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Weight the IQ2 encoders by the square root of GPTQ's Hessian diagonal - #2686

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What does this PR do?

Type of change: new feature

This is the second step of the IQ GPTQ + imatrix plan, after #2665: the IQ2 search now weights each input column by how much the layer's outputs depend on it.

Weighted encoders. quantize_iq2_xxs, quantize_iq2_xs and quantize_iq2_s take an optional per-input-column importance w. They then choose codes that minimise Σ w·(|x| − s·q)² instead of the plain squared error, which is how llama.cpp uses an imatrix.

  • The weighting covers the local-scale choice, the grid entry and the even-parity sign flip. The super-block scale predictor stays unweighted.
  • The CUDA kernel gets the weighting through a kWeighted template flag. The unweighted kernel compiles as before.
  • With uniform importance, the payload is identical to the unweighted one.
  • importance must be finite and non-negative, with one entry per input column; anything else raises.
  • The new GGMLFormat.weighted field marks the formats that accept it.

GPTQ feeds them, when the recipe asks. A new GPTQ option, importance_weighted (default false), turns it on. GGMLGPTQHelper then gives each GGML block of a weighted format its slice of sqrt(diag(H)). Here diag(H) = E[x_j²] is llama.cpp's imatrix statistic, floored at 1e-4 of its mean. GPTQ already accumulates H, so this needs no extra calibration pass. The built-in general/ptq/iq2_* presets set the option, so their output is the weighted one. Formats without a weighted search (IQ1_*, NVFP4, INT4) ignore it, so one recipe can set it for mixed formats.

Why the switch is on the algorithm, not the format. The right weight depends on what supplies the statistic. Under GPTQ, later columns absorb part of each block's error, and the square root measured best. A future imatrix-only calibration, with no error compensation, would want the raw E[x²] and would be its own method. The encoder's importance= argument and GGMLFormat.weighted are shared by both.

Why the square root. The raw diagonal is heavy-tailed, so a few outlier columns dominate the search. It made KL divergence worse than no weighting on IQ2_XXS and IQ2_XS (table below). The square root keeps the column ranking with half the log-range, and it beats both on all three formats.

IQ1_S and IQ1_M stay unweighted for now. Their encoders are the next step.

Usage

The built-in IQ2 recipes turn it on:

python examples/hf_ptq/hf_ptq.py --pyt_ckpt_path <model> --recipe general/ptq/iq2_xxs --export_path <dir>

In a custom recipe:

algorithm:
  method: gptq
  block_size: 256
  perc_damp: 0.3
  importance_weighted: true   # weight the IQ2 codebook search by sqrt(diag(H)); other formats ignore it
  layerwise:
    enable: true

Calling the encoder directly, for example with an imatrix from elsewhere:

from modelopt.torch.quantization.ggml.iq2_xxs import quantize_iq2_xxs

packed, shape = quantize_iq2_xxs(weight, importance=imatrix)  # imatrix: [in_features], finite, >= 0

Testing

Qwen3.8-27B, MLP-only IQ2 with GPTQ, using the default general/ptq/iq2_* recipes.

Mean KL divergence for each format and each choice of importance:

Format Unweighted (main) diag(H) sqrt(diag(H)) (this PR) sqrt − unweighted, paired (chunks where sqrt is lower)
IQ2_XXS 0.307 0.319 0.287 −0.0197 ± 0.0025 (80/100)
IQ2_XS 0.214 0.226 0.203 −0.0106 ± 0.0020 (71/100)
IQ2_S 0.159 0.152 0.139 −0.0195 ± 0.0015 (91/100)
Format Perplexity (BF16: 6.746): unweighted / diag(H) / sqrt Same top token: unweighted / diag(H) / sqrt
IQ2_XXS 8.57 / 8.51 / 8.32 77.2 / 75.8 / 77.1 %
IQ2_XS 7.90 / 7.87 / 7.84 81.3 / 80.4 / 81.3 %
IQ2_S 7.74 / 7.56 / 7.50 83.8 / 83.8 / 84.6 %
  • The square root beats the raw diagonal on every format. KL divergence is lower by 0.031, 0.023 and 0.012, and lower in 91, 88 and 82 of the 100 chunks. The raw diagonal is worse than no weighting on IQ2_XXS and IQ2_XS.
  • The exponent is fixed rather than exposed because the best one is the same on all three formats.
  • Paired differences compare both arms on the same tokens. The standard error is over the 100 chunks.
  • Same-top-token agreement is unchanged on IQ2_XXS and IQ2_XS and improves on IQ2_S.
  • Every unweighted arm ran with uniform importance. That gives byte-identical codes: the tests check it, and so did a run on Qwen3.8-27B's layer-10 MLP weights.
  • Runs are deterministic. Rerunning an arm reproduces its numbers exactly.
  • Earlier baseline: an earlier version of this description compared IQ2_XXS against 0.298. That baseline came from a pre-merge Support GPTQ for the GGML formats #2665 commit; every row here now runs on current main.

