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fix(gdn_decode): widen pool indices to Int64 to prevent int32 element-offset overflow #3230

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Merged
kahyunnam merged 1 commit into from
May 11, 2026
Merged

fix(gdn_decode): widen pool indices to Int64 to prevent int32 element-offset overflow #3230

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64 changes: 54 additions & 10 deletions flashinfer/gdn_kernels/gdn_decode_bf16_state.py
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Original file line number Diff line number Diff line change
Expand Up @@ -163,15 +163,28 @@ def gdn_decode_bf16state_mtp_ilp4_kernel(
i_n = tmp // HV
i_h = i_hv // (HV // H)

cache_idx = h0_indices[i_n]
# Widen pool indices to Int64 BEFORE they multiply ``stride[0]`` of
# ``h0_source``. The reshape ``[pool_size, HV, V, K] -> [pool_size * HV,
# V, K]`` (BF16) gives ``stride[0] = V * K = 16384`` elements, so the
# downstream offset ``(cache_idx * HV + i_hv) * 16384`` reaches 2**31 at
# ``cache_idx >= ceil(2**31 / (HV * V * K)) = 4096`` (HV=32, V=K=128;
# 2048 at HV=64). Past that boundary the Int32 multiplication wraps to
# a negative offset and ``cute.local_tile(h0_source, ...)`` issues a
# load/store to an unmapped global address. Propagating Int64 through
# ``flat_state_idx`` / ``flat_write_state_idx`` (computed below) keeps
# the offset multiplication 64-bit. See
# ``tests/gdn/test_decode_pretranspose_noncontiguous_pool.py
# ::test_decode_pretranspose_pool_int64_offset_bf16`` for the
# regression test.
cache_idx = cutlass.Int64(h0_indices[i_n])
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if cutlass.const_expr(same_pool):
# Single-pool: alias write to read; nvcc DCEs the write-side LDG /
# IMAD / local_tile entirely in this compile path.
write_cache_idx = cache_idx
else:
write_cache_idx = h0_out_indices[i_n]
write_cache_idx = cutlass.Int64(h0_out_indices[i_n])
if write_cache_idx < 0:
write_cache_idx = cutlass.Int32(0)
write_cache_idx = cutlass.Int64(0)

r_A_log = cutlass.Float32(A_log[i_hv])
r_dt_bias = cutlass.Float32(dt_bias[i_hv])
Expand Down Expand Up @@ -225,7 +238,7 @@ def gdn_decode_bf16state_mtp_ilp4_kernel(
)

if cache_idx < 0:
cache_idx = cutlass.Int32(0)
cache_idx = cutlass.Int64(0)

if cache_idx >= 0:
k_start = lane_in_group * vec_size
Expand Down Expand Up @@ -652,7 +665,16 @@ def gdn_decode_bf16state_mtp_ilp4_kernel(
# initial_state_indices points at slots >= B (i.e. any
# realistic pool_size > B serving config). Fix mirrors
# upstream PR #3145.
flat_idx = i_n * T * HV + i_t * HV + i_hv
# Defense-in-depth: widen to Int64 so the offset
# ``flat_idx * stride[0]`` (= ``flat_idx * V * K``
# = ``flat_idx * 16384`` BF16 elements) into the
# batch-scoped intermediate-states cache cannot
# wrap. This kernel is only reached at
# ``B * HV <= 128`` so the flat_idx itself stays
# well below the wrap threshold, but matching the
# wide_vec kernel below keeps the two paths
# bit-equivalent at large pool sizes.
flat_idx = cutlass.Int64(i_n) * T * HV + i_t * HV + i_hv
ita = cute.local_tile(
intermediate_states,
(1, 1, vec_size),
Expand Down Expand Up @@ -780,7 +802,18 @@ def gdn_wide_vec_kernel(
i_n = tmp // HV
i_h = i_hv // (HV // H)

cache_idx = h0_indices[i_n]
# Widen pool indices to Int64 BEFORE they multiply ``stride[0]`` of
# ``h0_source``. ``h0_source`` is reshaped to ``[pool_size * HV, V,
# K]`` (BF16), so ``stride[0] = V * K = 16384`` elements; the
# downstream offset ``(cache_idx * HV + i_hv) * 16384`` wraps int32
# at ``cache_idx >= ceil(2**31 / (HV * V * K)) = 4096`` (HV=32) /
# 2048 (HV=64). Propagating Int64 through ``flat_state_idx`` /
# ``flat_write_state_idx`` keeps the ``cute.local_tile`` offset
# arithmetic 64-bit at every reachable pool size. See
# ``tests/gdn/test_decode_pretranspose_noncontiguous_pool.py
# ::test_decode_pretranspose_pool_int64_offset_bf16`` for the
# regression test.
cache_idx = cutlass.Int64(h0_indices[i_n])

r_A_log = cutlass.Float32(A_log[i_hv])
r_dt_bias = cutlass.Float32(dt_bias[i_hv])
Expand Down Expand Up @@ -824,7 +857,7 @@ def gdn_wide_vec_kernel(
)

if cache_idx < 0:
cache_idx = cutlass.Int32(0)
cache_idx = cutlass.Int64(0)

