AIE kernel optimization (the "what to change" menu)

Fourth of four porting phases, alongside aie-dataflow-opt (see aie-model-baseline for the numeric oracle, aie-dataflow-presim for pre-hardware validation, aie-hw-bringup for getting to a correct baseline on real hardware — this skill assumes that baseline already exists and is correct). This skill is micro; aie-dataflow-opt is macro — this skill answers "how do I make this one compiled kernel faster"; aie-dataflow-opt answers "which op should run on which tile, how should data move between them, and is a kernel even the right thing to be optimizing." Use this once a kernel is confirmed to be on the critical path (see aie-dataflow-opt lever #1, NOOP ablation, if that hasn't been confirmed yet).

This is a catalog of the levers that actually move AIE/Peano kernels — in rough priority order, each with the constraint to respect so it pays off. Use it once you've identified a specific kernel worth optimizing.

The deltas cited are illustrative results from hand-optimizing example INT8 kernels (convs, 1x1 projections, depthwise, GEMM, softmax-style attention) — they show the headroom a given lever can unlock relative to a straightforward first implementation, not a benchmark of any shipping product. The mechanisms are general to AIE/Peano; the conv-shaped examples are just illustrations — read "inner reduction loop", "output loop", "epilogue" generically for your kernel. Concrete vector widths and mmul<...> shapes in the examples reflect one AIE generation (the numbers differ across AIE/AIE-ML/AIE2P); take them as worked examples and check the aie-api headers for your target's actual widths.

Architecture names

The codebase, marketing material, and silicon teams use different names for the same architecture. The internal name (left column) is what you pass to the toolchain (--target=aie2-none-unknown-elf, --dev npu2, etc.) and what gates code in the aie-api headers.

internal name(s)	marketing name	chips with this architecture
aie	AIE	Versal VCK190
aie2 (== NPU1)	AIE-ML (XDNA)	Phoenix
aie2p (== NPU2)	XDNA2	Strix Point, Strix Halo, Krackan
aie2ps	AIE-MLv2	Telluride

Before you start (methodology — don't skip)

Measure before modeling; verify before trusting. Static analysis of MLIR or kernel C is unreliable on AIE — paper-compute estimates have mispredicted real HW time by 5–300× depending on the kernel. Every claim about where time goes must be backed by an HW measurement or an ELF inspection, not a cycle count in your head.

1. Baseline first. Build clean, run on HW, record the median wall time over ≥20 iterations. If the std-dev is >1% of the mean, fix that (background load, throttling, noisy runtime) before optimizing — you can't measure a 5% win against 10% noise.
2. Establish a bit-exact gate from the start. Capture the exact command that produces a PASS against a reference. Every change below must keep it passing. Don't take wall-time numbers from a broken build — wrong data computes in a different amount of time than correct data.
3. Confirm the kernel is on the critical path before optimizing it. The cheapest reliable attribution is ablation: gate the kernel call behind a flag (leave the locks/acquires/releases intact, just skip the compute), re-time, and look at the delta. A kernel contributing 99 ms is 20× more worth your time than one contributing 4 ms. HW packet tracing is more difficult to interpret; prefer ablation for attribution.
4. One change at a time. If you edit the C source and the build flags and the dataflow at once, you can't tell which moved the needle.
5. Verify the mechanism in the .o before believing your cycle-model. In practice, both "obvious" explanations for a given regression (register spill; doubled scalar gather) have turned out wrong on objdump inspection. A regression you can't see in the codegen is one you don't understand yet. Useful checks:
6. llvm-nm --print-size build/X.o — spill ≈ noticeably bigger function.
7. llvm-objdump -d build/X.o | grep -coE '\bv(lda|ldb|mac|mul|st)\b' — confirm vector ops actually appear (scalar-only mac/lda/st means the kernel didn't vectorize at all — fix that before anything else).
8. llvm-nm build/X.o | grep __div — should be empty (see lever #3).

