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[GitHub] [tvm-rfcs] areusch commented on a diff in pull request #77: [RFC] Buffer Layout Padding
areusch commented on code in PR #77:
URL: https://github.com/apache/tvm-rfcs/pull/77#discussion_r891581768
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rfcs/0077-layout-transform-padding.md:
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+- Feature Name: Layout Transformation Padding Roadmap
+- Authors: [Eric Lunderberg](https://github.com/Lunderberg/),
+ [Chris Sullivan](https://github.com/csullivan),
+ [Wuwei Lin](https://github.com/vinx13/),
+ [Junru Shao](https://github.com/junrushao1994)
+- Start Date: 2022-06-06
+- RFC PR: [apache/tvm-rfcs#0077](https://github.com/apache/tvm-rfcs/pull/0077)
+- GitHub Issue: TBD
+
+# Table of contents
+- [Table of contents](#table-of-contents)
+- [Summary](#summary)
+- [Motivation](#motivation)
+- [Guide-level explanation](#guide-level-explanation)
+ - [Padded Transformations](#padded-transformations)
+ - [Defining Padded Values](#defining-padded-values)
+ - [Overcompute vs Branching](#overcompute-vs-branching)
+- [Reference-level explanation](#reference-level-explanation)
+ - [TIR Changes](#tir-changes)
+ - [Buffer Annotation of Padding Predicate/Constraint Pairs](#buffer-annotation-of-padding-predicateconstraint-pairs)
+ - [New TIR Op, `tir::builtin::arbitrary`](#new-tir-op-tirbuiltinarbitrary)
+ - [Buffer Annotation of Layout Transforms](#buffer-annotation-of-layout-transforms)
+ - [Transformations/Metaschedule Primitives](#transformationsmetaschedule-primitives)
+ - [Enhancement - transform_layout](#enhancement---transform_layout)
+ - [New Primitive - Add buffer constraint](#new-primitive---add-buffer-constraint)
+ - [New Primitive - Reorder Loops According to Buffer](#new-primitive---reorder-loops-according-to-buffer)
+ - [Enhancement - Predicate for DomainTouched](#enhancement---predicate-for-domaintouched)
+ - [Enhancement - Remove No Op](#enhancement---remove-no-op)
+ - [Enhancement - Simplify](#enhancement---simplify)
+ - [New Transform - Hoist Expression](#new-transform---hoist-expression)
+ - [New Transform - Reduce Loop Extents](#new-transform---reduce-loop-extents)
+ - [Utility - Merge Adjacent Loops](#utility---merge-adjacent-loops)
+ - [New Primitive - Remove Branching Through Overcompute](#new-primitive---remove-branching-through-overcompute)
+ - [New Primitive - Remove Overcompute Through Branching](#new-primitive---remove-overcompute-through-branching)
+ - [New Lowering Transform - Remove T.Arbitrary](#new-lowering-transform---remove-tarbitrary)
+ - [Implementation options](#implementation-options)
+ - [Never write to transformation padding](#never-write-to-transformation-padding)
+ - [Never read from transformation padding](#never-read-from-transformation-padding)
+ - [Allocate internal buffer containing transformation padding](#allocate-internal-buffer-containing-transformation-padding)
+ - [Explicitly write next operator's desired default at end of function](#explicitly-write-next-operators-desired-default-at-end-of-function)
+ - [Implicitly write default value of next operator](#implicitly-write-default-value-of-next-operator)
+ - [Apply operator element-wise over the transformation padding](#apply-operator-element-wise-over-the-transformation-padding)
+ - [Multiple Buffer Semantics](#multiple-buffer-semantics)
+ - [Points of Communication](#points-of-communication)
+- [Drawbacks](#drawbacks)
+- [Rationale and alternatives](#rationale-and-alternatives)
+- [Prior art](#prior-art)
+- [Unresolved questions](#unresolved-questions)
+- [Future possibilities](#future-possibilities)
+
+# Summary
+[summary]: #summary
+
+Buffer layout transformations can require padding in the transformed
+buffer. The efficiency of an operator depends on the semantics used
+for loads and stores to values in the required padding. The choice of
+buffer semantics can reduce branch divergence and avoid repeated
+setting of default values, but also imposes constraints between the
+producer and consumer of a buffer.
+
+This RFC discusses a general plan for specifying buffer semantics to
+be used, and the constraints imposed. Subsequent RFCs will follow
+describing the design for support of each of the semantics proposed in
+this roadmap.
