flydsl-kernel-authoring

name	flydsl-kernel-authoring
description	Comprehensive reference for authoring FlyDSL GPU kernels on AMD GPUs. Covers the layout algebra, tiled copy/MMA, buffer ops, loop-carried range loops, SharedAllocator (LDS), autotuning, and common patterns. Use when writing, reviewing, or understanding FlyDSL kernel code.
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FlyDSL Kernel Authoring Skill

Overview

FlyDSL is a Python DSL and MLIR-based compiler for writing high-performance GPU kernels on AMD GPUs (MI300X/MI350). It provides explicit layout algebra for controlling data movement, tiling, and memory access patterns. The layout system is the core abstraction that distinguishes FlyDSL from Triton/Gluon.

Repository: /FlyDSL/ (installed in editable mode) Target GPU: gfx942 (MI300X, CDNA3), gfx950 (MI350, CDNA4) Python: 3.12, ROCm 7.2

Scope (read this first): This skill is the reference — the full layout-algebra API surface, per-op tables, MFMA/copy-atom catalogs, environment variables, and an exhaustive troubleshooting list. Reach for it to look something up while writing or reviewing kernel code. If instead you want a guided, step-by-step procedure that turns a kernel requirement into a finished, tested kernel (classify -> skeleton -> compute -> control flow -> test), use the flydsl-tile-programming skill, which is the wizard companion to this reference. For diagnosing a kernel that already compiles but produces NaN/inf/wrong results, use the debug-flydsl-kernel skill. When modernizing or cleaning up an existing kernel — replacing legacy/deprecated constructs (ArithValue, fx.Index, buffer_ops, raw MLIR dialects, SmemPtr, per-tile *_atom_call, raw rocdl.mfma_*) with the current fx.* surface, trimming comments/dead code, or applying the _run_compiled fast launch path — use the flydsl-kernel-code-cleanup skill.

---

1. Architecture and Compilation

Pipeline

Python (@flyc.kernel/@flyc.jit)
  -> AST Rewriting (for/if -> scf.for/scf.if)
  -> MLIR Tracing (generates Fly dialect + gpu/arith/scf/memref/vector ops)
  -> MlirCompiler.compile() (Fly -> ROCDL -> LLVM -> HSACO binary)
  -> JITCFunction (ExecutionEngine wrapper)

Key Passes

Pipeline is built by RocmBackend._pipeline_parts() and split into three stages — see docs/architecture_guide.md §3 for the per-pass table. Highlights:

1. fly-rewrite-func-signature - Rewrite DSL types at function / SCF boundaries to packed LLVM structs
2. fly-layout-lowering - Lower layout algebra (fly.crd2idx, partitions, divides) to arithmetic
3. fly-convert-atom-call-to-ssa-form + fly-promote-regmem-to-vectorssa - Lift copy/MMA atom calls and register memory to vector SSA
4. convert-fly-to-rocdl - Fly ops -> ROCDL intrinsics
5. gpu-module-to-binary{format=fatbin} - Emit HSACO binary via LLVM AMDGPU backend

Key Source Paths

• python/flydsl/compiler/ - JIT compilation (jit_function.py, kernel_function.py)
• python/flydsl/expr/ - DSL expression API (primitive.py, derived.py, typing.py)
• python/flydsl/expr/primitive.py - All layout algebra functions
• python/flydsl/expr/derived.py - CopyAtom, MmaAtom, TiledCopy, TiledMma wrappers
• python/flydsl/expr/gpu.py - GPU operations (thread_idx, block_idx, barrier)
• python/flydsl/expr/buffer_ops.py - AMD buffer load/store intrinsics
• python/flydsl/expr/rocdl/ - MFMA/WMMA and other ROCm intrinsics (package: cdna4, cluster, inline_asm, tdm_ops, universal)
• python/flydsl/expr/gpu.py - SharedAllocator for LDS (shared memory), thread_id/block_id, barrier
• python/flydsl/utils/smem_allocator.py - legacy SmemAllocator (un-migrated kernels only)
• kernels/ - Pre-built kernels, organized into subpackages: gemm/ (preshuffle_gemm.py, mxfp4_preshuffle.py, ...), norm/ (layernorm/softmax/rmsnorm), attention/, moe/, mma/, common/, comm/, conv/

---

2. Layout System (Core Abstraction)

Core Types

Type	Description	Example
!fly.int_tuple	Integer tuple (can be nested)	(8, 16), (8, (4, 2))
!fly.layout	(Shape, Stride) pair	(8, 16):(1, 8) (col-major)
!fly.memref	Memory reference with layout	Typed pointer + layout info

Construction

import flydsl.expr as fx

shape = fx.make_shape(8, 16)              # IntTuple (8, 16)
stride = fx.make_stride(1, 8)             # IntTuple (1, 8)
layout = fx.make_layout(shape, stride)    # Layout (8,16):(1,8)

# Shorthand with Python tuples
layout = fx.make_layout((8, 16), (1, 8))

# Coordinates
coord = fx.make_coord(i, j)

# Nested shapes for hierarchical tiling
shape_nested = fx.make_shape(9, (4, 8))   # (9, (4, 8))

# Identity layout
identity = fx.make_identity_layout((M, N))

Coordinate Mapping

The fundamental operation maps logical coordinates to physical memory indices.