GPTQ's per-layer relative error (the Relative MSE error it logs), weighted ÷ unweighted:

Format Importance gate_proj up_proj down_proj
IQ2_XXS diag(H) 1.00 (38/64 layers lower) 1.01 (20/64) 0.91 (46/64)
IQ2_XXS sqrt(diag(H)) 0.97 (58/64) 0.98 (58/64) 0.90 (64/64)
IQ2_XS diag(H) 1.00 (36/64) 1.01 (32/64) 0.94 (44/64)
IQ2_XS sqrt(diag(H)) 0.97 (59/64) 0.97 (60/64) 0.90 (64/64)
IQ2_S diag(H) 1.00 (38/64) 1.01 (13/64) 0.95 (38/64)
IQ2_S sqrt(diag(H)) 0.97 (54/64) 0.98 (53/64) 0.93 (63/64)

These are geometric means over the 64 layers. With layerwise calibration, each run's later layers see different inputs, so read these as indicative.

The raw diagonal barely lowers GPTQ's own error on gate_proj and up_proj, while the square root lowers it on every projection. So the raw diagonal is a worse stand-in for GPTQ's per-block objective, not a case of over-fitting the calibration data.

The plan came from a prototype that used llama.cpp's own quantizers on the UD-IQ1_S layout. In it, Hessian-diagonal weighting stacked with block-GPTQ (wikitext-2 KL divergence):

Unweighted Weighted by our diag(H)
no GPTQ 2.360 0.619
GPTQ 0.584 0.482

Unit tests, on CPU. They pass with and without a GPU visible, 122 passed.

  • tests/unit/torch/quantization/test_iq_formats.py, for each weighted format:
    • uniform importance reproduces the unweighted payload exactly;
    • non-uniform importance lowers the importance-weighted error below 0.75 × that of the unweighted search;
    • invalid importance raises: wrong length, integer dtype, negative, or NaN.
  • tests/unit/torch/quantization/test_ggml_gptq.py:
    • GPTQ hands an IQ2 format sqrt(diag(H)) column-sliced per block;
    • an unweighted format (iq1_s) gets nothing.

GPU tests, 92 passed: tests/gpu/torch/quantization/test_iq_formats_cuda.py, test_ggml_gptq_cuda.py, test_gptq.py, test_iq1_s_cuda.py and test_iq2_xs_cuda.py.

  • The weighted CUDA packer matches the PyTorch weighted encoder. At most 1 block in 1,000 differs, which is float tie-breaking, and the weighted errors are equal.
  • With uniform importance, the CUDA packer reproduces the unweighted payload.
  • The CUDA packer rejects an importance that doesn't tile the blocks.

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Make sure you read and follow the Security Best Practices (e.g. avoiding hardcoded trust_remote_code=True, torch.load(..., weights_only=False), pickle, etc.).

  • Is this change backward compatible?: ✅ The API is compatible, since importance is optional. The codes GPTQ picks for the IQ2 recipes change, which is the point of this PR.
  • If you copied code from any other sources or added a new PIP dependency, did you follow guidance in CONTRIBUTING.md: N/A
  • Did you write any new necessary tests?: ✅
  • Did you update Changelog?: ✅
  • Did you get Claude approval on this PR?: ❌ Not yet.

