agx: document non-monolithic ABI

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28483>

src/asahi/compiler/README.md

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+# Special registers
+
+`r0l` is the hardware nesting counter.
+
+`r1` is the hardware link register.
+
+`r5` and `r6` are preloaded in vertex shaders to the vertex ID and instance ID.
+
+# ABI
+
+The following section describes the ABI used by non-monolithic programs.
+
+## Vertex
+
+Registers have the following layout at the beginning of the vertex shader
+(written by the vertex prolog):
+
+* `r0-r4` and `r7` undefined. This avoids preloading into the nesting counter or
+  having unaligned values. The prolog is free to use these registers as
+  temporaries.
+* `r5-r6` retain their usual meanings, even if the vertex shader is running as a
+  hardware compute shader. This allows software index fetch code to run in the
+  prolog without contaminating the main shader key.
+* `r8` onwards contains 128-bit uniform vectors for each attribute.
+  Accommodates 30 attributes without spilling, exceeding the 16 attribute API
+  minimum. For 32 attributes, we will need to use function calls or the stack.
+
+One useful property is that the GPR usage of the combined program is equal to
+the GPR usage of the main shader. The prolog cannot write higher registers than
+read by the main shader.
+
+Vertex prologs do not have any uniform registers allocated for preamble
+optimization or constant promotion, as this adds complexity without any
+legitimate use case.
+
+For a vertex shader reading $n$ attributes, the following layout is used:
+
+* The first $n$ 64-bit uniforms are the base addresses of each attribute.
+* The next $n$ 32-bit uniforms are the associated clamps (sizes). Presently
+  robustness is always used.
+* The next 32-bit uniform is the base instance. This must always be reserved
+  because it is unknown at vertex shader compile-time whether any attribute will
+  use instancing.
+* For a hardware compute shader, the next 32-bit uniform is the base/first
+  vertex.
+* For a hardware compute shader, the next 64-bit uniform is a pointer to the
+  input assembly buffer.
+
+In total, the first $6n + 2$ 16-bit uniform slots are reserved for a hardware
+vertex shader, or $6n + 8$ for a hardware compute shader.
+
+## Fragment
+
+When sample shading is enabled in a non-monolithic fragment shader, the fragment
+shader has the following register inputs:
+
+* `r0l = 0`. This is the hardware nesting counter.
+* `r0h` is the mask of samples currently being shaded. This usually equals to
+  `1 << sample ID`, for "true" per-sample shading.
+
+When sample shading is disabled, no register inputs are defined. The fragment
+prolog (if present) may clobber whatever registers it pleases.
+
+Registers have the following layout at the end of the fragment shader (read by
+the fragment epilog):
+
+* `r0l = 0` if sample shading is enabled. This is implicitly true.
+* `r0h` preserved if sample shading is enabled.
+* `r2` and `r3l` contain the emitted depth/stencil respectively, if
+  depth and/or stencil are written by the fragment shader. Depth/stencil writes
+  must be deferred to the epilog for correctness when the epilog can discard
+  (i.e. when alpha-to-coverage is enabled).
+* The vec4 of 32-bit registers beginning at `r(4 * (i + 1))` contains the colour
+  output for render target `i`. When dual source blending is enabled, there is
+  only a single render target and the dual source colour is treated as the
+  second render target (registers r8-r11).
+
+Uniform registers have the following layout:
+
+* u0_u1: 64-bit render target texture heap
+* u2...u5: Blend constant
+* u6_u7: Root descriptor, so we can fetch the 64-bit fragment invocation counter
+  address and (OpenGL only) the 64-bit polygon stipple address