pytorch

0032fa77 - Add a Functionalization pass in core (#64432)

Add a Functionalization pass in core (#64432) Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/64432 Original PR description + feedback here: https://github.com/pytorch/pytorch/pull/63048 I've addressed all of the feedback in the original PR and made some pretty large changes, listed below. **Table of Contents** - Starting points - List of the main changes from the original PR - Next Steps - Example codegen output (for a view, mutation, and view+mutation op) **Starting Points** A good place to start when looking through the PR: * Alban mentioned that this is a useful mental model (thanks Ed for originally making this clear to me). Semantically, the pass currently does THREE things, which are all needed by functorch - all fused together into one big pass. * (a) alias removal, which replaces {view} calls with {view}_copy calls, and manually tracks aliasing information, so that when one tensor is mutated, we re-apply the same mutation to all of the aliases. This is the bulk of the work - once this is done, the next 2 things are trivial to implement. * (b) mutation removal, which is easy to do once we know that there are no aliases. Every mutation `a.add_(b)` becomes `a.replace_(a.add(b))` * (c) reapplying views: all of the `{view}_copy` calls are replaced with `{view}` calls again. This is an optimization that we can make specifically for functorch (and strided backends), that only care about mutation removal and not alias removal * XLA and Vulkan only want (a), or (a) + (b). Later, we'll want to split this out so that you can actually opt into different versions of this logic. * There is currently no {view}_copy replacement, because the pass just <replace views with copies> and <replace copies with views> steps have been combined. Later, we'll want to actually implement {view}_copy variants of each view operator, probably with codegen. * documentation breadcrumb 1, in `FunctionalTensorWrapper.cpp`: https://github.com/pytorch/pytorch/pull/64432/files#diff-a0bac99bf205dba5b94cb64fc2466d3d55d991887572f9cd6a02e27b3a91dd60R59 (you might have to expand the `FunctionalTensorWrapper.cpp` file, which GitHub closes by default because it's large) * documentation breadcrumb 2, in `FunctionalTensorWrapper.h`: https://github.com/pytorch/pytorch/pull/64432/files#diff-c945c71a4ccac65871f24a912e8904f9a5088b24a32e636727ea9c8fe920708aR12 * Reading through the codegen output at the bottom of this description. **Main changes from the original PR** (1) I use lambdas instead of a giant enum to handle all of the different views. This results in less boilerplate per view op (and more stuff that can be codegen'd). Every `ViewMeta` object now contains a `forward` and `reverse` lambda, that knows how to replay the view and its inverse. This makes the actual code that executes the replaying logic a lot less boilerplate-y (see `Alias::sync_update_operations` and `FunctionalTensorWrapper::sync_`) (2) Every tensor during the functionalization pass is always wrapped in a `FunctionalTensorWrapper`. This is potentially unnecessary for Vulkan/XLA, and will have a mild perf impact, but for now this PR just targets the functorch use case. I previously had a complicated design a (`FunctionalTensorImplBase` class) to avoid needing the wrapper for XLA, but it had some subtleties that are gonna require more thought to fix, so I'm pushing that off for now. (3) `FunctionalTensorWrapper` objects accurately report stride information. It's a little annoying to do this though, because the logic that calculates stride info for each view isn't easily separated from the actual view kernels in core, `at::native::{view}`. I do this by adding logic in each `at::functionalization::{view}` kernel to call the reference implementation `at::native::{view}`. I don't do anything with the output aside from taking it's size/stride/storage_offset to set the actual output tensor's size/stride/storage_offset correctly. There's another annoying part to this: I'm pretty sure that we want to pass in the actual *wrapper* tensors directly into the native kernels, not their inner unwrapped values. But there are some `at::native::{view}` kernels that call other tensor methods, which re-invokes the dispatcher, calling functionalization/functorch kernels that try do the unwrapping. To do this, right now I have an `AutoDispatchDirectlyToNative` guard that basically ensures that any tensor methods called inside of the at::native::{view} op always redispatch straight to the CPU kernel (which will be another at::native:: kernel). This feels kind of heavy handed, but I'm not sure of a better way to do it. (4) `FunctionalTensorWrapper` objects accurately report aliasing information. There's a new `FunctionalStorageImpl` class (subclass of `StorageImpl`) that allows tensors in the functionalization pass to accurately alias storage. If two