Formula: Index = sum(coord_i * stride_i)

idx = fx.crd2idx(coord, layout)    # Coordinate -> linear index
coord = fx.idx2crd(idx, layout)    # Linear index -> coordinate
s = fx.size(layout)                # Total element count (product of shape)

Example: For layout (8, 16):(1, 8) (8x16, column-major):

• crd2idx((3, 5), layout) = 3*1 + 5*8 = 43
• idx2crd(43, layout) = (43 % 8, 43 / 8) = (3, 5)

Query Operations

fx.size(layout)           # Total element count
fx.get_shape(layout)      # Extract shape IntTuple
fx.get_stride(layout)     # Extract stride IntTuple
fx.get(int_tuple, i)      # Get i-th element
fx.rank(int_tuple)        # Number of top-level modes

Layout Algebra Operations

Composition: fx.composition(A, B)

Compose two layouts: result(x) = A(B(x)). Used to apply permutations or tile coordinate mappings.

Complement: fx.complement(tiler, target_size)

Compute remaining modes not covered by tiler, up to target_size. Internal building block for divides.

Coalesce: fx.coalesce(layout)

Simplify layout by merging adjacent modes. Preserves mapping but flattens structure.

Right Inverse: fx.right_inverse(layout)

Compute right inverse of layout mapping.

Recast: fx.recast_layout(layout, old_bits, new_bits)

Adjust layout for type width change (e.g., FP16->FP8).

Product Operations (Combine Layouts)

Products combine two layouts to create a larger layout:

fx.logical_product(layout, tiler)   # Basic mode-wise concatenation
fx.raked_product(thr, val)          # Interleaved access pattern (common for TiledCopy)
fx.blocked_product(layout, tiler)   # Blocked access pattern
fx.zipped_product(layout, tiler)    # Zipped modes
fx.tiled_product(layout, tiler)     # Hierarchical tiled structure
fx.flat_product(layout, tiler)      # Flattened result

Divide Operations (Partition Layouts)

Divides split a layout by a divisor, creating tile + rest dimensions:

fx.logical_divide(layout, divisor)  # Basic partitioning (uses complement internally)
fx.zipped_divide(layout, divisor)   # Zipped division
fx.tiled_divide(layout, divisor)    # Hierarchical tiled division
fx.flat_divide(layout, divisor)     # Flattened division

Structural Operations

fx.select(int_tuple, indices=[0, 2])      # Pick specific modes
fx.group(int_tuple, begin=1, end=3)        # Group modes into nested tuple
fx.append(base, elem)                      # Append mode
fx.prepend(base, elem)                     # Prepend mode
fx.slice(src, coord)                       # Slice at coordinate (None = keep mode)

---

3. Writing Kernels

Basic Pattern

import flydsl.compiler as flyc
import flydsl.expr as fx
from flydsl.expr import const_expr, gpu, range_constexpr, rocdl

@flyc.kernel
def my_kernel(
    A: fx.Tensor,         # GPU tensor (memref via DLPack)
    B: fx.Tensor,
    N: fx.Constexpr[int], # Compile-time constant
):
    tid = gpu.thread_id("x")
    bid = gpu.block_id("x")
    # ... kernel body ...

@flyc.jit
def launch(
    A: fx.Tensor,
    B: fx.Tensor,
    N: fx.Constexpr[int],
    stream: fx.Stream = fx.Stream(None),
):
    my_kernel(A, B, N).launch(
        grid=(N // 256,), block=(256,), stream=stream
    )

# Usage:
import torch
A = torch.randn(1024, device="cuda", dtype=torch.float32)
B = torch.empty(1024, device="cuda", dtype=torch.float32)
launch(A, B, 1024)

Current Syntax Quick Reference

Use the current public FlyDSL surface from kernels/gemm/preshuffle_gemm.py when writing new kernels:

Vec = fx.Vector

tx = gpu.thread_id("x")
bx = gpu.block_id("x")
by = gpu.block_id("y")

i32_m: fx.Int32
c_m = fx.Int64(i32_m)
c4 = fx.Int64(4)
zero_f = fx.Float32(0.0)

layout = fx.make_layout((4, 64), (64, 1))
coord = fx.idx2crd(tx, layout)
wave_id = fx.get(coord, 0)
lane_id = fx.get(coord, 1)

acc = Vec.filled(4, 0.0, fx.Float32)
v_i64 = Vec(raw_vec).bitcast(fx.Int64)
elem0 = v_i64[0]

buf = fx.rocdl.make_buffer_tensor(tensor)          # preferred buffer-resource view
tA  = fx.make_view(fx.get_iter(buf), fx.make_layout((M, N), (N, 1)))
# load/store tA via copy atoms (fx.copy_atom_call); raw buffer_ops is legacy

Older code may use gpu.thread_idx.x, gpu.block_idx.x, arith.constant(...), T.i32, raw vector.* helpers, the ArithValue wrapper, and buffer_ops. Keep those when editing existing code that already uses them heavily, but prefer gpu.thread_id/block_id, fx.Int64/fx.Int32/fx.Float32, fx.Vector, and fx.rocdl.make_buffer_tensor for new code. ArithValue, fx.Index, and buffer_ops are deprecated/legacy — for migrating an existing kernel see the flydsl-kernel-code-cleanup skill.

