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Pass input_output_alias to TritonAutotunedKernelCall #2814

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Pass input_output_alias to TritonAutotunedKernelCall #2814

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918df1e
Pass input_output_alias to TritonAutotunedKernelCall
nvjax Mar 31, 2026
7ed5f95
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Mar 31, 2026
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27 changes: 13 additions & 14 deletions transformer_engine/jax/triton_extensions/utils.py
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Original file line number Diff line number Diff line change
Expand Up @@ -474,23 +474,22 @@ def lowering(ctx, x, *, block_size):

kernel_calls.append((config_call, str(config)))

# IMPORTANT: We pass an empty tuple for input_output_aliases_with_sizes.
#
# Background:
# 1. jax.ffi.ffi_lowering(operand_output_aliases=...) is a HINT to XLA that an
# output can reuse an input's buffer. XLA may or may not honor this.
# 2. TritonAutotunedKernelCall's input_output_aliases_with_sizes triggers
# save/restore logic during autotuning (see jaxlib/gpu/triton_kernels.cc:630-701).
#
# The problem: The save phase (triton_kernels.cc:632) only saves if buffers[input_idx] == buffers[output_idx],
# but the restore phase (triton_kernels.cc:697-700) unconditionally iterates over all aliases and tries
# to access input_copies[input_idx]. If XLA didn't actually alias the buffers, input_copies[input_idx] doesn't exist, creating an empty vector whose .data() returns nullptr, causing CUDA_ERROR_INVALID_VALUE during the restore memcpy.
#
# WAR: Don't pass aliases to TritonAutotunedKernelCall.
input_output_aliases_with_sizes = ()
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@jberchtold-nvidia jberchtold-nvidia Mar 31, 2026

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We should guard this on the JAX version corresponding to the version that has the fixed XLA version as users may still be on older JAX/XLA. I think currently that will be a dev nightly release, so we can guard on that for now and then add a TODO(tdophung) to update to an official release in the future when it's included in the next release

if input_output_aliases:
num_inputs = len(ctx.avals_in)
aliases = []
for input_idx, output_idx in input_output_aliases.items():
aval = ctx.avals_in[input_idx]
size_bytes = aval.size * jnp.dtype(aval.dtype).itemsize
# AutotunedKernelCall expects buffer indices (inputs + outputs).
buffer_output_idx = num_inputs + output_idx
aliases.append((input_idx, buffer_output_idx, size_bytes))
input_output_aliases_with_sizes = tuple(aliases)

kernel_call = gpu_triton.TritonAutotunedKernelCall(
f"{actual_kernel_fn.__name__}_autotuned",
kernel_calls,
(), # Empty to avoid buggy save/restore in jaxlib/gpu/triton_kernels.cc
input_output_aliases_with_sizes,
)

else:
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