Read the disassembly — for a compute-bound kernel it can replace HW timing. AIE cores don't stall (no cache, no dynamic scheduling, no branch misprediction), so the hot loop's instruction count ÷ core clock ≈ its execution time. This is not the source-level paper-compute modeling warned against above: that fails because you can't know what the compiler emitted, whereas counting instructions in the emitted assembly is counting exactly what runs. Disassemble with relocations (the -r marks the start/end of the zero-overhead loop):

$PEANO_INSTALL_DIR/bin/llvm-objdump -d -r build/X.o

Or compile straight to assembly, skipping the object file:

$PEANO_INSTALL_DIR/bin/clang++ -O2 -std=c++20 \
  -I$MLIR_AIE_INSTALL_DIR/include --target=aie2-none-unknown-elf \
  -S my_kernel.cc -o kernel.s
# (swap aie2 for aie2p/etc. per the architecture table above)

In the zero-overhead loop body (a hardware loop whose bounds are programmed once, so it repeats a fixed instruction block with no per-iteration branch/counter overhead), count two things: total instructions, and inserted nops — padding emitted when the compiler can't fill a VLIW slot, i.e. visible evidence the loop isn't packed tight. Fewer of each is directly faster. 6. Don't trust "I cleaned the build." Make tracks file mtimes, not flag changes, and .prj/ dirs cache stale ELFs. Confirm the .o actually rebuilt and contains your edit (see "Verifying any change took effect" at the end).

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Priority order

Bigger, more reliable levers first. Don't jump to the wider-mmul lever before doing the loop-hint / branch / epilogue levers — the structural wins live there.

1. Loop hints — cheapest, often biggest

Include aie_kernel_utils.h. Decorate every non-trivial for-loop. The macro goes before the for statement, never inside the parens (it's a clang pragma, not the chess attribute form).

• AIE_PREPARE_FOR_PIPELINING on the loop you want software-pipelined (usually the outermost loop over output elements/tiles). Its effect is toolchain-dependent; leave it in for portability across toolchains, but treat the other hints below as the ones doing the work.
• AIE_LOOP_RANGE(N, N) on every fixed-trip-count loop. This is a trip-count hint only — Peano still emits a runtime loop, it just knows the bounds.
• AIE_LOOP_RANGE(lo, hi) on bounded-but-variable loops (e.g. a reduction whose count depends on a border/edge condition).
• AIE_LOOP_UNROLL_FULL on small fixed-count inner loops (trip count is a small constant like 3) — mandatory if the loop body switches or branches on the loop variable (a helper that does switch(i) on the index, an index-dependent address calc). AIE_LOOP_RANGE alone leaves the switch as a runtime branch and blocks VLIW pipelining of the body. Verified on a 3x3 conv: changing the window loop from AIE_LOOP_RANGE(3,3) to AIE_LOOP_UNROLL_FULL was a one-line diff for −47% kernel time. RANGE alone was insufficient.

2. Make shapes and trip counts compile-time constants

This is the lever that unlocks most of the others, so do it early. Peano optimizes far more aggressively when the bounds, strides, and shapes it needs are visible as constants at compile time rather than arriving as runtime kernel arguments. constexpr (not just const) is what guarantees that visibility: a const int initialized from a runtime argument is still a runtime value, whereas a constexpr — or a value threaded in through a template parameter or a -D compile definition — is a literal the optimizer can fold against.

What you get when the shapes are constexpr: - Loop hints actually bind. The AIE_LOOP_RANGE / AIE_LOOP_UNROLL_FULL hints from lever #1 only help when the trip count is a known constant; fed a runtime bound, they have nothing to fold. - Address arithmetic folds to immediates. ic_t * stride + x * 64 with constant strides becomes immediate-offset loads instead of runtime multiplies. - Power-of-2 divides and multiplies become shifts instead of __divsi3 software calls (see lever #3).

How to apply: prefer template parameters or constexpr for per-call-site shapes. If the same kernel serves multiple shapes, pass them as -D compile definitions (one specialization per shape) with a runtime-argument fallback path, so the hot configurations get the constant-folded code and the rest still build. Compile-time shapes alone are typically a small direct win (~2%), but they are the precondition that makes levers #1, #3, and the addressing in #6 pay off.