# Split-pool write index: distinct slot to write the updated H state.
# When same_pool=True (compile-time, set by the dispatcher whenever the
Expand All @@ -835,9 +868,9 @@ def gdn_wide_vec_kernel(
if cutlass.const_expr(same_pool):
write_cache_idx = cache_idx
else:
write_cache_idx = h0_out_indices[i_n]
write_cache_idx = cutlass.Int64(h0_out_indices[i_n])
if write_cache_idx < 0:
write_cache_idx = cutlass.Int32(0)
write_cache_idx = cutlass.Int64(0)

if cache_idx >= 0:
flat_state_idx = cache_idx * HV + i_hv
Expand Down Expand Up @@ -1169,7 +1202,18 @@ def gdn_wide_vec_kernel(
# initial_state_indices points at slots >= B (i.e. any
# realistic pool_size > B serving config). Fix mirrors
# upstream PR #3145.
flat_idx = i_n * T * HV + i_t * HV + i_hv
# Widen to Int64: ``intermediate_states`` is
# reshaped to ``[B * T * HV, V, K]`` (BF16) with
# ``stride[0] = V * K = 16384`` elements. The
# offset ``flat_idx * 16384`` reaches 2**31 at
# ``flat_idx >= 131072``; with HV=64 + T=8 that's
# already hit at ``i_n >= 256`` (i.e. any
# production-scale MTP decode batch with caching
# enabled). Without the widening the Int32
# multiplication wraps and the
# ``cute.local_tile(intermediate_states, ...)``
# writes corrupt unrelated GMEM.
flat_idx = cutlass.Int64(i_n) * T * HV + i_t * HV + i_hv
it0 = cute.local_tile(
intermediate_states,
(1, 1, vec),
Expand Down
27 changes: 21 additions & 6 deletions flashinfer/gdn_kernels/gdn_decode_pretranspose.py
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Original file line number Diff line number Diff line change
Expand Up @@ -134,11 +134,20 @@ def gdn_decode_kernel_small_batch_pretranspose(

# Compute state index: use pool indexing if enabled.
if cutlass.const_expr(use_pool_indexing):
pool_idx = h0_indices[i_n]
out_pool_idx = h0_out_indices[i_n]
# Widen pool indices to Int64 BEFORE they multiply ``stride[0]`` of
# ``h0_source``. With Int32 indices, the per-slot element offset
# ``pool_idx * stride[0]`` silently wraps once it exceeds INT32_MAX,
# which makes the kernel issue loads/stores to unmapped global
# addresses (illegal memory access). For example, the padded slot
# stride 540672 used by vLLM for Qwen3.5-class GDN models crosses the
# threshold at pool_idx >= ceil(2**31 / 540672) = 3972. See
# ``tests/gdn/test_decode_pretranspose_noncontiguous_pool.py::
# test_decode_pretranspose_pool_int64_offset`` for the regression test.
pool_idx = cutlass.Int64(h0_indices[i_n])
out_pool_idx = cutlass.Int64(h0_out_indices[i_n])
# Redirect negative write indices to null buffer (slot 0)
if out_pool_idx < 0:
out_pool_idx = cutlass.Int32(0)
out_pool_idx = cutlass.Int64(0)
else:
pool_idx = 0
out_pool_idx = 0
Expand Down Expand Up @@ -442,11 +451,17 @@ def gdn_decode_kernel_big_batch_pretranspose(

# Compute state index: use pool indexing if enabled.
if cutlass.const_expr(use_pool_indexing):
pool_idx = h0_indices[i_n]
out_pool_idx = h0_out_indices[i_n]
# Widen pool indices to Int64 BEFORE they multiply ``stride[0]`` of
# ``h0_source``. With Int32 indices, the per-slot element offset
# ``pool_idx * stride[0]`` silently wraps when it exceeds 2**31, which
# makes the kernel issue loads/stores to unmapped global addresses
# (illegal memory access). See the small-batch kernel above for the
# full rationale.
pool_idx = cutlass.Int64(h0_indices[i_n])
out_pool_idx = cutlass.Int64(h0_out_indices[i_n])
# Redirect negative write indices to null buffer (slot 0)
if out_pool_idx < 0:
out_pool_idx = cutlass.Int32(0)
out_pool_idx = cutlass.Int64(0)
else:
pool_idx = 0
out_pool_idx = 0
Expand Down
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