parallel processing - OpenCL barrier of finding max in a block -


i've found piece of opencl kernel sample code in nvidia's developer site purpose function maxoneblock find out biggest value of array maxvalue , store maxvalue[0].

i understand looping part, confused unroll part: why unroll part not need sync thread after each step done?

e.g: when 1 thread done comparison of localid , localid+32, how ensure other thread have stored result localid+16?

the kernel code:

void maxoneblock(__local float maxvalue[],                  __local int   maxind[]) {     uint localid   = get_local_id(0);     uint localsize = get_local_size(0);     int idx;     float m1, m2, m3;      (uint s = localsize/2; s > 32; s >>= 1)     {         if (localid < s)          {             m1 = maxvalue[localid];             m2 = maxvalue[localid+s];             m3 = (m1 >= m2) ? m1 : m2;             idx = (m1 >= m2) ? localid : localid + s;             maxvalue[localid] = m3;             maxind[localid] = maxind[idx];         }         barrier(clk_local_mem_fence);     }      // unroll final warp reduce loop , sync overheads     if (localid < 32)     {         m1 = maxvalue[localid];         m2 = maxvalue[localid+32];         m3 = (m1 > m2) ? m1 : m2;         idx = (m1 > m2) ? localid : localid + 32;         maxvalue[localid] = m3;         maxind[localid] = maxind[idx];           m1 = maxvalue[localid];         m2 = maxvalue[localid+16];         m3 = (m1 > m2) ? m1 : m2;         idx = (m1 > m2) ? localid : localid + 16;         maxvalue[localid] = m3;         maxind[localid] = maxind[idx];          m1 = maxvalue[localid];         m2 = maxvalue[localid+8];         m3 = (m1 > m2) ? m1 : m2;         idx = (m1 > m2) ? localid : localid + 8;         maxvalue[localid] = m3;         maxind[localid] = maxind[idx];          m1 = maxvalue[localid];         m2 = maxvalue[localid+4];         m3 = (m1 > m2) ? m1 : m2;         idx = (m1 > m2) ? localid : localid + 4;         maxvalue[localid] = m3;         maxind[localid] = maxind[idx];          m1 = maxvalue[localid];         m2 = maxvalue[localid+2];         m3 = (m1 > m2) ? m1 : m2;         idx = (m1 > m2) ? localid : localid + 2;         maxvalue[localid] = m3;         maxind[localid] = maxind[idx];          m1 = maxvalue[localid];         m2 = maxvalue[localid+1];         m3 = (m1 > m2) ? m1 : m2;         idx = (m1 > m2) ? localid : localid + 1;         maxvalue[localid] = m3;         maxind[localid] = maxind[idx];     } }

why unroll part not need sync thread after each step done?

the sample incorrect, barrier indeed required after each step.

it looks sample written in warp-synchronous style, way of exploiting underlying execution mechanism of warps on nvidia hardware, incorrect synchronization cause break if underlying execution mechanism changes or in presence of compiler optimizations.


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