Parameter Types

Type	Description	At host boundary
fx.Tensor	GPU tensor (memref)	Auto-converted from torch.Tensor via DLPack
fx.Constexpr[int]	Compile-time constant	Different values -> different compiled kernels
fx.Int32	Runtime i32	Auto-converted from Python int
fx.Stream	CUDA/HIP stream	fx.Stream(None) for default stream

Thread/Block Hierarchy

from flydsl.expr import gpu

tid_x = gpu.thread_id("x")   # Preferred current spelling
bid_x = gpu.block_id("x")
bid_y = gpu.block_id("y")

# Legacy spelling still appears in older kernels:
tid_x = gpu.thread_idx.x
bid_x = gpu.block_idx.x

gpu.barrier()                # Workgroup synchronization

Control Flow

from flydsl.expr import range_constexpr

# Compile-time unrolled loop (emitted inline in IR)
for i in range_constexpr(N):
    ...

# Runtime loop (lowered by AST rewriting)
for i in range(runtime_value):
    ...

Runtime vs Compile-Time Conditions (Current Style)

Use Python/DSL operators for runtime SSA comparisons. The AST rewriter lowers dynamic if conditions to scf.IfOp, and comparison operators like ==, <, >= generate the needed MLIR predicates.

tid = gpu.thread_id("x")
lane = tid % fx.Int64(64)
c_zero = fx.Int64(0)
c_limit = fx.Int64(8)

# Preferred: readable DSL comparisons
if lane == c_zero:
    ...

in_range = lane < c_limit
val = in_range.select(good_val, zero_val)

# Avoid for simple integer comparisons
in_range = arith.cmpi(arith.CmpIPredicate.slt, lane, c_limit)

Use const_expr(...) only for values known at trace/compile time, such as Python booleans, constexpr arguments, loop-unroll choices, or type/layout branches:

if const_expr(trans_v):
    ...

if const_expr(max_context_partition_num <= WARP_SIZE):
    ...

Do not wrap GPU runtime values in const_expr. Even with @flyc.kernel(known_block_size=(256, 1, 1)), gpu.thread_id("x"), lane, and warp_id are runtime SSA values; the compiler knows their range, not the current lane instance.

# Wrong: lane depends on gpu.thread_id("x")
if const_expr(lane == c_zero):
    ...

# Correct
if lane == c_zero:
    ...

Frontend Semantic Restrictions

When writing or reviewing @flyc.kernel / @flyc.jit code, proactively avoid these patterns because they can conflict with MLIR construction even if they look valid in plain Python.

1. Do not define values inside if/else and use them later outside the branch. Keep a single explicit definition path.

   if cond:
       dst = a
   else:
       dst = b
   use(dst)  # avoid this pattern
   

2. Do not mutate captured outer variables inside nested helper functions. Read-only closure capture is acceptable, but writes should go through explicit parameters and return values.

   def kernel():
       acc = fx.Float32(0.0)
   
       def helper(acc):
           acc = acc + fx.Float32(1.0)
           return acc
   
       acc = helper(acc)
   

3. Avoid early return, and do not place return / yield inside if/else branches. Prefer a single explicit exit so the frontend can determine result types.

   if cond:
       out = v0
   else:
       out = v1
   return out
   

4. Compile-time conditions must use const_expr(...). Use if const_expr(flag): ... for constexpr flags and other static decisions. A plain Python if is only safe when the condition is already a Python bool.

5. Runtime branches inside helper functions should be dispatched via local @flyc.jit. When a branch body has side effects, loop-carried values, or branch-local definitions, split the branch bodies into local helpers and wrap the if in a local JIT helper:

   def then_path():
       ...
   
   def else_path():
       ...
   
   @flyc.jit
   def dispatch():
       if runtime_cond:
           then_path()
       else:
           else_path()
   
   dispatch()
   

Runtime Loops with Loop-Carried Values (Software Pipelining)

Use init= on range() to create a runtime loop with explicit SSA phi nodes for loop-carried state. This is required for software pipelining (prefetch patterns) where data must flow across iterations.

Pattern (from preshuffle_gemm.py):

# Prologue: load first tile
tile_0 = prefetch(0)
init_state = [acc_init, tile_0_flat_val1, tile_0_flat_val2, ...]

# Runtime loop with loop-carried state
# Use typed DSL integer bounds (fx.Int64) so the AST rewriter does not treat this as a Python unrolled range.
_start = fx.Int64(0)
_stop = fx.Int64(N - 1)
_step = fx.Int64(1)
for iv, state in range(_start, _stop, _step, init=init_state):
    acc_in = state[0]
    tile_in = state[1:]

    next_tile = prefetch(iv + 1)      # load NEXT data
    acc_in = compute(acc_in, tile_in)  # compute CURRENT

    results = yield [acc_in] + next_tile  # carry to next iter

# Epilogue: process last tile from results
acc_final = results[0]
tile_final = results[1:]
compute(acc_final, tile_final)

How it works in MLIR:

Element	Meaning
init=init_state	List of SSA values that seed the runtime loop block arguments for iteration 0
state	The loop-carried block arguments (phi nodes) for THIS iteration
yield [...]	Feeds values back as next iteration's state
results	After loop exits, holds the last yielded values

Three critical pitfalls (all verified by debugging):

1. Loop bounds must be a typed DSL integer such as fx.Int64(...), NOT a plain Python int. A plain int makes the AST rewriter unroll the loop and silently ignore init=. If you write range(0, 15, 1, init=...), the AST rewriter treats the constant bounds as a Python range and unrolls; only plain Python-int bounds are unrolled, so a typed bound still produces a runtime scf.for. Use fx.Int64(0), fx.Int64(15), fx.Int64(1) instead.

