[Codegen][CPU] Add design doc for the LLVMCPU C-ukernel framework.

Adds README.md for compiler/plugins/target/LLVMCPU/builtins/ukernel/ as a
single final documentation commit, rather than iterating the doc across
the earlier framework commits (less churn for review).

Covers: scope (the framework is op-agnostic in principle, with
`inner_tiled` as the initial and primary case getting the detailed
treatment — inner-K-loop-only contract, and how specialization splits
between symbol selection on the `MMAIntrinsic` and post-inlining loop
unrolling on the `intrinsics_{m,n,k}` factors); the two structural
pillars (lives in compiler/, bitcode-only; self-contained IR via
`hal.executable_object`); the end-to-end lowering flow; the
`--iree-llvmcpu-enable-llvm-ukernels` user flag; bring-your-own-ukernel;
an authoring guide; testing at both the lit (compiler IR) and
`tests/e2e/matmul` (numerical) levels; and the three seed examples.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Benoit Jacob <jacob.benoit.1@gmail.com>

Author: bjacob

compiler/plugins/target/LLVMCPU/builtins/ukernel/README.md

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+# LLVMCPU C microkernels
+
+This directory holds **C-based microkernels** (ukernels) for the LLVMCPU
+target backend. They are compiled to LLVM bitcode, embedded as static data
+in `iree-compile`, and at compile time are copied into the IR as
+`hal.executable_object` attributes on the dispatch's HAL executable variant.
+That representation is the same as the GPU C ukernels under
+`compiler/plugins/target/ROCM/builtins/ukernel/`.
+
+This is **not** the same framework as the legacy CPU ukernels under
+`runtime/src/iree/builtins/ukernel/` — see the design pillars below for the
+differences. The two coexist; this directory is purely additive.
+
+## Scope
+
+The framework itself is **not tied to any one op**. A ukernel is just a C
+function, compiled to bitcode, that codegen may substitute for some piece
+of IR it would otherwise lower itself. Any op that codegen knows how to
+match and replace with a `iree_codegen.ukernel.generic` call can have a
+ukernel; the build/embed/lookup/lowering machinery below is generic.
+
+The **initial and primary** use case — and the only one wired up so far —
+is `iree_codegen.inner_tiled`. The rest of this document leans on that case
+for its concrete examples, but keep the distinction in mind: the pillars
+about *where ukernels live* and *how their bitcode reaches the IR* are
+general, while the specifics of *what a ukernel computes* (the inner K
+loop, the `intrinsics_{m,n,k}` contract) are particular to `inner_tiled`.
+
+### The `inner_tiled` case
+
+For an `iree_codegen.inner_tiled` op with a
+`#iree_cpu.data_tiled_mma_layout`, a ukernel implements the **inner K
+loop** of one data-tiled MMA tile. Outer M/N looping is handled by ordinary
+IREE tiling *before* the ukernel runs; the ukernel sees a single
+(M, N) tile and walks K itself.
+
+A ukernel is specific to **one `MMAIntrinsic`** but **generic over the
+`intrinsics_{m,n,k}` unrolling factors**. Those factors are passed to the C
+function as ordinary parameters and look like runtime values inside the
+ukernel's translation unit — but they are *always* compile-time constants
+in the calling context (the caller passes the `DataTiledMMAAttr`'s
+constants), and the ukernel is *always* inlined into that caller (anything
+else is a bug). Linking the ukernel as bitcode into the dispatch lets the
+post-inlining optimizer fully specialize the body to each call site's
+concrete `(intrinsics_m, intrinsics_n, intrinsics_k)` triple: loops over
+those counts unroll, the unrolled tile lives in registers, and nothing of
+the "runtime parameter" framing survives. In effect the bitcode + inlining
++ LTO chain gives C++-template-like specialization without the template
+syntax — specialization on the `MMAIntrinsic` happens via *symbol
+selection* (one ukernel function per intrinsic), and specialization on the
+unrolling factors happens via *post-inlining loop unrolling*.
+
+In particular, this is a **lower-level interface than the legacy
+`mmt4d` ukernels**:
+
+| | Legacy (`runtime/.../ukernel/`) | New (this directory) |
+|---|---|---|
+| What an entry point is | A whole-matmul library: handles arbitrary `mmt4d` shapes, walks outer M/N itself, internally dispatches to arch-variant inner kernels | The inner K-loop of one specific (intrinsic, arch-variant) configuration |
+| Outer M/N looping | Inside the ukernel | Outside, done by IREE tiling |
+| Per-intrinsic specialization | Hidden behind a dispatching front door | Exposed — one entry point per (intrinsic, arch-variant) |
+| Caller responsibility | Almost nothing | Tile to the ukernel's exact data-tiled shape |
+
+## Design pillars
