[BUG]: tileiras SIGSEGV when occupancy=2 is requested for a 32-wide fused tile kernel

[robobryce]

cuTile Python version

1.5.0. The same reproducer also fails with 1.4.0.

CUDA Toolkit version

13.3 (tileiras V13.3.36)

Which installation method does this occur on?

Pip

Describe the bug

tileiras terminates with SIGSEGV when occupancy=2 is requested for the
kernel below. Automatic occupancy and occupancy=1 compile. The crash happens
during compilation, before the intentionally small input can execute.

I expected the occupancy request either to compile, to be treated as a hint that
cannot be met, or to produce a clear resource diagnostic. A scheduling/resource
request should not terminate the native compiler.

This was reduced from two Cholesky _left_superpanel failures. Their source
inputs were float32[1,4096,4096]; the compiler-only reproducer needs one
float32[1,4,4] tensor while preserving the 32x32, 32x64, and 64x64 compile-time
tiles.

Minimum reproducible example

import torch
import cuda.tile as ct


ConstInt = ct.Constant[int]
ZERO = ct.PaddingMode.ZERO


def factor(a, block: ConstInt):
    cols = ct.arange(block, dtype=ct.int32)[None, :]
    for p in range(block):
        pivot = ct.extract(a, (p, p), shape=(1, 1))
        column = ct.extract(a, (0, p), shape=(block, 1)) / pivot
        outer = column * column.transpose(0, 1)
        a = ct.where(cols > p, a - outer, a)
    return a


def solve(panel, diagonal, block: ConstInt):
    cols = ct.arange(block, dtype=ct.int32)[None, :]
    for p in range(block):
        pivot = ct.extract(diagonal, (p, p), shape=(1, 1))
        solved = ct.extract(panel, (0, p), shape=(block, 1)) / pivot
        column = ct.extract(diagonal, (0, p), shape=(block, 1))
        panel = ct.where(
            cols > p,
            panel - solved * column.transpose(0, 1),
            panel,
        )
    return panel


@ct.kernel(opt_level=2, occupancy=2)
def kernel(a, step, block: ConstInt):
    rows = block
    width = 2 * block
    next_step = step + 1

    work = ct.load(a, (0, 0, 0), shape=(1, rows, width), padding_mode=ZERO)
    work = work.reshape((rows, width))
    cross = ct.load(a, (0, 1, 0), shape=(1, block, block), padding_mode=ZERO)
    cross = cross.reshape((block, block))
    diagonal = ct.load(a, (0, 1, 1), shape=(1, block, block), padding_mode=ZERO)
    diagonal = diagonal.reshape((block, block))

    for prior in range(step):
        left = ct.load(a, (0, 0, prior), shape=(1, rows, width), padding_mode=ZERO)
        right = ct.load(a, (0, 0, prior), shape=(1, width, width), padding_mode=ZERO)
        left = left.reshape((rows, width))
        right = right.reshape((width, width))
        work = ct.mma(
            left.astype(ct.tfloat32),
            (-right.transpose(0, 1)).astype(ct.tfloat32),
            work,
        )
        diagonal = ct.mma(
            left.astype(ct.tfloat32),
            (-left.transpose(0, 1)).astype(ct.tfloat32),
            diagonal,
        )

    first_panel = ct.extract(work, (0, 0), shape=(rows, block))
    second_panel = ct.mma(
        first_panel.astype(ct.tfloat32),
        (-cross.transpose(0, 1)).astype(ct.tfloat32),
        first_panel,
    )
    second_panel = solve(second_panel, diagonal, block)

    ct.store(a, (0, 0, 0), first_panel.reshape((1, rows, block)))
    if ct.bid(1) == 0:
        ct.store(a, (0, 1, 1), diagonal.reshape((1, block, block)))

    if ct.bid(1) == next_step:
        next_diagonal = ct.load(
            a,
            (0, next_step, next_step),
            shape=(1, block, block),
            padding_mode=ZERO,
        ).reshape((block, block))
        next_diagonal = ct.mma(
            second_panel.astype(ct.tfloat32),
            (-second_panel.transpose(0, 1)).astype(ct.tfloat32),
            next_diagonal,
        )
        next_diagonal = factor(next_diagonal, block)
        ct.store(
            a,
            (0, next_step, next_step),
            next_diagonal.reshape((1, block, block)),
        )


a = torch.empty((1, 4, 4), device="cuda", dtype=torch.float32)
ct.launch(torch.cuda.current_stream(), (1, 1), kernel, (a, 0, 32))

Run it in a fresh process and compiler cache. Crash dumps are disabled only to
avoid the separate masking problem in #92.

run=$(mktemp -d)
CUDA_TILE_CACHE_DIR=off \
CUDA_TILE_TEMP_DIR="$run" \
CUDA_TILE_ENABLE_CRASH_DUMP=0 \
python repro.py

Relevant log output

subprocess.CalledProcessError: Command '['/usr/local/cuda/bin/tileiras',
  '/tmp/.../kernel....bytecode', '-o',
  '/tmp/.../kernel....cubin', '--gpu-name', 'sm_120',
  '-O2', '--lineinfo']' died with <Signals.SIGSEGV: 11>.

cuda.tile._exception.TileCompilerExecutionError: Return code -11
Unknown location

The corresponding direct tileiras invocation exits 139 and emits no cubin.
Two fresh 1.4.0 processes produced identical failing bytecode; a fresh 1.5.0
process also produced a failing compiler input.

Environment

OS: Ubuntu 22.04.5 LTS, Linux 6.8.0-90-generic x86_64
GPU: NVIDIA RTX PRO 6000 Blackwell Server Edition, compute capability 12.0
Driver: 580.126.09
CUDA toolkit: 13.3; nvcc 13.3.33; tileiras V13.3.36
Python: 3.13.14
PyTorch: 2.12.0+cu130 (bundled CUDA runtime 13.0)
cuTile Python: 1.5.0; also reproduced on 1.4.0
CPU: AMD EPYC 9355, 16 vCPUs

Other details

The two original failing source variants were:

• occupancy=2;
• occupancy=2, num_worker_warps=4.

Both original inputs compiled the same _left_superpanel body and failed at
n=4096. The reduced controls isolate the request:

• automatic occupancy passes;
• occupancy=1 passes;
• occupancy=2, num_worker_warps=4 has the same crash;
• occupancy=2, num_worker_warps=8 passes at this reduced boundary;
• occupancy=2 with block=16 passes;
• shrinking the allocated tensor from 4x4 to 3x3 changes its alignment signature
  and passes.

The original no-worker-warp and four-worker-warp compiler inputs differ only by
worker-warp metadata and both exit 139. This makes the occupancy-two request the
common trigger for this compact configuration.

Contributing Guidelines

• I agree to follow cuTile Python's contributing guidelines
• I searched the open bugs and found no duplicate for this report