2. Prefer internal types, but unwrap at hard boundaries. Most range(..., init=...) uses accept DSL numeric/vector values. If a lower-level helper explicitly expects raw ir.Value, unwrap with v.ir_value() / _raw(v) at that boundary only.

3. Clear SmemPtr._view_cache before epilogue. SmemPtr.get() caches the view it creates. If called inside the runtime loop body, the cached view is defined in the loop scope. Using it in the epilogue (outside the loop) causes an SSA dominance error. Fix:

   # After the runtime loop, before epilogue compute:
   my_smem_ptr._view_cache = None
   

Arithmetic Operations

c42 = fx.Int64(42)                              # typed integer constant (preferred)
c3_14 = fx.Float32(3.14)                        # f32 constant (preferred)
mask = fx.Int32(0xFF)                            # i32 constant (preferred)

# Prefer operators / Numeric methods
result = a + b
result = a * scale
result = cond.select(true_val, false_val)

# Keep direct arith.*FOp only when explicit fastmath flags are required.

Internal Types: Vector and Numeric (PREFERRED)

Use FlyDSL's internal typed system instead of raw MLIR ops. The Vector class wraps vector<NxTy> with operator overloading and type-safe methods.

Vec = fx.Vector

# Wrap raw vector values
acc = Vec(frag_C.load())      # vector<Nxf32> → Vector with * / + operators

# Indexing (replaces vector.extract)
val = acc[idx]                 # returns Float32 scalar

# Bitcast (replaces vector.bitcast)
v_f32 = Vec(raw_vec).bitcast(fx.Float32)  # vector<Nxi32> → vector<Nxf32>

# Type conversion (replaces arith.trunc_f / arith.ext_f)
bf16_val = f32_val.to(fx.BFloat16)     # f32 → bf16

# Arithmetic — use Python operators, not arith.mulf/addf
result = (val * scale_a) * scale_b

# Splat constant vector
zeros = Vec.filled(N, 0.0, fx.Float32)

# Index cast — use fx.Int32 instead of arith.index_cast
idx = fx.Int32(gpu.block_id("x") * tile_m)

Prefer internal types over raw ops:

Raw MLIR op	Internal type equivalent
vector.extract(v, static_position=[i], ...)	Vec(v)[i]
vector.bitcast(target_ty, v)	Vec(v).bitcast(Float32)
arith.trunc_f(ty, v)	v.to(BFloat16)
arith.mulf(a, b)	a * b
arith.addf(a, b)	a + b
arith.index_cast(T.i32, v)	fx.Int32(v)

Use Vec.filled(...) for splats and Vec.from_elements(...) for vectors from scalars.

Arith Ops Availability Table

Operation	Function	Works on Vectors	Notes
Add	a + b	Yes	Use direct FOp only for explicit fastmath
Multiply	a * b	Yes	Use direct FOp only for explicit fastmath
Negate	-a	Yes
Max	a.maximumf(b)	Yes	Good for ReLU
Compare	a < b, a == b, a > b	Yes	Boolean result (i1 / vec)
Select	cond.select(t, f)	Yes
Abs	no direct helper	Use -v, comparison, and cond.select(...)
FMA	a * b + c	Yes	Use direct FOp only when explicit fastmath is needed
Splat const	Vec.filled(width, val, dtype)	Creates vector	For scalar broadcast

Printf Debugging

fx.printf("tid={} bid={} val={}", tid, bid, value)

---

4. Data Movement Patterns

Layout-Based Copy (Preferred for Element-wise Kernels)

The standard pattern: divide tensor by tile size, slice by block/thread, copy via atoms.

@flyc.kernel
def my_kernel(A: fx.Tensor, B: fx.Tensor, BLOCK_DIM: fx.Constexpr[int]):
    bid = fx.block_idx.x
    tid = fx.thread_idx.x

    # 1. Divide tensor into blocks
    tA = fx.logical_divide(A, fx.make_layout(BLOCK_DIM, 1))
    tB = fx.logical_divide(B, fx.make_layout(BLOCK_DIM, 1))

    # 2. Select this block's tile
    tA = fx.slice(tA, (None, bid))
    tB = fx.slice(tB, (None, bid))

    # 3. Further divide for per-thread access
    tA = fx.logical_divide(tA, fx.make_layout(1, 1))  # 1 element per thread
    tB = fx.logical_divide(tB, fx.make_layout(1, 1))

    # 4. Allocate registers
    copyAtom = fx.make_copy_atom(fx.UniversalCopy32b(), fx.Float32)
    rA = fx.make_rmem_tensor(1, fx.Float32)

    # 5. Copy: global -> register -> compute -> global
    fx.copy_atom_call(copyAtom, fx.slice(tA, (None, tid)), rA)
    # ... compute on register values ...
    fx.copy_atom_call(copyAtom, rA, fx.slice(tB, (None, tid)))

Vectorized Loads (Wide Copies)

VEC_WIDTH = 4
copy_bits = VEC_WIDTH * 32   # 128 bits
copyAtom = fx.make_copy_atom(fx.UniversalCopy(copy_bits), fx.Float32)

rA = fx.make_rmem_tensor(VEC_WIDTH, fx.Float32)