Additional Information

Follows #2665 (block-GPTQ for the GGML formats). Next: weighted IQ1_S / IQ1_M encoders.

🤖 Generated with Claude Code

Summary by CodeRabbit

  • New Features
    • IQ2_S, IQ2_XS, and IQ2_XXS quantization now support optional per-column importance weights to prioritize accuracy for more important values.
    • GPTQ can derive weights from the square root of the Hessian diagonal. Built-in IQ2 recipes enable this option by default; quantization without weights remains supported.
  • Bug Fixes
    • Invalid importance weights, including unsupported types, shapes, negative values, or NaNs, are rejected.

…Hessian diagonal

The IQ2_XXS, IQ2_XS and IQ2_S encoders take an optional per-input-column
importance w and search for the codes minimising sum w (|x| - s q)^2
instead of the unweighted squared error: the local-scale choice, the grid
entry and the even-parity sign flip all use the weighted terms. This is how
llama.cpp uses an imatrix. The CUDA kernel gets it through a kWeighted
template flag, so the unweighted kernel compiles as before, and uniform
importance reproduces the unweighted payload exactly. The super-block
scale predictor is unchanged.

GGMLGPTQHelper passes each GGML block its slice of the square root of the
Hessian diagonal, E[x_j^2] (llama.cpp's imatrix statistic), floored at
1e-4 of its mean, for any format whose record declares a weighted search.
GPTQ already collects that Hessian, so the built-in IQ2 recipes get the
weighting with no extra calibration. On Qwen3.8-27B IQ2_XXS the raw
diagonal raised wikitext-2 KL from 0.298 to 0.319 while its root lowered
it to 0.287.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Signed-off-by: Chenjie Luo <chenjiel@nvidia.com>
@cjluo-nv
cjluo-nv requested review from a team as code owners October 7, 2026 07:15
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  • modelopt/torch/quantization/ggml/gptq.py
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  • modelopt/torch/quantization/ggml/iq2_xxs.py
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📝 Walkthrough

Walkthrough

IQ2_S, IQ2_XS, and IQ2_XXS accept optional per-column importance weights in Torch and CUDA encoding. GPTQ can derive weights from the Hessian diagonal for supported formats. The change adds an opt-in GPTQ setting and enables it in the built-in IQ2 presets.

Changes

Weighted IQ2 Quantization

Layer / File(s) Summary
Importance preparation and GPTQ integration
modelopt/torch/quantization/ggml/common.py, modelopt/torch/quantization/ggml/gptq.py, modelopt/torch/quantization/config.py, modelopt/torch/quantization/model_calib.py, modelopt/torch/quantization/utils/calib_utils.py, tests/unit/torch/quantization/test_ggml_gptq.py, modelopt_recipes/configs/ptq/presets/model/iq2_*.yaml, CHANGELOG.rst
GGMLFormat gains a weighted flag. Helpers validate and reshape importance values and select rows for block chunks. GPTQ can pass importance derived from the floored Hessian diagonal to weighted formats. The IQ2 presets enable the option.
Weighted IQ2 Torch encoders
modelopt/torch/quantization/ggml/iq2_{s,xs,xxs}.py, modelopt/torch/quantization/ggml/common.py, tests/unit/torch/quantization/test_iq_formats.py
The three IQ2 quantizers accept optional importance and apply it to Torch encoding and parity correction. Their format metadata marks them as weighted. Unit tests check uniform weights, weighted reconstruction error, and invalid inputs.
Weighted IQ2 CUDA packing
modelopt/torch/kernels/quantization/ggml/{ggml.cpp,common.cuh,iq2_family.cuh,iq2_s.cu,iq2_xs.cu,iq2_xxs.cu}, modelopt/torch/quantization/extensions.py, tests/gpu/torch/quantization/test_iq_formats_cuda.py
CUDA bindings and packers accept optional importance. The shared CUDA encoder validates importance and uses block-row indexing for weighted scoring and parity correction. GPU tests compare CUDA and Torch results and check invalid input.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant GPTQ as GGMLGPTQHelper
  participant Format as GGMLFormat
  participant Quantizer as IQ2 quantizer
  participant Packer as IQ2 CUDA packer
  participant Encoder as IQ2 encoding kernel
  GPTQ->>Format: Quantize group with column slice
  Format->>Quantizer: Pass group and Hessian-derived importance
  Quantizer->>Packer: Pass blocks and prepared importance
  Packer->>Encoder: Validate and encode with weighted scoring
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✅ Passed checks (5 passed)
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Title check Passed The title clearly and concisely describes the main change: applying the square root of GPTQ's Hessian diagonal to IQ2 encoder weighting.
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Inline comments:
Review comments at @modelopt/torch/quantization/ggml/gptq.py:
- Around line 99-108: Update `_blockwise_update` so `importance` remains `None`
when the Hessian diagonal mean is zero or negative; only compute the clamped
square-root weights for a positive mean. This lets weighted IQ2 quantization use
unweighted error when no positive calibration importance is available.

