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Adept I
Adept I

Get element from an array in a paticular lane

Say I have 8 threads, I want to get array tsum[laneID%8] from lane 0/8 for thread of laneID.

One possible way is to use 16 move dpp instructions with row shift, like this:

__asm ( \

"s_nop 1\n" \

  "v_mov_b32 %[dst0], %[src0]\n" \

  "v_mov_b32_dpp %[dst1], %[src1] row_shr:1\n" \

  "v_mov_b32_dpp %[dst2], %[src2] row_shr:2\n" \

  "v_mov_b32_dpp %[dst3], %[src3] row_shr:3\n" \

  "v_mov_b32_dpp %[dst4], %[src4] row_shr:4\n" \

  "v_mov_b32_dpp %[dst5], %[src5] row_shr:5\n" \

  "v_mov_b32_dpp %[dst6], %[src6] row_shr:6\n" \

  "v_mov_b32_dpp %[dst7], %[src7] row_shr:7\n" \

"s_nop 1\n" \

: [dst0] "=&v" (s[0]), \

  [dst1] "=&v" (s[1]), \

  [dst2] "=&v" (s[2]), \

  [dst3] "=&v" (s[3]), \

  [dst4] "=&v" (s[4]), \

  [dst5] "=&v" (s[5]), \

  [dst6] "=&v" (s[6]), \

  [dst7] "=&v" (s[7]) \

: [src0] "v" (tsum[0]), \

  [src1] "v" (tsum[1]), \

  [src2] "v" (tsum[2]), \

  [src3] "v" (tsum[3]), \

  [src4] "v" (tsum[4]), \

  [src5] "v" (tsum[5]), \

  [src6] "v" (tsum[6]), \

  [src7] "v" (tsum[7])); \

Can I do this job in one instruction, that select different src from lane 0, based on my thread id?

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Re: Get element from an array in a paticular lane

If I've understood the above correctly, you want to read an element from location (thread id mod 8). If so, I think, cross lane instructions particularly ds_bpermute_b32 can be used here. I also checked with the compiler team and they have suggested this builtin:  __builtin_amdgcn_ds_bpermute .

If you are looking for something else, please provide some more details for clarification.


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