# Divide for VEC_WIDTH elements per thread
tA = fx.logical_divide(tA, fx.make_layout(VEC_WIDTH, 1))
fx.copy_atom_call(copyAtom, fx.slice(tA, (None, tid)), rA)

# Load/store as vectors
vec = fx.memref_load_vec(rA)     # Load vector from register memref
fx.memref_store_vec(vec, rA)     # Store vector to register memref

TiledCopy Abstraction (for 2D Copies)

# Define thread and value layouts
thr_layout = fx.make_layout((4, 1), (1, 1))    # 4 threads
val_layout = fx.make_layout((1, 8), (1, 1))    # 8 values per thread

# Create copy atom
copy_atom = fx.make_copy_atom(fx.rocdl.BufferCopy128b(), fx.Float32)

# Build tiled copy with raked product layout
layout_thr_val = fx.raked_product(thr_layout, val_layout)
tile_mn = fx.make_tile(4, 8)
tiled_copy = fx.make_tiled_copy(copy_atom, layout_thr_val, tile_mn)

# Get this thread's slice and partition
thr_copy = tiled_copy.get_slice(tid)
partition_src = thr_copy.partition_S(src_tensor)
partition_dst = thr_copy.partition_D(dst_tensor)
frag = fx.make_fragment_like(partition_src)

# Execute copy: src -> fragment -> dst
fx.copy(copy_atom, partition_src, frag)
fx.copy(copy_atom, frag, partition_dst)

Buffer Load/Store

Preferred: build a buffer-resource view with make_buffer_tensor and move data through copy atoms — the OOB-checked V# descriptor is built for you.

buf = fx.rocdl.make_buffer_tensor(tensor)
tA  = fx.make_view(fx.get_iter(buf), fx.make_layout((M, N), (N, 1)))
copy = fx.make_copy_atom(fx.rocdl.BufferCopy128b(), fx.Float32)
fx.copy_atom_call(copy, fx.slice(tA, (None, tid)), rA)   # after partitioning tA

Legacy raw intrinsics (buffer_ops.create_buffer_resource / buffer_load / buffer_store, offset in elements) remain for un-migrated kernels; see the flydsl-kernel-code-cleanup skill to migrate them.

Copy Atom Types

Type	Bits	Usage
fx.UniversalCopy32b()	32	1x f32 element copy
fx.UniversalCopy(64)	64	2x f32 elements
fx.UniversalCopy(128)	128	4x f32 elements
fx.rocdl.BufferCopy128b()	128	AMD buffer load 4xf32 (CDNA)
fx.rocdl.make_tdm_atom(...)	whole tile	gfx1250 TDM async Global↔LDS DMA (1–5D) — see below

gfx1250 TDM async copy (fx.rocdl.make_tdm_atom): a whole-tile DMA whose descriptor (base pointer, per-dim extent for HW OOB handling, per-dim stride) is carried as atom state. The global operand of copy_atom_call is a shape/direction token only — its layout gives the compile-time N-D tile shape and its address space picks load vs store; its pointer is unused (base comes from state). Needs a raw VA (not make_buffer_tensor).

lds = fx.SharedAllocator().allocate(fx.Array[fx.Float16, M * N]).peek()
lds2d = fx.make_view(lds.ptr, fx.make_layout((M, N), (N, 1)))       # note: lds.ptr
g2d = fx.make_view(fx.get_iter(A), fx.make_layout((M, N), (N, 1)))
atom = fx.rocdl.make_tdm_atom(g2d, [M, N], num_warps=4)            # rank = len(extents), 1–5D
fx.copy_atom_call(atom, g2d, lds2d)                               # Global → LDS
fx.rocdl.tdm_ops.tensor_wait(0)                                  # await async DMA
atom = fx.rocdl.advance_tdm_atom(atom, k_tile * k_stride_bytes)  # K-loop tile bump (imm_offset)

---

5. Shared Memory (LDS)

SharedAllocator Pattern (preferred for new kernels)

Declare the LDS storage as an @fx.struct of fx.Array fields, allocate it inside the kernel with fx.SharedAllocator, then .view() each field as a layout. In the default static=True mode the compiler sizes a per-leaf static LDS global, so launch(smem=...) is left unset.

import flydsl.expr as fx

@fx.struct
class SharedStorage:
    a: fx.Array[fx.Float16, 8192]
    b: fx.Array[fx.Float16, 8192]

@flyc.kernel
def my_kernel(A: fx.Tensor, ...):
    lds = fx.SharedAllocator().allocate(SharedStorage).peek()
    lds_a = lds.a.view(fx.make_layout((64, 128), (128, 1)))
    lds_b = lds.b.view(fx.make_layout((64, 128), (128, 1)))
    # ds_write / ds_read happen through copy atoms or view loads/stores

For the dynamic mode (static=False), the launch wrapper auto-infers smem from SharedAllocator.allocated_bytes when smem=None.

Legacy SmemAllocator

flydsl.utils.smem_allocator.SmemAllocator remains for un-migrated kernels. Its surface is __init__/finalize/get_base plus SmemPtr.get/load/store; prefer SharedAllocator for anything new.