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  • modelopt/torch/quantization/ggml/gptq.py
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✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files
@@            Coverage Diff             @@
##             main    #2686      +/-   ##
==========================================
+ Coverage   71.70%   78.12%   +6.41%     
==========================================
  Files         642      642              
  Lines       71312    71345      +33     
==========================================
+ Hits        51135    55739    +4604     
+ Misses      20177    15606    -4571     
Flag Coverage Δ
examples-diffusers 20.25% <15.51%> (+<0.01%) ⬆️
examples-gpt-oss 13.48% <15.51%> (+0.01%) ⬆️
examples-hf_ptq 23.44% <55.17%> (+0.03%) ⬆️
examples-llm_distill 13.56% <15.51%> (+0.03%) ⬆️
examples-llm_eval 17.32% <15.51%> (+<0.01%) ⬆️
examples-llm_qat 17.56% <15.51%> (+0.02%) ⬆️
examples-llm_sparsity 15.85% <15.51%> (+0.01%) ⬆️
examples-megatron_bridge 26.46% <15.51%> (-0.15%) ⬇️
examples-specdec_bench 13.25% <15.51%> (+0.01%) ⬆️
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gpu 58.84% <93.10%> (+25.29%) ⬆️
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Weighting the IQ2 search by GPTQ's Hessian diagonal is now opted into
with a GPTQ config field, importance_weighted (default False), instead of
following from the format alone. The right weighting depends on the
algorithm that supplies the statistic -- the square root of diag(H) under
GPTQ, while an imatrix-only calibration without error compensation would
want the raw E[x^2] -- so the switch belongs with the algorithm. Formats
without a weighted search ignore it, so a recipe can set it once.

The built-in iq2_xxs, iq2_xs and iq2_s presets set it, so their output
does not change.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Signed-off-by: Chenjie Luo <chenjiel@nvidia.com>
@cjluo-nv
cjluo-nv requested a review from a team as a code owner October 8, 2026 06:06
@cjluo-nv
cjluo-nv requested a review from h-guo18 October 8, 2026 06:06

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Bot review (bedrock-claude-opus-5-5) — DM the bot to share feedback.

Requesting changes: the weighted encoders look correct and are well tested, but an MoE expert that gets no calibration tokens ends up with an all-zero importance, and its IQ2 codes come out degenerate.

Needs action:

  • Fix GGMLGPTQHelper._blockwise_update in ggml/gptq.py. When diag(H) is all zero, or its mean is not positive, fall back to importance=None. Today clamp_min(1e-4 * 0) leaves every weight at 0, so every candidate scores zero error and the search picks entry 0 at the smallest local scale. See the inline comment.
  • Add a unit test in test_ggml_gptq.py for that case. Give a weighted format a zero Hessian and assert it reproduces the unweighted payload.