tensors `a` and `b` in a functionalized program are views of one another, then `a.storage.is_alias_of(b.storage)` should return true. I added this in a pretty similar way to how meta tensors allocate storage, although I don't pass in an actual allocator (I think this is fine because you should never resize a functional tensor's storage). One thing I'm not sure about - should `FunctionalTensorWrapper` set `storage_access_should_throw_`: (a) always, (b) never, (c) only if its wrapped tensor has it set. Right now I have it not set, mostly because calling the reference view functions (`at::native::{view}`) requires looking at the storage. But that means that if you try to access storage from python in a functionalized program, you'll get silent garbage instead of an error. Related question: are we planning on exposing meta tensor storage to python in the future (even though it contains garbage)? (5) better docs :) **View operator coverage** (6) The functionalization pass now gets math-composite view ops for free. I didn't add the `Functionalize` dispatch key to the composite set, because I don't want composite ops like `torch.ones` to get decomposed before hitting the functionalization pass. Instead, I added codegen to manually register the `at::native::` kernels of composite view ops. This is a little hairy, because the names of the `at::native::` kernels aren't easily accessible. They're stored in a `Dict[DispatchKey, BackendIndex]`. I made a best-effort attempt to get each view kernel's name, basically by assuming that every view op has either a composite or cpu implementation. There's also a hardcoded list of composite view ops in `gen_inplace_or_view_type.py`, but it looks like it's wrong. This is probably worth rationalizing later, but instead I created a new list of the "complete" set of composite view ops, and preserved the old set by hardcoding the delta between the two sets. (7) I've added codegen for ops that are both views AND mutations, like `transpose_()` (why do we even have these {emoji:1f622}). From some light testing, it looks like they work correctly with one caveat: I had a hard time ensuring that functorch programs that mutate their inputs using ops like `transpose_()` preserve the input mutations after the program finishes running. For (in my corresponding functorch branch) I emit a warning when this happens, and just don't preserve the mutation (8) I added `{view}_inverse` implementations for every view op, in `FunctionalInverses.cpp`. These are needed to take mutations made to views and replay them back onto the base. To reduce boilerplate, the codegen generates function declarations for each `{view}_inverse` function, so you get a nice compiler error when someone eventually adds a new view op. The only view ops currently not supported are (a) as_strided, and (b) the sparse view ops (values()/indices()). I can add support for as_strided, but it needs an `as_strided_inverse()` function. That will look really similar to the `as_strided_backward()` function in FunctionsManual.cpp, but it has some noticeable differences: we basically want an `as_strided_embed` for autograd and `as_strided_scatter` for functionalization. We also will probably need them to be primitives w.r.t to autograd, since the currently implementation for autograd uses view().copy_() calls that XLA won't be able to handle. I'm wondering if anyone has any objections, but otherwise I can make those change (which will require writing backward formulas for `as_strided_embed` and `as_strided_scatter`). I did a bunch of manual testing that all looks pretty good, but it's definitely not fully tested. Ed pointed out that once XLA uses this pass (or at least once there's a POC), we can just run the existing xla view test suite. Hopefully that delay is okay - if it's not, maybe we can think about using OpInfos similar to how functorch uses them for testing. Note: there's some duplication with autograd's view code. Every `{view}_inverse` implementation is really similar to the implementation for that view listed in `derivatives.yaml`. There are some major differences though: * the autograd implementations over those backwards functions (like `permute_backwards()`, in `FunctionsManual.cpp`) internally call other view ops. For functoinalization, we want them to (eventually call `{view}_copy` operators). * For view ops that take a subset of the original storage, like `slice/select/diagonal/as_strided()`, the autograd backward functions fill the "spaces" in the inverse call with zeroes. For functionalizations, we want to fill them with the value of `base` at those positions. It looks like this currently applies to 6 total ops (since we can ignore composites): * select * slice * diagonal * as_stridied * split * split_with_sizes A nice end state would probably be for the autograd + functoinalization codegen to both look at the same yaml (either `derivatives.yaml`, or