LDS Capacity

Architecture	GPU	LDS per CU
gfx942	MI300X	64 KB
gfx950	MI350	160 KB

---

6. MFMA Integration (Matrix Math)

Available MFMA Instructions

from flydsl.expr import rocdl

# FP16/BF16 MFMA
result = rocdl.mfma_f32_16x16x16_f16(a, b, acc)

# FP8 MFMA
result = rocdl.mfma_f32_16x16x32_fp8(a, b, acc)

# INT8 MFMA
result = rocdl.mfma_i32_16x16x32i8(a, b, acc)

gfx1250 WMMA (wave32)

gfx1250 uses WMMA (not MFMA), M=N=16. Build the atom with rocdl.WMMA (arch dispatched: gfx11 v16 ABI, gfx12 / gfx1250 v8 ABI) and issue it via fx.make_mma_atom + fx.gemm / fx.mma_atom_call:

dtype (A,B → Acc)	K	notes
f32 → f32	4
f16 / bf16 → f32 or same	32
fp8 / bf8 (OCP E4M3FN / E5M2, any mix) → f32 or f16	64, 128
i8 → i32	64	sign_a / sign_b / clamp kwargs
i4 → i32	32	sign_a / sign_b / clamp kwargs

mma = fx.make_mma_atom(rocdl.WMMA(16, 16, 128, fx.Float8E4M3FN))       # fp8 → f32
mma = fx.make_mma_atom(rocdl.WMMA(16, 16, 32, T.i4, T.i32, sign_a=True, sign_b=True, clamp=True))

MX-scaled WMMA — rocdl.WMMAScale(m, n, k, elem_ty_a, ..., block_size=32) for the E8M0 block-scaled f8/f6/f4 format (16x16x128, or 32x16x128 fp4-only). Per-operand scales are atom state; block_size 32 → i32 scale (i64 for 16):

mma = fx.make_mma_atom(rocdl.WMMAScale(16, 16, 128, fx.Float8E4M3FN))
mma = fx.atom_set_value(mma, "scale_a", fx.Int32(scale_a))
mma = fx.atom_set_value(mma, "scale_b", fx.Int32(scale_b))
fx.gemm(mma, frag_C, frag_A, frag_B, frag_C)

GEMM Pattern (Preshuffle)

The preshuffle GEMM pattern in kernels/gemm/preshuffle_gemm.py:

1. B matrix is pre-shuffled to layout: (N/16, K/64, 4, 16, kpack_bytes)
2. A tiles loaded from global to LDS with XOR16 swizzle for bank-conflict avoidance
3. K64-byte micro-steps: each step issues 2x K32 MFMA operations
4. Ping-pong LDS (lds_stage=2) for overlapping loads with compute
5. Epilogue: either direct row-major store or CShuffle via LDS for packing

---

7. Reduction Patterns

Warp Reduction (AMD wave64)

XOR-shuffle-based intra-wave reduction:

val = fx.Float32(val)                                        # typed once, not per-iter
for sh in [32, 16, 8, 4, 2, 1]:
    peer = val.shuffle_xor(fx.Int32(sh), fx.Int32(64))       # typed numeric method
    val = val + peer                                         # or val.addf(peer, fastmath=...) for fastmath

Block Reduction

1. Intra-wave XOR shuffle (shifts: 32, 16, 8, 4, 2, 1)
2. Lane 0 writes per-wave partial to LDS
3. gpu.barrier()
4. Wave 0 reads and reduces NUM_WAVES partials from LDS

See the norm kernels in kernels/norm/ (e.g. rmsnorm_kernel.py, which defines local wave_reduce_add / block_reduce_add closures) for worked reductions.

---

8. Common Patterns and Recipes

Element-wise Kernel Template

@flyc.kernel
def elementwise_kernel(In: fx.Tensor, Out: fx.Tensor, BLOCK: fx.Constexpr[int], VEC: fx.Constexpr[int]):
    bid, tid = fx.block_idx.x, fx.thread_idx.x
    tile = BLOCK * VEC
    tIn = fx.logical_divide(In, fx.make_layout(tile, 1))
    tOut = fx.logical_divide(Out, fx.make_layout(tile, 1))
    tIn = fx.slice(tIn, (None, bid))
    tOut = fx.slice(tOut, (None, bid))
    tIn = fx.logical_divide(tIn, fx.make_layout(VEC, 1))
    tOut = fx.logical_divide(tOut, fx.make_layout(VEC, 1))
    copy = fx.make_copy_atom(fx.UniversalCopy(VEC * 32), fx.Float32)
    rIn = fx.make_rmem_tensor(VEC, fx.Float32)
    rOut = fx.make_rmem_tensor(VEC, fx.Float32)
    fx.copy_atom_call(copy, fx.slice(tIn, (None, tid)), rIn)
    # Transform
v = Vec(fx.memref_load_vec(rIn))
v = v * v  # example: square
    fx.memref_store_vec(v, rOut)
    fx.copy_atom_call(copy, rOut, fx.slice(tOut, (None, tid)))

Element-wise Kernel Cookbook (GPU-Verified)

All recipes below follow the same vectorized copy_atom pattern (256 threads, vec_width=4, 128-bit loads). Only the compute section between memref_load_vec and memref_store_vec differs.