No action needed:

  • About 300 core-logic lines, under the size budget. No new files and no licensing changes.
  • The design question is covered. This adds an optional argument to the existing encoders and GPTQ helper rather than a new system, and the PR body explains why the switch sits on the algorithm and why it uses the square root.
  • I checked the weighted parity-flip math (dot − 2·min w|x|q), the block % blocks_per_row column mapping in the CUDA kernel, and the per-group slicing of importance[columns].
  • The test edits are justified: the _round_to_tenths and _round_in_groups_of_4 helpers only gained the new columns parameter.
  • Minor: check_importance repeats the finite and non-negative check that the Python side already does, which adds a host sync. Optional.

importance = None
if self.importance_weighted and ggml_format.weighted:
diagonal = self.hessian.diagonal().to(self.weight.device, torch.float32)
importance = diagonal.clamp_min(_IMPORTANCE_FLOOR * diagonal.mean()).sqrt()

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Bot comment.

An MoE expert that receives no calibration tokens keeps self.hessian at zero (update_hessian skips empty batches). Then diagonal.mean() is 0, the floor is 0, and importance is all zeros. In the weighted search every candidate then scores exactly zero error, so each vector gets the lowest local scale and grid entry 0. That is far worse than the unweighted encode the expert gets today, which GPTQ's identity h_inv fallback keeps close to RTN. The built-in IQ2 recipes target MoE weights, so this will happen in practice. Suggested fix: if not diagonal.mean() > 0: importance = None, plus a test.

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Fixed in 340e12b: _blockwise_update in ggml/gptq.py now weights the search only when the Hessian diagonal has a positive mean, so a module no calibration token reached keeps the plain IQ2 search. The new test_a_module_calibration_never_reached_gets_the_plain_search in test_ggml_gptq.py leaves one layer out of the calibration loop and checks its codes match the unweighted encode exactly; it failed on every value before the fix.

The optional item is in 3b2c5c5: the CUDA packer no longer repeats the finite and non-negative check (the Python encoders already do it), so the extra host sync per encode is gone. It keeps its dtype, shape and device checks.

cjluo-nv and others added 2 commits October 8, 2026 06:21
An MoE expert that no calibration token is routed to keeps a zero
Hessian. Its importance was then all zeros, so every candidate code
scored zero error and each 8-value group took grid entry 0 at the
smallest local scale -- far worse than the plain encode GPTQ's identity
fallback gives such a module today. Weight the search only when the
Hessian diagonal has a positive mean.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Signed-off-by: Chenjie Luo <chenjiel@nvidia.com>
The Python encoders already reject a non-finite or negative importance
before calling the CUDA packer, so the packer's own value check only
added a host sync to every encode, which GPTQ runs once per 256-column
group. The packer keeps its dtype, shape and device checks. Also list
the packers' optional importance argument in the extension docstring.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Signed-off-by: Chenjie Luo <chenjiel@nvidia.com>

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Bot review (bedrock-claude-opus-5-5) — DM the bot to share feedback.

Approving: the zero-Hessian blocker from the last review is fixed, it has a regression test, and nothing new came up on re-review.

No action needed.

No action needed:

  • ✔️ Resolved since the last review:
    • The idle-expert fallback: _blockwise_update now passes importance=None unless diag(H).mean() > 0.
    • Its regression test: test_a_module_calibration_never_reached_gets_the_plain_search checks the codes match the unweighted encode exactly.
    • The optional CUDA host-sync item: check_importance no longer repeats the finite and non-negative checks.
  • New since the last review: the importance_weighted GPTQ option. It defaults to False and is threaded through GPTQCalibConfig, gptq() and GPTQHelper.__init__. GGMLGPTQHelper is the only subclass of GPTQHelper, so the new keyword breaks nothing.
  • Tests cover all three settings: weighted on an IQ2 format, flag off, and an unweighted format (iq1_s). Each format with a weighted search ignores the flag when it isn't set.
  • Size is about 320 core-logic lines, within the budget. No licensing changes.
  • Design: this adds an optional argument to the existing encoders and GPTQ helper rather than a new system. The PR body explains why the switch sits on the algorithm and why it uses the square root.

Complex PR: 20 files changed (≥ 20); spans 7 directories (≥ 5); 1 existing test file modified or removed. Looping in a human for approval.