something else), and automatically generate the right thing. I didn't leave that in scope for this PR though. **Current State + Next Steps** There are a bunch of followups after this PR eventually lands. Roughly in order: * Use the current pass to register problematic composite ops in functorch. Also, nested `functionalize()` calls aren't supported yet (I mostly just need to remove some debug asserts and test it). * Work on freeing up dispatch key space in the by deduplicating the `{backend}`/`Autograd{backend}`/`Sparse{backend}`/`Quantized{backend}` keys * Once we have more dispatch keys, split up this pass into 3 pieces - it's currently fused, and doesn't do the right thing for vulkan/XLA. Specifically, all of the `{view}` calls in the current pass's view-replay logic should turn into `{view}_copy` calls that vulkan/XLA know how to implement, and there will be separate passes for (a) removing mutations, and (b) turning `{view}_copy` calls back into `{view}` calls. For Vulkan, we eventually want a pass that ONLY removes aliasing and view calls, and doesn't remove mutations. We can also probably make the 2 new passes user dispatch keys to save dispatch key space, if they'll only be used by functorch anyway. * Do more of a dive on perf for the vulkan/xla use cases. There are several areas to improve perf with varying levels of effort required. The simplest one that I'll probably do regardless is to codegen the out-of-place kernels instead of using a boxed fallback. Getting a POC working for xla will also be useful to test the view operator coverage. **Example Codegen Output** View Op: ``` ::std::vector<at::Tensor> split_Tensor(c10::DispatchKeySet ks, const at::Tensor & self, int64_t split_size, int64_t dim) { auto self_ = at::functionalization::impl::unwrapFunctionalTensor(self); ::std::vector<at::Tensor> out; { at::AutoDispatchBelowFunctionalize guard; auto tmp_output = at::redispatch::split(ks & c10::after_func_keyset, self_, split_size, dim); out = at::functionalization::impl::wrapFunctionalTensor(tmp_output); // I'm fusing the [alias removal], [mutation removal], [add views back] passes together. // Later, we'll want to turn them into separate passes (since e.g. vulkan only cares about alias removal). } at::functionalization::ViewMeta view_meta = at::functionalization::ViewMeta( [split_size, dim](const at::Tensor& base, int64_t mutated_view_idx) -> at::Tensor { return base.split(split_size, dim)[mutated_view_idx]; }, [split_size, dim](const at::Tensor& base, const at::Tensor& mutated_view, int64_t mutated_view_idx) -> at::Tensor { return at::functionalization::impl::split_inverse(base, mutated_view, mutated_view_idx, split_size, dim); } ); at::functionalization::impl::set_view_meta(out, self, view_meta); at::AutoDispatchDirectlyToNative native_guard; ::std::vector<at::Tensor> reference_tensor_output = at::native::split(self, split_size, dim); at::functionalization::impl::set_strides(out, reference_tensor_output); return out; } ``` Mutation Op: ``` at::Tensor & add__Tensor(c10::DispatchKeySet ks, at::Tensor & self, const at::Tensor & other, const at::Scalar & alpha) { at::functionalization::impl::sync(self); at::functionalization::impl::sync(other); auto self_ = at::functionalization::impl::unwrapFunctionalTensor(self); auto other_ = at::functionalization::impl::unwrapFunctionalTensor(other); at::Tensor tmp_output; { at::AutoDispatchBelowFunctionalize guard; // The functionalization pass explicitly doesn't pass out= parameters to the redispatch tmp_output = at::redispatch::add( ks & c10::after_func_keyset, self_, other_, alpha); } self.replace_(tmp_output); at::functionalization::impl::maybe_add_update(self); return self; } ``` View + Mutation Op: ``` at::Tensor & transpose_(c10::DispatchKeySet ks, at::Tensor & self, int64_t dim0, int64_t dim1) { at::functionalization::ViewMeta view_meta = at::functionalization::ViewMeta( [dim0, dim1](const at::Tensor& base, int64_t mutated_view_idx) -> at::Tensor { return base.transpose(dim0, dim1); }, [dim0, dim1](const at::Tensor& base, const at::Tensor& mutated_view, int64_t mutated_view_idx) -> at::Tensor { return at::functionalization::impl::transpose_inverse(base, mutated_view, dim0, dim1); } ); at::functionalization::impl::mutate_view_meta(self, view_meta); // See Note [Propagating strides in the functionalization pass] // Directly update the sizes/strides/storage_offset fields on self using the inplace call. // I need the guard because I don't want the at::native kernel to end up calling more functionalization/functorch kernels. // Its only job is to directly compute the output size/stride/storage_offset metadata. at::AutoDispatchDirectlyToNative native_guard; at::native::transpose_(self, dim0, dim1); return self; } ``` Test Plan: Imported from OSS Reviewed By: albanD Differential Revision: D31942093 Pulled By: bdhirsh fbshipit-source-id: b95598dae35dd1842fa8b1d8d1448332f3afaadf