3. Eliminate __divsi3 / signed power-of-2 ops on the hot path

A signed power-of-2 divide is not equivalent to a shift (int32_t a / 8 ≠ a >> 3 — rounding toward zero differs for negatives), so it lowers to a __divsi3 software call rather than a single shift. That external call also acts as a vectorization barrier for the whole enclosing function, so removing it can unlock much more than the divide itself.

const int n_tiles = (uint32_t)channel_count / 8u;  // → shift, not __divsi3

The same applies to signed multiply by a power of 2 when the operand can be negative: idx * STRIDE where idx can be −1 (e.g. a left/top border offset) won't fold to a shift even with STRIDE a compile-time power-of-2 (negative-shift would be UB). Fix by peeling the borders out of the hot loop — handle the edge iterations separately so the interior path has a provably non-negative index and the address arithmetic folds to shifts. Verify with llvm-nm build/X.o | grep __div — the call should disappear.

4. No branches inside the inner compute loop

A per-iteration if/else/ternary inside the VLIW inner loop blocks Peano's back-to-back vld + vld + vmac (or vld + vop) issue — even when the predicate is a compile-time-known constant array indexed by the loop variable.

The common offender: a kernel that reduces over inputs from mixed sources (some operands in one memory layout, some in another) and selects per-iteration with if (is_packed[i]) vec_load else scalar_pack. Don't. Split into K straight-line per-source loops, each body uniform:

for (int i = 0; i < n_src0; ++i) { /* pure vec_load + op */ }
for (int i = 0; i < n_src1; ++i) { /* pure scalar-pack + op */ }

Verified: branched → split was −7% on a 3-source reduction kernel, on top of staying bit-exact. A (i < n) ? a : b ternary inside the loop counts as a branch too — split per-source even when both branches load.

5. Vectorize the epilogue (bias + requantize/SRS + clamp + activation)

Once the inner loop is tight, the scalar tail becomes the bottleneck: processing the output elements one at a time (add bias, shift-round- saturate, clamp, activation lookup). In practice, vectorizing the epilogue was the single biggest per-kernel lever — roughly 20–25% per kernel even after loop hints (#1) and compile-time shapes (#2) were already in. In one measured case a pipeline stage went 27.75 → 15.99 ms (−42% / +74% fps) across three kernels, almost entirely from epilogue vectorization.

Pattern (for any kernel with a scalar acc[i] + bias; round_shift; clamp; out[i] = lut[...] tail):

1. Match the rounding mode to your reference. If the scalar reference uses round-half-to-even (banker's rounding), set aie::set_rounding(aie::rounding_mode::conv_even) so the vector acc.to_vector<int8>(shift) is bit-identical. A mismatched mode (e.g. positive_inf) produced ~0.2% element mismatch with max diff 3 — small, but an activation LUT amplifies ±1 rounding diffs into larger output errors, so it fails a bit-exact gate.
2. Bias-init / fold the add into a vector op. Load the bias values, broadcast/concat them to the accumulator width, aie::add on the int32 vector, then acc.from_vector(sum); acc.to_vector<int8>(shift) to do shift-round-saturate in one shot. Replaces N scalar bias-adds + N scalar SRS.
3. Keep the epilogue helper always_inline and pass the accumulator by reference. A non-inline pass-by-ref of an aie::mmul/aie::accum breaks the ABI. Only break it out of inline (and watch tile program memory) if PM pressure forces it.
4. The activation-table lookup (lut[...]) stays scalar. Vectorize the bias + SRS + clamp around it; the per-element table lookup and store remain a scalar loop.
5. For a residual/skip add, use a wide-enough intermediate. Accumulate into an int16 vector and add non-saturating when the value range fits, then acc.to_vector<int8>(0) for the saturating narrow. aie::add on int8 vectors wraps rather than saturates, so the saturation has to come from the narrowing step.

