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[Pallas] Device Id dict to mesh fastpath for power of twos #33817

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Merged
copybara-service merged 1 commit into from
Dec 9, 2025
Merged

[Pallas] Device Id dict to mesh fastpath for power of twos #33817

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25 changes: 20 additions & 5 deletions jax/_src/pallas/primitives.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1350,11 +1350,26 @@ def _device_id_dict_to_mesh(mesh_context: pallas_utils.MeshInfo, device_id_dict,
)
axes_dimensions = [mesh_axis_sizes[name] for name in axis]
for axis_index, axis_name in enumerate(axis):
axis_size = arith.constant(i32, mesh_axis_sizes[axis_name])
minor_divisor = arith.constant(
i32, math.prod(axes_dimensions[axis_index + 1 :])
)
device_idx = arith.remsi(arith.divsi(idx, minor_divisor), axis_size)
axis_size = mesh_axis_sizes[axis_name]
inner_mesh_size = math.prod(axes_dimensions[axis_index + 1 :])
minor_divisor = arith.constant(i32, inner_mesh_size)

# Fast path for power of 2s
if inner_mesh_size & (inner_mesh_size - 1) == 0:
shift_len = (inner_mesh_size & -inner_mesh_size).bit_length() - 1
partial_device_idx = arith.shrui(idx, arith.constant(i32, shift_len))
else:
partial_device_idx = arith.divsi(idx, minor_divisor)

if axis_size & (axis_size - 1) == 0:
device_idx = arith.andi(
partial_device_idx,
arith.constant(i32, mesh_axis_sizes[axis_name] - 1),
)
else:
device_idx = arith.remsi(
partial_device_idx, arith.constant(i32, axis_size)
)
physical_axis_dict[axis_name] = device_idx
else:
physical_axis_dict[axis] = idx
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