# --- Scale: C = A * scalar ---
vA = Vec(fx.memref_load_vec(rA))
scale = Vec.filled(vec_width, 2.0, fx.Float32)
vC = vA * scale

# --- Multiply: C = A * B ---
vC = Vec(fx.memref_load_vec(rA)) * Vec(fx.memref_load_vec(rB))

# --- FMA: D = A * B + C ---
vAB = Vec(fx.memref_load_vec(rA)) * Vec(fx.memref_load_vec(rB))
vD = vAB + Vec(fx.memref_load_vec(rC))

# --- ReLU: C = max(A, 0) ---
vA = Vec(fx.memref_load_vec(rA))
zero_vec = Vec.filled(vec_width, 0.0, fx.Float32)
vC = vA.maximumf(zero_vec)

# --- Abs: C = |A| (arith.absf does NOT exist) ---
vA = fx.memref_load_vec(rA)
zero_vec = Vec.filled(vec_width, 0.0, fx.Float32)
neg_vA = -vA
is_neg = vA < zero_vec
vC = is_neg.select(neg_vA, vA)

Naive GEMM Template (for understanding, not performance)

@flyc.kernel
def naive_gemm(A: fx.Tensor, B: fx.Tensor, C: fx.Tensor,
               M: fx.Constexpr[int], N: fx.Constexpr[int], K: fx.Constexpr[int],
               BM: fx.Constexpr[int], BN: fx.Constexpr[int]):
    tid, bid = gpu.thread_id("x"), gpu.block_id("x")
    bm, bn = bid // (N // BN), bid % (N // BN)
    tm, tn = tid // BN, tid % BN
    row, col = bm * BM + tm, bn * BN + tn

    # Buffer-resource views with logical row-major layouts
    tA = fx.make_view(fx.get_iter(fx.rocdl.make_buffer_tensor(A)), fx.make_layout((M, K), (K, 1)))
    tB = fx.make_view(fx.get_iter(fx.rocdl.make_buffer_tensor(B)), fx.make_layout((K, N), (N, 1)))
    tC = fx.make_view(fx.get_iter(fx.rocdl.make_buffer_tensor(C)), fx.make_layout((M, N), (N, 1)))

    copy = fx.make_copy_atom(fx.UniversalCopy32b(), fx.Float32)
    rA, rB, rC = (fx.make_rmem_tensor(1, fx.Float32) for _ in range(3))

    acc = fx.Float32(0.0)
    for k in range_constexpr(K):
        fx.copy(copy, fx.slice(tA, (row, k)), rA)
        fx.copy(copy, fx.slice(tB, (k, col)), rB)
        acc = acc + fx.Vector(fx.memref_load_vec(rA))[0] * fx.Vector(fx.memref_load_vec(rB))[0]

    fx.memref_store_vec(fx.Vector.from_elements([acc], fx.Float32), rC)
    fx.copy(copy, rC, fx.slice(tC, (row, col)))

---

9. Environment and Debugging

IR Dump

FLYDSL_DUMP_IR=1 FLYDSL_DUMP_DIR=./dumps python my_kernel.py

Produces numbered .mlir files per pipeline stage plus final_isa.s.

Key Environment Variables

Variable	Default	Description
FLYDSL_DUMP_IR	false	Dump IR at each stage
FLYDSL_DEBUG_ENABLE_DEBUG_INFO	false	Emit DWARF debug info (source-to-asm mapping)
FLYDSL_RUNTIME_ENABLE_CACHE	true	Enable kernel disk caching (in-memory cache is always active)
FLYDSL_RUNTIME_CACHE_DIR	~/.flydsl/cache	Cache directory
FLYDSL_COMPILE_OPT_LEVEL	2	Optimization level (0-3)
ARCH	auto-detect	Override GPU architecture

Disk Cache Invalidation

The JIT disk cache auto-invalidates when kernel source or closure values change. Set FLYDSL_RUNTIME_ENABLE_CACHE=0 only when modifying C++ passes or non-closure helper functions:

FLYDSL_RUNTIME_ENABLE_CACHE=0 python my_kernel.py  # or: rm -rf ~/.flydsl/cache

Source-to-Assembly Debug Info

FlyDSL supports source-to-assembly mapping for rocprofv3 ATT traces via the MLIR ensure-debug-info-scope-on-llvm-func pass (equivalent to Triton's add_di_scope).

How it works:

1. FlyDSL's FuncLocationTracker generates MLIR loc() metadata pointing to Python source lines
2. The ensure-debug-info-scope-on-llvm-func{emission-kind=LineTablesOnly} pass converts MLIR locations into LLVM DISubprogramAttr / DICompileUnitAttr metadata
3. The -g flag in gpu-module-to-binary preserves this metadata as .debug_line in the HSACO binary
4. rocprofv3 ATT reads .debug_line to produce code.json with "source_file:line" entries

Pipeline position: After reconcile-unrealized-casts, before gpu-module-to-binary:

... -> reconcile-unrealized-casts
    -> ensure-debug-info-scope-on-llvm-func{emission-kind=LineTablesOnly}  (conditional on enable_debug_info)
    -> gpu-module-to-binary{format=fatbin opts=-g}

Verification: With FLYDSL_DUMP_IR=1, check final_isa.s for .file and .loc directives. The PA decode kernel achieves 99.9% coverage (1109/1110 ISA instructions mapped to source).

Key insight: Without this pass, MLIR loc() metadata is silently dropped during MLIR-to-LLVM-IR translation. The -g flag alone is useless — it preserves debug info, but there's none to preserve without the DI scope pass.