6. Pre-pack operand layout to kill the scalar gather feeding mmul

The scalar operand-build that feeds a matmul — for(p,k) a_buf[p*W+k] = src[...]; load_v<...>(a_buf) into a stack scratch — is frequently the dominant cost of an mmul-based kernel: the per-mmul cost is several times what the same mmul achieves once the operand is delivered in mmul-ready layout. It's a non-issue in the canonical matmul example (aie_kernels/aie*/mm.cc) because there DMA delivers the operand already in mmul-ready layout; kernels that hand-assemble the operand from L1-shared buffers (not via DMA) pay the repack cost instead.

Root cause: aie::concat has a minimum operand width (on current AIE it won't concat sub-128-bit vectors), so a wide operand vector can't be assembled from strided narrow loads — the scalar byte-copy fallback is the only option when the source layout doesn't match the mmul's expected operand layout.

Fix — requires controlling BOTH the producer and the consumer of the intermediate buffer: - Producer (free): an mmul result acc.to_vector<int8>(shift) is already in the byte order the next mmul wants as its A operand. Have the producer write it as one vector store into the intermediate buffer in that layout. No transform on the producer side. - Consumer: reads the operand with a single aligned vld. If the next kernel needs a shifted window of that data (e.g. a sliding conv window), build the shifted vector with concat + shuffle_down + extract rather than re-gathering scalar-wise:

auto combined = aie::concat(lo_block, hi_block);   // ≥128-bit
return aie::shuffle_down(combined, shift_amt).template extract<N>(0);

Total bytes are identical to the naive layout; both ends must agree on the stride.

Verified: a multi-kernel design went 14.97 → 11.44 ms (−23.6%) standalone; a standalone conv went 207 → 521 fps (+135%). The estimate (0.8–1.5 ms) badly undershot the actual (3.5 ms) because removing the scalar pack also let Peano schedule the surrounding loop more tightly — a secondary register-allocation effect invisible in any bytecount model. Only applies when both ends are in your code (not a library/external consumer that expects the naive layout).

7. Pack wide copies explicitly

For a small fixed-size element copy, write the wide move explicitly rather than relying on a scalar byte loop to be widened for you — in deeply-nested or stack-allocated-buffer contexts the explicit form is what guarantees a single wide load/store:

*reinterpret_cast<uint64_t *>(d) = *reinterpret_cast<const uint64_t *>(src);  // 8 bytes
// 4 bytes → uint32_t; 16 → two uint64; 32 → four. alignas(N) both ends.

Verified: a per-row scratch pack went 3.44 → 2.95 ms (+14%) from this change alone. If the explicit form produces wrong output, check for a Worker stack overflow into an adjacent buffer — bump the worker's stack size (e.g. Worker(stack_size=2048)).

8. Wider mmul shape — only where it amortizes

An int8 mmul<r, s, t> comes in several widths (the exact set is arch-dependent; check the aie-api headers for your target — e.g. AIE2P int8 offers mmul<4,8,8>, <4,16,8>, <8,8,8>, <8,16,8>). A wider M (r) doubles output rows per call; a wider K (s=16) needs the weight operand re-laid-out (e.g. ...I8O8 → ...I16O8), which isn't free.

• When there's a deep inner reduction loop per output (e.g. 3x3+ conv with a kx·ky·channels reduction): usually a direct win — halving the outer call count cuts a large per-output workload, swamping any extra operand-gather cost.
• When the per-output work is tiny (1x1 conv / GEMV-ish) and the operand is built by scalar gather: it can regress. mmul<M,8,8> does M×8 byte copies per reduction step; going M=4→8 doubles the gather while only halving an already-small call count. Verified: a 1x1 kernel went 3.34 → 3.58 ms (+7%, reverted). Vectorize the operand gather (lever #6) first; only then does the wider shape pay back.
• Non-contiguous operands (strided/gathered inputs): tried + reverted. The wider shape can't help if the operand can't be loaded contiguously.

