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Speed up nvfp4 pack/unpack w/ torch.compile #400

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Merged
merged 2 commits into from
Aug 25, 2025
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Speed up nvfp4 pack/unpack w/ torch.compile #400

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c2ffbac
Speed up nvfp4 pack/unpack w/ torch.compile
fynnsu Jul 22, 2025
6bb69c1
Add `dynamic=True` to torch.compile call in nvfp4 packing
fynnsu Jul 30, 2025
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9 changes: 5 additions & 4 deletions src/compressed_tensors/compressors/quantized_compressors/nvfp4_quantized.py
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Original file line number Diff line number Diff line change
Expand Up @@ -105,6 +105,7 @@ def decompress_weight(
return decompressed_weight


@torch.compile(fullgraph=True)
def pack_fp4_to_uint8(x: torch.Tensor) -> torch.Tensor:
"""
Packs a tensor with values in the fp4 range into uint8.
Expand All @@ -127,12 +128,11 @@ def pack_fp4_to_uint8(x: torch.Tensor) -> torch.Tensor:

# Find closest valid FP4 value index for each element
abs_x = torch.abs(x)
abs_indices = torch.zeros_like(abs_x, dtype=torch.long)
for i, val in enumerate(kE2M1):
abs_indices = torch.where(torch.isclose(abs_x, val), i, abs_indices)
abs_diff_x = torch.abs(abs_x.unsqueeze(-1) - kE2M1) # [m, n, 8]
abs_indices = torch.argmin(abs_diff_x, dim=-1) # [m, n]

# Apply sign bit (bit 3) to get final 4-bit representation
indices = abs_indices + (torch.signbit(x) << 3).to(torch.long)
indices = abs_indices + (torch.signbit(x).to(torch.long) << 3)

# Reshape to prepare for packing pairs of values
indices = indices.reshape(-1)
Expand All @@ -155,6 +155,7 @@ def pack_fp4_to_uint8(x: torch.Tensor) -> torch.Tensor:
)

# reference: : https://github.com/vllm-project/vllm/pull/16362
@torch.compile(fullgraph=True)
def unpack_fp4_from_uint8(
a: torch.Tensor, m: int, n: int, dtype: Optional[torch.dtype] = torch.bfloat16
) -> torch.Tensor:
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