The ancillary input's sample number (bits 11:8) was the SampleId
builtin in every pixel shader. Vulkan enables sample shading for a
shader that uses SampleId, so a shader that reads the sample field or
the whole ancillary word ran once per sample on a multisampled draw
even when the guest runs it once per pixel (PS_ITER_SAMPLE clear), and
FragCoord, and with it POS_X_FLOAT and POS_Y_FLOAT, moved to the
sample. The sample number is now SampleId only when the request says
the guest iterates the shader, and otherwise 0, the iterated sample
number of a pixel the hardware does not iterate.
The other inputs that depend on the sample follow where the console
evaluates them:
- PERSP_SAMPLE and LINEAR_SAMPLE are interpolated with
InterpolateAtOffset at sample 0's offset in a shader the guest does
not iterate, and in an iterated one at the offset of the host's
SampleId, read from a table of the guest's locations, because Vulkan
leaves plain barycentrics under sample shading at an
implementation-defined position (lavapipe uses the pixel centre).
- PERSP_CENTER and LINEAR_CENTER are interpolated at offset 0 in an
iterated shader.
- POS_X_FLOAT and POS_Y_FLOAT follow SPI_BARYC_CNTL: the centre is
floor(FragCoord) + 0.5 under sample shading, a sample position in a
shader the guest does not iterate is the centre plus sample 0's
offset, and POS_FLOAT_ULC gives the upper-left corner.
The offsets need InterpolationFunction (sampleRateShading) and the
fragment-shader barycentric path; devices without them keep the host's
default positions. The shader disk cache format goes to 39.
Without VK_KHR_fragment_shader_barycentric the recompiler dropped
v_interp_p1_f32 and made v_interp_p2_f32 read the attribute Vulkan
interpolated, but never wrote the I/J VGPRs: a pixel shader that read
them for anything else got 0, and v_interp_mov_f32 of P10/P20 (or P0 of
an interpolated input) needed PerVertexKHR and failed pipeline
validation.
The fallback translation now loads the I/J VGPRs and marks v_interp_p1
and v_interp_p2 with witnesses. After the front-end simplification
(SSA, folding, DCE, read-lane and masked-select elimination),
HostInterpolationChecker requires every p2 to read its J, and its p1's
I, from a barycentric of the parameter's kind, every p1 result to reach
only the matching p2 (through a write masked by the p2's EXEC, whose
inactive lanes keep the old register), no parameter read as raw
vertices and no slot shared by different interpolations. It then
lowers the witnesses to the host-interpolated attribute as before,
reruns DCE and masked-select elimination, and accepts the program only
when no barycentric read remains, so reusing the I/J VGPRs after
interpolating keeps host interpolation and the previous SPIR-V. Any other
program is translated with the barycentric path and marked as emulated:
its barycentrics become vec3 inputs (NoPerspective for the linear
weights) and its per-vertex parameters flat vec4[3] inputs, placed in
the free locations by LayoutBarycentricEmulation. The driver then
inserts a generated geometry stage between the vertex and fragment
shaders that copies the position and parameters, writes the one-hot
weights of each vertex and the three vertices' parameter values, so the
rasterizer interpolates the same weights the extension provides.
The stage needs geometryShader and a vertex shader before it;
tessellation, mesh and rect-list pipelines, other primitive types and
interfaces beyond 32 locations or the device's geometry limits throw.
CompiledShaderArtifact carries the emulation (shader disk cache format 21).
Hardware snaps window corners to 1/256 pixel before interpolating
(measured by yahya308 on an RTX 4070 Laptop and a Radeon 780M in #1547);
lavapipe snaps only for coverage (lp_rast.h FIXED_ORDER 8), so the test
accepts either reference per pixel at the original 1e-4 margin.
Sources:
- v_interp_p1_f32/v_interp_p2_f32/v_interp_mov_f32 semantics (P0,
P10, P20, I/J): AMD RDNA2 ISA, "Parameter Interpolation".
- BaryCoordKHR/BaryCoordNoPerspKHR weights and PerVertexKHR vertex order:
SPV_KHR_fragment_shader_barycentric, VK_KHR_fragment_shader_barycentric.
- Geometry shader triangle input order and provoking vertex: Vulkan
specification, "Primitive Topologies" and "Geometry Shading".
- Interpolation decorations decided by the fragment input: Vulkan
specification, "Interpolation Decorations".
- lavapipe interpolation: Mesa src/gallium/drivers/llvmpipe/lp_rast.h
(FIXED_ORDER 8), lp_setup_tri.c (subpixel_snap for coverage only),
lp_state_setup.c (init_args, plane setup from float positions).
Yunus Shaikh