Undocumented Instructions from GCN ISA used in RDNA2 #93

AquaGeneral · 2022-04-25T02:19:14Z

Both S_SETVSKIP and S_SET_GPR_IDX_ON seem to still exist in RDNA 2, and the RGA command line tool at the very least reports these instructions as ENC_SOPC_invalid_16. I've managed to create a small reproducible OpenGL compute shader that only produces two instances of S_SETVSKIP (targeting gfx1031):

#version 430 core

#ifdef GL_AMD_gpu_shader_half_float
#extension GL_AMD_gpu_shader_half_float : require
#else
#extension GL_NV_gpu_shader5 : require
#endif

layout(local_size_x = 8, local_size_y = 8, local_size_z = 8) in;

layout(binding = 0)           uniform usampler3D samp;
layout(binding = 0) writeonly uniform uimage3D   img;

// Total 17408 bytes per work group
shared float16_t b[16*16*16]; // 8192 bytes

void main() {
    uint lindex = gl_LocalInvocationIndex;
    uvec4 b_xyzw = texelFetchOffset(samp, ivec3(gl_WorkGroupID), 0, ivec3(0));
    f16vec2 b011_111 = unpackFloat2x16(b_xyzw.w);

    b[lindex + 500] = b011_111.y;

    memoryBarrierShared();
    barrier();
    
    imageStore(img, ivec3(gl_GlobalInvocationID), uvec4(1, 2, 3, 4));
}

And the binary representation of these two invalid instructions is 0xBF109F07 and 0xBF108080 respectively which looking at the GCN ISA, seem to map directly to S_SETVSKIP.

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Undocumented Instructions from GCN ISA used in RDNA2 #93

Undocumented Instructions from GCN ISA used in RDNA2 #93

AquaGeneral commented Apr 25, 2022

Undocumented Instructions from GCN ISA used in RDNA2 #93

Undocumented Instructions from GCN ISA used in RDNA2 #93

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AquaGeneral commented Apr 25, 2022