+
+### 1. Lives in `compiler/`, built only as LLVM bitcode
+
+Two structural decisions, both intentionally different from the legacy
+framework:
+
+- **`compiler/`, not `runtime/`.** The legacy ukernels live under
+  `runtime/src/iree/builtins/ukernel/` because at the time they served two
+  consumers: the llvm-cpu backend *and* VMVX. The VMVX ukernel path is now
+  effectively defunct, so the runtime-side / shared-with-VMVX
+  justification no longer applies. The new ukernels are llvm-cpu-only and
+  live in the plugin that uses them, mirroring the GPU C ukernels under
+  `compiler/plugins/target/ROCM/builtins/ukernel/`.
+
+- **Bitcode only.** The legacy ukernels are also built with the native
+  toolchain, to be testable/benchmarkable in isolation via the runtime's
+  `tools/` directory. That property turned out not to be worth the extra
+  build complexity. The new ukernels are built only as bitcode, embedded
+  as static data in `iree-compile`, and tested via the self-contained-IR
+  property described next.
+
+### 2. IR representation: self-contained via `hal.executable_object`
+
+When `iree-compile` decides to use one of these built-in ukernels, the
+ukernel's bitcode is **copied into the IR** as a `hal.executable_object`
+attached to the dispatch's executable variant. From that point on the
+module is self-contained: everything needed to compile and run the
+dispatch is in the IR.
+
+This is **different** from the legacy framework, where the ukernel was an
+opaque external symbol the runtime was expected to provide. Two
+consequences fall out:
+
+- **Tests are trivially writable.** A lit test is just an MLIR file that
+  carries its own `hal.executable_object`. The test owns its ukernel
+  bitecode bytes outright — no runtime-side fixturing, no special CI
+  wiring, no need to actually rerun the bitcode-library build for the test
+  to run.
+- **Bring-your-own-ukernel is just `hal.executable_object`.** A user who
+  wants to override or supply their own ukernel attaches their own
+  `hal.executable_object` to their input MLIR. `iree-compile` honors it on
+  equal footing with the built-in ones (and the lookup logic, when it
+  walks up looking for executable objects, finds the user's one first).
+  No fork of IREE, no runtime patching.
+
+## How a built-in ukernel works end to end
+
+This walks through the `inner_tiled` path; an analogous flow would apply to
+any future op the framework grows to cover.
+
+1. **Source.** A C file under this directory implements the ukernel as a
+   single `__attribute__((always_inline))` function. For `inner_tiled`, the
+   function takes tile pointers + the `intrinsics_{m,n,k}` unrolling factors
+   as scalar arguments, and emits the unrolled inner K-loop. The file does
+   `#include "common.h"` for no-stdlib stdint replacements and pulls in
+   architecture-specific intrinsic headers (e.g. `<immintrin.h>` for x86).
+
+2. **Build.** A bazel/CMake rule (`iree_bitcode_library` with `ARCH=x86_64`
+   and feature-specific `COPTS` like `-mavx512bf16`) compiles the C file
+   to an LLVM bitcode file, e.g.
+   `iree_uk_mma_x86_avx512bf16_1x16x2_f32_bf16.x86_64_avx512bf16.bc`.
+
+3. **Embed.** `iree_c_embed_data` collects all such `.bc` files into a
+   single TOC blob (`iree_uk_cpu_bitcode.{c,h}`), which is linked into
+   `iree-compile`.
+
+4. **Register at compile time.** On LLVMCPU target init, the TOC is
+   iterated and the bitcode files are inserted into the global
+   `EmbeddedDataDirectory`.
+
+5. **Match.** During LLVMCPU's kernel configuration step
+   (`LLVMCPUSelectUKernels`), an `iree_codegen.inner_tiled` op carrying a
+   `#iree_cpu.data_tiled_mma_layout` is matched against the available
+   ukernels by intrinsic + element types + arch + features. If a match is
+   found, the op gets a `iree_codegen.ukernel = "<name>"` descriptor
+   attribute set on it, and the matching bitcode is attached.
+
+6. **Lower.** The generic pass
+   `iree-codegen-lower-bitcode-ukernels` finds annotated ops, looks up the
+   `iree_codegen.ukernel_provider` (a
+   `#iree_cpu.ukernel_provider` attribute installed on the target config),
+   and calls its `createAndReplaceWithUkernelOp`. That method:
+   - Looks up the matching `hal.executable_object` (first in the
+     dispatch's existing `hal.executable.objects`, then in the global
+     `EmbeddedDataDirectory`).
+   - Attaches it to the executable variant.
+   - Replaces the op with an `iree_codegen.ukernel.generic` call to the
+     ukernel, with `fn_def_attrs = {hal.import.bitcode = true}` so the
+     eventual call resolves directly against the linked bitcode rather
+     than through the runtime import table.
+
+7. **Lower again.** The existing
+   `iree-codegen-lower-ukernel-ops-to-calls` pass lowers