9. Move pure data-movement into DMA (free a whole tile)

If a kernel body is only a data rearrangement — a strided copy, transpose (dst[a,b] = src[b,a]), deinterleave, or gather, with no arithmetic — it's exactly what the memtile DMA dims_to_stream / dims_from_stream machinery was built for. Move it onto the upstream ObjectFifo (cons().forward(dims_to_stream=…)) instead of running a kernel. This drops an extern call + a .o and frees the consumer tile's program memory — worth doing even when the kernel isn't on the critical path (verified: pure-transpose packer kernels that ablation showed were off the critical path were still worth removing for the PM/tile savings).

Don't do this if the kernel also computes (mac, activation, requant) — the rearrangement is essentially free when overlaid on real compute, so leave it in the kernel.

Related DMA layout wins: - Deinterleave a strided input so the kernel reads aligned vectors instead of a strided scalar gather (e.g. split a stride-2 input into even/odd halves per row). Verified +12% fps on a strided-input conv. - A layout transform can live on the shim tap, the compute toStream/fromStream, or the memtile forward — pick the cheapest for your dataflow, it's not memtile-only.

Constraints to respect when forwarding (otherwise expect wrong or zero data): - Aligned vector loads need their natural alignment (a load of an N-byte vector wants N-byte alignment, e.g. load_v<int8,32> → 32-byte). If a deinterleave lands data at an unaligned offset, do 2 aligned loads + shuffle_down rather than one unaligned load. - In a multi-kernel chain, pin the forward to a specific column memtile (.forward(tile=Tile(col, 1))) so the ObjectFifoLink has a shared tile; a placement that works standalone can otherwise fail in the chain.

Decision rule: pure-DMA offload needs ≥4-byte elements. Shim DMA's minimum stride-1 access granularity is 4 bytes, so a pure-DMA rearrangement (no compute core) only works when the element size is ≥4 bytes (int32/uint32); sub-4-byte types (int8/int16) can't be expressed as a DMA-only stride pattern and need a hybrid instead — a coarse DMA reshuffle plus a small kernel (e.g. VSHUFFLE) doing the fine-grained rearrangement in place. programming_examples/basic/transposes/ (--strategy dma vs --strategy combined) is a worked example of both ends of this tradeoff.

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Register-pressure / power note

More compute isn't always the goal. On a kernel that was off the critical path, removing an explicit AIE_LOOP_UNROLL(2) from a 4- accumulator reduction loop cut in-loop register-spill traffic by ~87% and .text by ~8% at net-zero throughput — pure power savings, which can matter for deployment. When adding explicit unroll to a kernel with 4+ simultaneous accumulators, count in-loop spill stores/loads (vst/vld of accumulator-state registers) in the disasm before adopting: spill inside a hot loop is a power tax even when wall time is flat.

The same mechanism isn't always throughput-neutral, though: on a kernel that is on the critical path, adding unroll to a loop that already keeps several accumulators live can regress wall time directly — each unrolled iteration wants its own accumulator, and once the live set exceeds the register file the compiler spills, and the spilled vst/vld traffic lands inside the hot loop where every extra instruction is pure added latency, not just a power cost. Symptom: you add unroll expecting a speedup and measure a slowdown instead. Diagnosis is the same disasm check — compare spill-instruction counts with and without the unroll — before deciding whether to revert, reduce the unroll factor, or restructure to use fewer live accumulators.

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Verifying any change took effect

Make's incremental rebuild tracks file mtimes, not flag changes, and .prj/ directories cache stale ELFs — a source edit may not reach the .o. After each change: 1. Confirm the .o rebuilt (check mtime) and, when in doubt, make clean / rm -rf build/*.prj. 2. Confirm the change is in the binary: did the vmac count change, did __divsi3 disappear, did the function size move the way you expected? 3. Only then trust the wall-time delta.

The aie-api headers are the ground truth for what each intrinsic lowers to. In this repo they're vendored at third_party/aie_api/include/aie_api/ (aie.hpp is the umbrella header; per-arch lowerings live under detail/aie*/); after a build they're also copied to install/include/aie_api/. The matmul programming example (programming_examples/.../matrix_multiplication/) is a good reference for DMA-delivered mmul-ready operand layouts.