Eliminate redundant NCHW↔NHWC permute_copy and NHWC-safe view_copy transposes in ToTosaMemoryFormatPass (#18314) by 3l1 · Pull Request #18314 · pytorch/executorch

3l1 · 2026-03-19T08:46:23Z

Summary:

Two optimizations in ToTosaMemoryFormatPass to reduce TOSA TRANSPOSE nodes:

NHWC-safe reshape detection: When a 4D→4D view_copy has monotonic
shape_indices on the raw shapes and preserves both the batch dim (index 0)
and the last dimension (NHWC channel) alone in their output groups, skip
inserting input/output transposes. The view_copy can operate directly on
NHWC data.
Redundant permute_copy elimination: Model-level permute_copy ops whose
permutation matches channels_last_order (NCHW→NHWC) or its inverse
(NHWC→NCHW) AND whose input already has NHWC tosa_dim_order are redundant
with the tosa_dim_order annotation. Replace them with view_copy (identity
reshape) to avoid generating TOSA TRANSPOSE nodes. Standalone permute
models (NCHW input from placeholder) are not affected.

Differential Revision: D97266678

pytorch-bot · 2026-03-19T08:46:29Z

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meta-codesync · 2026-03-19T08:46:33Z

@3l1 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97266678.

github-actions · 2026-03-19T08:47:12Z

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…ansposes in ToTosaMemoryFormatPass (#18314) Summary: Two optimizations in ToTosaMemoryFormatPass to reduce TOSA TRANSPOSE nodes: 1. **NHWC-safe reshape detection:** When a 4D→4D view_copy has monotonic shape_indices on the raw shapes and preserves both the batch dim (index 0) and the last dimension (NHWC channel) alone in their output groups, skip inserting input/output transposes. The view_copy can operate directly on NHWC data. 2. **Redundant permute_copy elimination:** Model-level permute_copy ops whose permutation matches channels_last_order (NCHW→NHWC) or its inverse (NHWC→NCHW) AND whose input already has NHWC tosa_dim_order are redundant with the tosa_dim_order annotation. Replace them with view_copy (identity reshape) to avoid generating TOSA TRANSPOSE nodes. Standalone permute models (NCHW input from placeholder) are not affected. Differential Revision: D97266678

AdrianLundell · 2026-03-19T13:08:28Z

Hi, thanks for the PR! This is a complex topic to get right in all cases and FYI we are also planning on improving this internally so it is very nice to get some help with that. I see there are some errors in our unittests so looks like there are a few edge-cases to iron out before a proper review. Let us know if you have any questions about the current logic to help with this.

In the meanwhile that I have two comments:

The first optimization seems like a slighly more narrow condition of the "is_channel_reshape" function, do you think it could fit into there rather than a separate function?
For the second optimization, have you observed performance regressions due to noop transposes? Our working assumption has been that this is optimized away by the compiler.

3l1 · 2026-03-19T17:00:23Z

hey @AdrianLundell thanks for the feedback

yes that is something I can look into wrapping up into the existing function
yes we see on u55 (not u85) issues where regor was not optimizing away all of them, causing perf issues on firmware

(also fixing failing unit tests)

…ansposes in ToTosaMemoryFormatPass (#18314) Summary: Two optimizations in ToTosaMemoryFormatPass to reduce TOSA TRANSPOSE nodes: 1. **NHWC-safe reshape detection:** When a 4D→4D view_copy has monotonic shape_indices on the raw shapes and preserves both the batch dim (index 0) and the last dimension (NHWC channel) alone in their output groups, skip inserting input/output transposes. The view_copy can operate directly on NHWC data. 2. **Redundant permute_copy elimination:** Model-level permute_copy ops whose permutation matches channels_last_order (NCHW→NHWC) or its inverse (NHWC→NCHW) AND whose input already has NHWC tosa_dim_order are redundant with the tosa_dim_order annotation. Replace them with view_copy (identity reshape) to avoid generating TOSA TRANSPOSE nodes. Standalone permute models (NCHW input from placeholder) are not affected. Differential Revision: D97266678

