_TYPES;
	      float r2inv = delx*delx + dely*dely + delz*delz;
	      
	      if (r2inv < cutsqSh[mtype]) {
		r2inv = 1.0f/r2inv;
		float r6inv = r2inv * r2inv * r2inv;
		float force = r2inv*r6inv*(lj1Sh[mtype]*r6inv - lj2Sh[mtype]);
		f.x += delx * force;
		f.y += dely * force;
		f.z += delz * force;

		if (eflag) {
		  float e=r6inv*(lj3Sh[mtype]*r6inv - lj4Sh[mtype]);				
		  ener += (e-offsetSh[mtype]); 
		}
		if (vflag) {
		  v0.x += delx*delx*force;
		  v0.y += dely*dely*force;
		  v0.z += delz*delz*force;
		  v1.x += delx*dely*force;
		  v1.y += delx*delz*force;
		  v1.z += dely*delz*force;
		}
	      }
	    }
	  }
	  __syncthreads();
	}
      }
    }

    if (answer_pos < inum) {
      force3[answer_pos] = f;
      if (eflag)
	energy[answer_pos] = ener;
      if (vflag) {
	virial[2*answer_pos] = v0;
	virial[2*answer_pos+1] = v1;
      }
    }
  }

}

#endif