Resolve the import conflicts in the IQ2 encoders (iq2_s, iq2_xs, iq2_xxs):
main removed the IQFormat compatibility alias in favour of GGMLFormat, and
this branch added the importance helpers to the same import list. Keep
GGMLFormat together with chunk_importance, importance_blocks and
iq2_tile_terms.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Signed-off-by: Chenjie Luo <chenjiel@nvidia.com>
@cjluo-nv
cjluo-nv enabled auto-merge (squash) October 8, 2026 22:32
@cjluo-nv
cjluo-nv merged commit bc9f439 into main Oct 9, 2026
55 checks passed
@cjluo-nv
cjluo-nv deleted the chenjiel/iq-weighted-encoders branch October 9, 2026 16:31
cjluo-nv added a commit that referenced this pull request Oct 9, 2026
…#2723)

### What does this PR do?

Type of change: new feature

#2686 let the IQ2 encoders weight their codebook search by a
per-input-column importance, which GPTQ fills with the square root of
its Hessian diagonal. This PR does the same for **IQ1_S and IQ1_M**.

**Weighted encoders.** `quantize_iq1_s` and `quantize_iq1_m` take the
same optional `importance` `w` as the IQ2 encoders. Each 8-value group
is approximated by `s·(q + δ)`, with `q` from the ternary grid and `δ =
±1/8`. The search now minimises `Σ w·(x − s·(q + δ))²` instead of the
plain squared error.
- The weighting covers each group's grid entry, its shift `δ`, and the
local scale it shares with neighbouring groups (4 groups in IQ1_S, 2 in
IQ1_M).
- The super-block scale predictors stay unweighted, as in IQ2.
- The CUDA kernel gets the weighting through the same `kWeighted`
template flag as IQ2, so the unweighted kernel compiles as before.
- With uniform importance, the payload is identical to the unweighted
one.
- Both format records now declare `weighted=True`, so GPTQ's
`importance_weighted` option reaches them.

**The built-in `general/ptq/iq1_s` and `iq1_m` recipes now set
`importance_weighted: true`**, as the IQ2 recipes do in #2686.

### Usage

The built-in IQ1 recipes turn it on:

```bash
python examples/hf_ptq/hf_ptq.py --pyt_ckpt_path <model> --recipe general/ptq/iq1_s --export_path <dir>
```

Calling the encoder directly, for example with an imatrix from
elsewhere:

```python
from modelopt.torch.quantization.ggml.iq1_s import quantize_iq1_s

packed, shape = quantize_iq1_s(weight, importance=imatrix)  # imatrix: [in_features], finite, >= 0
```

### Testing

**Qwen3.8-27B, MLP-only IQ1 with GPTQ**, using the default
`general/ptq/iq1_*` recipes. The setup is the same as #2686:
- Calibration: `hf_ptq.py` defaults (`cnn_nemotron_v2_mix`, 1024 × 512).
- Scoring: KL divergence from BF16 on wikitext-2 test, 100 × 512-token
chunks, scored as llama.cpp's `llama-perplexity --kl-divergence` does.
- All runs use this branch, before its rebase onto the merged #2686. The
only difference between them is the importance passed to the search.

Mean KL divergence for each format and each choice of importance:

| Format | Unweighted | `diag(H)` | **`sqrt(diag(H))` (this PR)** | sqrt
− unweighted, paired (chunks where sqrt is lower) |
|---|---|---|---|---|
| IQ1_S | 0.926 | 0.941 | **0.847** | −0.079 ± 0.007 (95/100) |
| IQ1_M | 0.560 | 0.587 | **0.544** | −0.016 ± 0.004 (66/100) |

| Format | Perplexity (BF16: 6.746): unweighted / `diag(H)` / sqrt |
Same top token: unweighted / `diag(H)` / sqrt |
|---|---|---|
| IQ1_S | 15.44 / 15.65 / **14.20** | 60.7 / 59.2 / **61.1** % |
| IQ1_M | 10.81 / 11.06 / **10.62** | 69.5 / 67.8 / 69.4 % |