…ansposes in ToTosaMemoryFormatPass (#18314) Summary: Pull Request resolved: #18314 Two optimizations in ToTosaMemoryFormatPass to reduce TOSA TRANSPOSE nodes: 1. **NHWC-safe reshape detection:** When a 4D→4D view_copy has monotonic shape_indices on the raw shapes and preserves both the batch dim (index 0) and the last dimension (NHWC channel) alone in their output groups, skip inserting input/output transposes. The view_copy can operate directly on NHWC data. 2. **Redundant permute_copy elimination:** Model-level permute_copy ops whose permutation matches channels_last_order (NCHW→NHWC) or its inverse (NHWC→NCHW) AND whose input already has NHWC tosa_dim_order are redundant with the tosa_dim_order annotation. Replace them with view_copy (identity reshape) to avoid generating TOSA TRANSPOSE nodes. Standalone permute models (NCHW input from placeholder) are not affected. Differential Revision: D97266678

…ansposes in ToTosaMemoryFormatPass (#18314) Summary: Two optimizations in ToTosaMemoryFormatPass to reduce TOSA TRANSPOSE nodes: 1. **NHWC-safe reshape detection:** When a 4D→4D view_copy has monotonic shape_indices on the raw shapes and preserves both the batch dim (index 0) and the last dimension (NHWC channel) alone in their output groups, skip inserting input/output transposes. The view_copy can operate directly on NHWC data. 2. **Redundant permute_copy elimination:** Model-level permute_copy ops whose permutation matches channels_last_order (NCHW→NHWC) or its inverse (NHWC→NCHW) AND whose input already has NHWC tosa_dim_order are redundant with the tosa_dim_order annotation. Replace them with view_copy (identity reshape) to avoid generating TOSA TRANSPOSE nodes. Standalone permute models (NCHW input from placeholder) are not affected. Differential Revision: D97266678

…ansposes in ToTosaMemoryFormatPass (#18314) Summary: Pull Request resolved: #18314 Two optimizations in ToTosaMemoryFormatPass to reduce TOSA TRANSPOSE nodes: 1. **NHWC-safe reshape detection:** When a 4D→4D view_copy has monotonic shape_indices on the raw shapes and preserves both the batch dim (index 0) and the last dimension (NHWC channel) alone in their output groups, skip inserting input/output transposes. The view_copy can operate directly on NHWC data. 2. **Redundant permute_copy elimination:** Model-level permute_copy ops whose permutation matches channels_last_order (NCHW→NHWC) or its inverse (NHWC→NCHW) AND whose input already has NHWC tosa_dim_order are redundant with the tosa_dim_order annotation. Replace them with view_copy (identity reshape) to avoid generating TOSA TRANSPOSE nodes. Standalone permute models (NCHW input from placeholder) are not affected. Differential Revision: D97266678

…ansposes in ToTosaMemoryFormatPass (#18314) Summary: Two optimizations in ToTosaMemoryFormatPass to reduce TOSA TRANSPOSE nodes: 1. **NHWC-safe reshape detection:** When a 4D→4D view_copy has monotonic shape_indices on the raw shapes and preserves both the batch dim (index 0) and the last dimension (NHWC channel) alone in their output groups, skip inserting input/output transposes. The view_copy can operate directly on NHWC data. 2. **Redundant permute_copy elimination:** Model-level permute_copy ops whose permutation matches channels_last_order (NCHW→NHWC) or its inverse (NHWC→NCHW) AND whose input already has NHWC tosa_dim_order are redundant with the tosa_dim_order annotation. Replace them with view_copy (identity reshape) to avoid generating TOSA TRANSPOSE nodes. Standalone permute models (NCHW input from placeholder) are not affected. Differential Revision: D97266678

3l1 requested a review from digantdesai as a code owner March 19, 2026 08:46

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meta-codesync bot changed the title ~~Eliminate redundant NCHW↔NHWC permute_copy and NHWC-safe view_copy transposes in ToTosaMemoryFormatPass (#18167)~~ Eliminate redundant NCHW↔NHWC permute_copy and NHWC-safe view_copy transposes in ToTosaMemoryFormatPass (#18314) Mar 19, 2026

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Eliminate redundant NCHW↔NHWC permute_copy and NHWC-safe view_copy transposes in ToTosaMemoryFormatPass (#18314)#18314

Eliminate redundant NCHW↔NHWC permute_copy and NHWC-safe view_copy transposes in ToTosaMemoryFormatPass (#18314)#18314
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