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kshyatt
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Oct 28, 2026
| # and store that instead of repeating the twist in the pullback each time. | ||
| # effectively, we replace `twist!` by `add ∘ twist`. | ||
| Ap = if !isa(α, Const) | ||
| Ap = twist(A.val, inds.val; inv, copy = true) |
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similar to permute and transform, is there any chance this could secretly be aliasing A.val?
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That's what the copy=true is for right?
kshyatt
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Oct 28, 2026
| shadow = EnzymeRules.needs_shadow(config) ? C.dval : nothing | ||
| return EnzymeRules.AugmentedReturn(primal, shadow, cache) | ||
| end | ||
| function EnzymeRules.reverse( |
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Was Enzyme not able to make its own rules for these? Kind of sad if so...
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These changes make twist go through the same path as permute and transpose etc, so even if it was able to do this I think this path is way more friendly on both compile time and possibly runtime?
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Before I start reviewing, since twist only depends on the uncoupled indices of the trees, it is just a constant scalar factor (or thus a UniformScaling) for every FusionBlock. Id that being used here? |
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It is, but it's slightly subtle: the code uses |
…ernel `twist!` looped over all fusion trees and scaled every subblock separately, with a cost that grows with the number of subblocks rather than the data size, and one kernel launch per subblock for GPU tensors (#595). - Add `twist(src, inds; inv)` for fusion tree pairs and blocks, returning the scalar twist factor, with `inds` given as indices or as a `Bool` mask. - Add the cached `treetwister`, a `UniqueTreeTransformer` for all sectors since a twist maps every tree pair onto itself, keyed on a fixed-length mask. - `twist!` and `twist` on a `TensorMap` go through `add_transform!` (in place for `twist!`, a single pass for `twist`); other tensor types keep the loop. - Move the shortcut for trivial permutations from `add_transform!` into `_braid!` and `_transpose!`, so `add_transform!` always applies the transformer. - Cache a device `treetwister` in the GPUArrays extension. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Adjoint destinations and sources are unwrapped as in the other index manipulations: the twist mask of an adjoint destination is relabeled onto the legs of its parent with the inverse twist, and `treetwister` takes a `conjsrc` flag, with the destination space and the permutation of the source legs following from the source space. An in-place twist of an adjoint therefore uses the same transformer as the equivalent twist of its parent. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
… α, β, ...)` `twist!(t, inds)` and `twist(t, inds)` are now thin wrappers around the three-argument form, which goes through `unwrap_adjoints` and a `_twist!` kernel entry like `_braid!`. Adds `twist_pb` with Mooncake and Enzyme rules, and backend/allocator keywords for `twist`. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
…ist!` on Julia 1.10 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
…h on Julia 1.10 Enzyme aborts in `fixup_callconv!` when compiling the wrapper while it returned the result of the three-argument `twist!`, which also broke the fermionic contraction rules. The in-place Enzyme rules call the wrapper again. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Addresses the standalone
twist!part of #595.twist!looped overfusiontrees(t)and calledscale!on every subblock, so its cost grew with the number of subblocks rather than with the data size, and a twist of a GPU tensor launched one kernel per subblock. This PR routestwist!/twistthrough the existing index manipulation kernel with a cached tree transformer.Changes
twist(src::Union{FusionTreePair, FusionTreeBlock}, inds; inv = false) -> (src, θ): fusion-tree level twist, next topermute/braid, withindsgiven as indices or as anNTuple{N, Bool}mask (indices are converted to a fixed-length mask, so all forms are inferable).twist!(tdst, tsrc, inds, α = 1, β = 0, [backend], [allocator]; inv = false)computingtdst = β * tdst + α * twist(tsrc, inds; inv), laid out likebraid!: it unwraps adjoints throughunwrap_adjointsand calls a_twist!kernel entry mirroring_braid!.twist!(t, inds)andtwist(t, inds)are thin wrappers around it, andtwistnow also acceptsbackend/allocatorkeywords.treetwister(A, Vdst, Vsrc, p, conjsrc, twistmask, inv), with the same arguments astreebraider(the mask taking the place of the levels): always aUniqueTreeTransformer, also for non-abelian sectors, since a twist maps every tree pair onto itself with a scalar factor (no recoupling matrices / pack-matmul-unpack needed).twist!(t, inds)runs throughadd_transform!(aliasing is allowed for trivial permutations in TensorOperations), andtwistis a single out-of-place pass instead ofcopy!followed bytwist!.TensorMap/AdjointTensorMapdestinations and sources go through the twister. The twist mask is relabeled like the levels ofbraid!, and an adjoint destination flipsinv(just as it conjugatesα).twist_pbpullback with Mooncake and Enzyme rules for the three-argumenttwist!, plus Enzyme rules for the in-placetwist!(t, inds). The existing ChainRulestwistrrule already forwards keywords and is now also tested withinvandbackend/allocator.add_transform!into_braid!/_transpose!(which also skips the transformer lookup there), soadd_transform!always applies its transformer.treetwister, registered and sized like the other device caches, so a GPUtwist!is a single batched kernel launch.Benchmarks (CPU, single thread)
fℤ₂ × U(1) tensors
Vχ ⊗ VD ⊗ VD' ← Vχ ⊗ VD ⊗ VD', twisted on indices(2, 3, 5, 6), minimum of 20 runs:twist!beforetwist!aftercopyBuilding the twister on a cache miss costs ~3× a cached call in the smallest case. There is still a per-subblock cost from the individual
tensoradd!calls on the CPU, which is left for a follow-up. GPU timings were not measured.Testing
tensors/indexmanipulations.jl,tensors/contractions.jl,chainrules/linalg.jl,mooncake/indexmanipulations.jl,enzyme/flip.jlandcuda/tensors.jl(RTX A6000) pass.test/cuda/tensors.jlcomparingtwist/twist!onCuTensorMaps (and their adjoints) against the host results, and checking that the cached device transformer is used.test/tensors/indexmanipulations.jl, covering adjoint sources, adjoint destinations and in-place twists of adjoints, plus α/β accumulation.Follow-up
Folding the twist into the permutations that precede a contraction (the second part of #595) will be done on top of this, through a cached contraction plan holding the transformers.
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