- **IQ1 follows the same pattern as all three IQ2 formats.**
- The square root beats the raw diagonal on both formats: KL divergence
is lower by 0.094 (IQ1_S) and 0.042 (IQ1_M), in 95 and 91 of the 100
chunks.
  - The raw diagonal is worse than no weighting on both.
  - So the fixed square root from #2686 holds for all five IQ formats.
- **The gain is largest on IQ1_S:** 8.5% lower KL divergence and 1.2
lower perplexity.
- **Same-top-token agreement** improves on IQ1_S and is unchanged within
noise on IQ1_M (−0.13 ± 0.23 points).
- **Paired differences** compare both arms on the same tokens. The
standard error is over the 100 chunks.
- **Every unweighted arm ran with uniform importance.** That gives
byte-identical codes: the tests check it, and so did a run on
Qwen3.8-27B's layer-10 MLP weights (0 of 348,160 blocks differ for
either format).

GPTQ's per-layer relative error (the `Relative MSE error` it logs),
weighted ÷ unweighted, geometric mean over 64 layers:

| Format | Importance | `gate_proj` | `up_proj` | `down_proj` |
|---|---|---|---|---|
| IQ1_S | `diag(H)` | 0.98 (42/64 layers lower) | 1.00 (24/64) | 0.93
(41/64) |
| IQ1_S | `sqrt(diag(H))` | 0.96 (63/64) | 0.99 (60/64) | 0.91 (62/64) |
| IQ1_M | `diag(H)` | 0.99 (41/64) | 1.01 (23/64) | 0.92 (42/64) |
| IQ1_M | `sqrt(diag(H))` | 0.96 (61/64) | 0.98 (59/64) | 0.90 (63/64) |

**Unit tests:** 137 passed in `test_iq_formats.py` and
`test_ggml_gptq.py`, with and without a GPU visible, plus the recipe
tests. The weighted tests are driven by the format registry, so they now
run for IQ1_S and IQ1_M too:
- uniform importance reproduces the unweighted payload exactly;
- importance lowers the importance-weighted error below 0.75 × that of
the unweighted search;
- invalid importance is rejected.

The GPTQ test now covers IQ1_S as a weighted format and uses Q8_0 as the
unweighted one.

**GPU tests:** 96 passed.
- The weighted CUDA packers match the PyTorch weighted encoders, with at
most 1 block in 1,000 differing.
- With uniform importance, the CUDA packers reproduce the unweighted
payload.
- They reject an importance that doesn't tile the blocks.

`pre-commit` passes.

### Before your PR is "*Ready for review*"

Make sure you read and follow [Contributor
guidelines](https://github.com/NVIDIA/Model-Optimizer/blob/main/CONTRIBUTING.md)
and your commits are signed (`git commit -s -S`).

Make sure you read and follow the [Security Best
Practices](https://github.com/NVIDIA/Model-Optimizer/blob/main/SECURITY.md#security-coding-practices-for-contributors)
(e.g. avoiding hardcoded `trust_remote_code=True`, `torch.load(...,
weights_only=False)`, `pickle`, etc.).

- Is this change backward compatible?: ✅ The API is compatible, since
`importance` is optional. The codes GPTQ picks for the built-in IQ1
recipes change, which is the point of this PR.
- If you copied code from any other sources or added a new PIP
dependency, did you follow guidance in `CONTRIBUTING.md`: N/A
- Did you write any new necessary tests?: ✅
- Did you update
[Changelog](https://github.com/NVIDIA/Model-Optimizer/blob/main/CHANGELOG.rst)?:
✅ This extends #2686's entry to the IQ1 formats and recipes.
- Did you get Claude approval on this PR?: ❌ Not yet.

### Additional Information

Follows #2665 (block-GPTQ for the GGML formats) and #2686 (weighted IQ2
encoders).

🤖 Generated with [Claude Code](https://claude.com/claude-code)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* IQ1_S and IQ1_M quantization now support optional per-column
importance weights, helping prioritize values that matter most during
codebook search.
* GPTQ calibration presets for IQ1_S and IQ1_M now enable
importance-weighted quantization by default.
* Importance-weighted calibration is supported across the built-in IQ1
and IQ2 recipes.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Chenjie Luo <chenjiel@nvidia.com>
Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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