Decodes IMAGE_GATHER4H_PCK (MIMG opcode 98) and IMAGE_GATHER8H_PCK
(opcode 99), implements gather4h_pck through translation, the SPIR-V
backend and a GPU execution test, and documents gather8h_pck as a
desktop null-op.
Semantics were measured on gfx1031 hardware with hand-encoded MIMG
words (LLVM rejects the PCK gather mnemonics on every RDNA1/RDNA2
target). The measured rule for gather4h_pck: the four-texel horizontal
window is floor(u*width-0.5)+{-1,0,+1,+2} clamped to the width, the row
is floor(v*height) clamped, the raw bits of the window texels are
concatenated into a little-endian bitstream (texel k at bit offset
k*texelBits), and output register j - the j-th set DMASK bit in
ascending order - receives stream word j, zero past the end of the
stream. Exactly popcount(dmask) consecutive VGPRs are written from vdst
ascending; texels wider than 32 bits write zeros (hardware limit); rows
past the height read zeros; negative coordinates clamp to zero. The
window anchor matches the repo's existing PS5-validated HorizontalOffsets
rule, so this reuses the established convention.
The decoder accepts multi-bit DMASKs for these two opcodes only and
sizes their data span as popcount(dmask) (measured), unlike gather4h
which always writes four components. Translation carries the packed
flag through imageMemoryInfoFromInstruction; the SPIR-V emitter
reassembles the bitstream per lane with per-texel OpImageFetch and
byte assembly. D16 data throws until measured.
gather8h_pck never returns texel data on this hardware: across dmasks
0x1-0xFF including the upper nibble, RGBA8/RGBA16/RGBA32 formats, 1D/2D
and 3D descriptors, normalized and unnormalized sampling, sentinel-
prefilled destination registers confirm it only ever zero-fills
popcount(dmask) registers. It decodes but throws at translation.
The measured semantics and the gather8h_pck null-op are recorded in
docs/dev/TechnicalDebt.md.
arcwiser
Mark Kuebel
asimplestray