+   `ukernel.generic` to a `func.call` against the (now-linked) bitcode
+   function. The LLVM `always_inline` attribute then specializes the
+   ukernel body into each call site at the LLVM optimization stage.
+
+## How to use as an IREE end user
+
+Pass `--iree-llvmcpu-enable-llvm-ukernels=inner_tiled` to `iree-compile`.
+The flag takes a comma-separated list of categories to enable (currently
+just `inner_tiled`; the name leaves room for future categories). When a
+category is enabled, any matching op implemented by one of the built-in
+ukernels listed below will be rewritten to a ukernel call, and the bitcode
+will appear in the resulting MLIR as a `hal.executable_object` on the
+executable variant.
+
+## How to bring your own ukernel
+
+Attach a `hal.executable_object` to the input MLIR carrying your bitcode
+(with a function whose name matches a known ukernel name). When
+`iree-compile` walks up looking for executable objects, it finds your one
+first and uses it in place of the built-in. Built-in ukernels are
+overridden seamlessly; new ukernels (functions IREE has no built-in for)
+work the same way as long as the user-supplied IR also annotates the
+matched op with `iree_codegen.ukernel = "<name>"`.
+
+## How to author a new built-in ukernel
+
+1. Write the C source under this directory. Function name (and file name)
+   is the corresponding `MMAIntrinsic` enum value, lowercased, with the
+   `iree_uk_` prefix — e.g. `MMA_X86_AVX512BF16_1x16x2_F32_BF16` becomes
+   `iree_uk_mma_x86_avx512bf16_1x16x2_f32_bf16`. This mirrors the AMDGPU C
+   ukernels: those carry the AMDGPU intrinsic name (`mfma_i32_16x16x32_i8`,
+   etc.) verbatim, and on CPU the arch tag (`x86`, ...) is already part of
+   the intrinsic name, so this convention drops in cleanly. One ukernel per
+   intrinsic — the `intrinsics_{m,n,k}` unrolling factors are *function
+   arguments*, not part of the name (see Scope above for why this fully
+   specializes anyway). Body is `always_inline`, no allocations, no stdlib
+   (just `common.h` and intrinsic headers). Loops driven by the unrolling
+   factors should unroll spontaneously after inlining; if Clang doesn't
+   manage on its own, add a targeted `#pragma clang loop unroll(...)`.
+2. Add a `iree_bitcode_library` entry in `BUILD.bazel` / `CMakeLists.txt`
+   for the new file, with the appropriate `COPTS` feature flags, and add
+   the resulting `.bc` to the embedded TOC.
+3. If the ukernel implements an `MMAIntrinsic` that the codegen pipeline
+   does not lower, add a new enum value to `IREECPUEnums.td` and minimal
+   metadata (`getRowMajorTilesMNKShape`, `getABCElementTypes`) in
+   `IREECPUAttrs.cpp` — *but no `lowerX86…` function*, because the
+   ukernel is the sole implementation by design.
+4. Extend `LLVMCPUSelectUKernels.cpp` to recognize the new ukernel by
+   intrinsic + element types + arch features.
+
+## How to test
+
+Two complementary levels:
+
+- **Lit tests (compiler IR).** Use the self-contained-IR property: write a
+  lit test that carries the ukernel bitcode directly as a
+  `hal.executable_object` literal in the MLIR. `iree-opt` running the
+  lowering passes can verify the resulting IR without ever invoking the
+  bitcode-library build. See the lit tests under `test/` for examples —
+  ranging from single-pass checks (`lower_inner_tiled_to_bitcode_ukernel*`,
+  `select_ukernel`) up to a full-codegen-pipeline check
+  (`e2e_inner_tiled_pipeline`) that drives an `inner_tiled` dispatch all the
+  way to a direct `llvm.call` against the ukernel symbol.
+
+- **End-to-end numerical tests (`tests/e2e/matmul`).** For actually running
+  a ukernel and checking its results against a reference, add a variant to
+  the matmul e2e runner tests under `tests/e2e/matmul/` (the
+  `iree_tests_e2e_matmul_*` family), compiling with
+  `--iree-llvmcpu-enable-llvm-ukernels=inner_tiled`. This is the level that
+  catches a wrong inner loop — a lit test only checks that the *call* is
+  emitted, not that the ukernel *computes* the right thing.
+
+## Seed examples
+
+The directory is intentionally **sparse**. Ukernels are no longer the
+majority path under the modernized CPU codegen; codegen handles most
+cases on its own. The seeds here illustrate two categories that justify
+a ukernel at all:
+
+| Seed | What it shows |
+|---|---|
+| `iree_uk_mma_x86_avx512bf16_1x16x2_f32_bf16.c` | Clean example. Codegen does the same thing at parity; this is the canonical "simplest new-style ukernel" reference for new authors. |
+| `iree_uk_mma_x86_avx512vnni_16x16x2_i32_i8_casti16.c` | Practically useful. i8×i8→i32 via VNNI is a workhorse for quantized inference, and codegen has a residual perf gap on this case. |
+
+This README is kept up to date as new seeds and framework pieces land.