Autotune Module

FlyDSL includes a Triton-style autotune module at /FlyDSL/python/flydsl/autotune.py:

from flydsl.autotune import autotune, Config, do_bench

@autotune(
    configs=[
        Config(block_dim=64, vec_width=4),
        Config(block_dim=128, vec_width=4),
        Config(block_dim=256, vec_width=4),
    ],
    key=['const_n'],     # re-tune when these arg values change
    warmup=5, rep=25,    # benchmark timing params
)
@flyc.jit
def myKernel(A, C, n: fx.Int32, const_n: fx.Constexpr[int],
             block_dim: fx.Constexpr[int], vec_width: fx.Constexpr[int],
             stream: fx.Stream = fx.Stream(None)):
    ...

• Config kwargs become Constexpr args injected into @jit call
• Config.num_warps, waves_per_eu, maxnreg are special compiler-level options
• First call benchmarks all configs; subsequent calls use cached best
• Disk cache at ~/.flydsl/autotune/{func_name}.json
• do_bench(fn, warmup=5, rep=25) benchmarks using CUDA/HIP events, returns median ms

IMPORTANT: waves_per_eu does NOT work via gpu-module-to-binary opts=. It needs to be set as an LLVM function attribute or through rocdl-attach-target. This is a known limitation.

DLTensorAdaptor bug: Do NOT use flyc.from_dlpack() with pre-wrapped tensors when calling a @jit function with varying Constexpr values. The DLTensorAdaptor caches MLIR types from the first ir.Context, which become invalid when a new context is created (causes segfault). Pass raw torch.Tensor objects instead.

---

10. Troubleshooting

Common Issues

1. Constants/casts: Prefer fx.Int32(...), fx.Int64(...), and fx.Float32(...) (fx.Index(...) is being deprecated in favor of fx.Int64(...)). Use arith.constant(...) only at low-level boundaries.

2. buffer_ops.buffer_load offset: The offset parameter is in ELEMENTS, not bytes.

3. Cache stale after code changes: The disk cache auto-invalidates on source/closure changes. Only set FLYDSL_RUNTIME_ENABLE_CACHE=0 or clear ~/.flydsl/cache/ if you changed C++ passes or non-closure helpers.

4. LDS overflow: Check capacity (64KB on gfx942, 160KB on gfx950). SharedAllocator sizes the LDS global for you; the compiler errors if a static allocation exceeds the arch limit.

5. Dynamic vs Constexpr: Constexpr[int] values are baked into IR -- different values produce different compiled kernels. Use Int32 for truly dynamic values.

6. Tensor layout marking: For dynamic shapes or alignment, use flyc.from_dlpack(tensor).mark_layout_dynamic(leading_dim=0, divisibility=4).

7. Legacy SmemAllocator finalize: only for the legacy SmemAllocator path — call allocator.finalize() inside the GPU module body (CompilationContext.get_current().gpu_module_body). SharedAllocator needs no finalize step.

8. AMD wavefront size: Always 64 on gfx9xx. Use shifts [32, 16, 8, 4, 2, 1] for full-wave reduction.

9. tile_k alignment for GEMM: tile_k * elem_bytes must be divisible by 64 (K64-byte micro-step).

10. INT4 (W4A8): A matrix is int8, B matrix is packed int4 (2 values/byte), unpacked to int8 in-kernel.

11. arith.absf does not exist: Prefer fx.Vector / typed fx operators (not the deprecated ArithValue): neg = -v, is_neg = v < zero, out = is_neg.select(neg, v).

12. Scalar broadcast to vector: Use Vec.filled(width, value, fx.Float32) to create a splat constant vector. Do NOT use raw vector ops for ordinary arithmetic.

---

11. Comparison with Triton/Gluon

Aspect	FlyDSL	Triton	Gluon
Layout control	Explicit layout algebra (Shape, Stride, Layout)	Implicit via block pointers	Implicit
Tiling	Manual via divide/product operations	Auto-tiling with tl.program_id	Auto-tiling
Memory access	Copy atoms, buffer load/store, TiledCopy	tl.load/tl.store	gluon.load/gluon.store
MFMA	Direct rocdl.mfma_* intrinsics	tl.dot	gluon.dot
Shared memory	SharedAllocator + @fx.struct explicit management	Implicit scratchpad	Implicit
Abstraction level	Low (near hardware)	Medium	Medium-High
Compilation	MLIR (Fly dialect -> LLVM -> HSACO)	MLIR (Triton dialect -> LLVM)	MLIR
Control	Maximum control over data layout and movement	Less control, more automation	Least control

FlyDSL gives maximum control at the cost of verbosity. The layout algebra is the key differentiator -- it enables precise control over how data is arranged in registers, shared memory, and global memory, and how threads map to data.

---

12. Running Kernels

SSH to Remote Host

# Run a kernel
ssh -o LogLevel=ERROR hjbog-srdc-39.amd.com 'docker exec hungry_dijkstra bash -c "cd /FlyDSL && python3 my_kernel.py"'

# Run existing tests
ssh -o LogLevel=ERROR hjbog-srdc-39.amd.com 'docker exec hungry_dijkstra bash -c "cd /FlyDSL && python3 tests/kernels/test_vec_add.py"'

# Run benchmarks
ssh -o LogLevel=ERROR hjbog-srdc-39.amd.com 'docker exec hungry_dijkstra bash -c "cd /FlyDSL && bash scripts/run_benchmark.sh"'