+
+# Motivation
+[motivation]: #motivation
+
+Suppose a buffer of shape `[14]` is transformed such that each index
+`i` is mapped to `[i//4, i%4]`. The first index can range from 0
+(`0//4`) to 3 (`14//4`), and the second index can range from 0 (`0%4`)
+to 3 (`3%4`). Therefore, the transformed shape is `[4,4]`. However,
+this has 16 elements, because the transformed coordinates `(3,2)` and `(3,3)` do
+not have a corresponding index on the workload range `0 <= i < 14`. The final
+result in these locations is not determined by the compute definition,
+so we have flexibility in what to store in the padding that is
+introduced by the transformation, and what assumptions can be made
+when reading from those locations.
+
+For example, an element-wise function may be most efficiently written
+using vectorized instructions over all values, regardless of whether
+they exist in the compute definition. Or a maxpool may be most
+efficiently written if input tensors have `-INF` stored in the
+transformation padding. Satisfying both of these at the same time may
+not be possible. While the compute definition doesn't impose
+constraints on the values in the transformation padding, there are
+still constraints imposed by the usage of those values by different
+operators.
+
+
+```
+ ┌─Logical-index-space───────────────────┐
+ │ │
+┌▼─┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬─▼┌──┬──┐
+│00│01│02│03│04│05│06│07│08│09│10│11│12│13│14│15│
+└▲─┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴─▲┘
+ │ │
+ └─Physical-index-space────────────────────────┘
+
+ ┌─Transformed-index-space─┐
+ │ │
+ │ ┌────┬────┬────┬───▼┐
+ │ │ 00 │ 01 │ 02 │ 03 │
+ │ ├────┼────┼────┼────┤
+ │ │ 04 │ 05 │ 06 │ 07 │
+ │ ├────┼────┼────┼────┤
+ │ │ 08 │ 09 │ 10 │ 11 │
+ │ ├────┼────┼────┼────┤
+ └──────► 12 │ 13 │ 14 │ 15 │
+ └────┴────┴────┴────┘
+```
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+## Padded Transformations
+
+In general, a transformation will introduce the minimum amount of
+padding such that all values in the original buffer can be stored in
+the layout specified. As a result, whether a transformation
+introduces padding depends on the transformation being applied and the
+buffer shape on which it is being applied. For example, consider a
+schedule that contains tensor `A` with shape `[16]` and tensor `B` with shape
+`[14]`.
+
+```python
+# This transformation does not introduce padding. The original shape
+# of [16] produces the transformed shape [2,8], which contains the
+# original 16 values no additional padding.
+sched[A].transform_layout(lambda i: [i//8, i%8])
+
+# This transform introduces padding. The original shape of [14] also
+# produces the transformed shape [2,8], which contains the original 14
+# values and an additional 2 values of padding. These are located at
+# transformed indices [1,6] and [1,7].
+sched[B].transform_layout(lambda i: [i//8, i%8])
+```
+
+The above example introduces padding at the end of a buffer. By
+including an offset in the layout transformation, we can instead place
+the padding at the beginning of a buffer.
+
+```python
+# This transform introduces padding. For 0 <= i < 14, the transformed
+# index (i+2)//8 can have values of 0 or 1, so the transformed shape
+# is [2,8]. There are no valid values of i that would produce [0,0]
+# or [0,1], so these transformed indices contain padding.
+sched[B].transform_layout(lambda i: [(i+2)//8, (i+2)%8])
+```
+
+In addition to moving the location of the padded indices, use of an
+offset in a layout transformation can introduce additional padding.
+
+```python
+# This transformation introduces padding. For 0 <= i < 16, the
+# transformed index (i+2)//8 can have values of 0, 1, or 2, so the
+# transformed shape is [3,8]. Padding is introduced from [0,0] to
+# [0,1], and from [2,2] to [2,7].
+sched[A].transform_layout(lambda i: [(i+2)//8, (i+2)%8])
+```
+
+
+## Defining Padded Values
+
+When a buffer is transformed, the majority of values in the
+transformed buffer are constrained to have the corresponding value in
+the original buffer. However, when a buffer is padded to meet some
+alignment criteria, these additional padded values have no such
+constraint.
+
+To specify the values stored in the padding, the `transform_layout`
+function takes an optional argument `pad_value` that
+specifies the value that should be present in the padding. This
+should be a function that maps from transformed indices to an
+`Optional[PrimExpr]`.
+
+```python
+# B.shape is [14]
+transform = lambda i: [i//4, i%4]
+
+# Three equivalent calls to perform the same layout transformation.
+# Padding is introduced, but access of the padding is forbidden.
+sched[B].transform_layout(transform)
+sched[B].transform_layout(transform, pad_value=None)
+sched[B].transform_layout(transform, pad_value=lambda io,ii: None)
+
+# Padding is introduced, and contains zeros.
+sched[B].transform_layout(transform, pad_value=0.0)
+sched[B].transform_layout(transform, pad_value=lambda io,ii: 0.0)
+
+# Padding is introduced, and contains arbitrary values.
+sched[B].transform_layout(transform, pad_value=tir.arbitrary(dtype="float32"))
+sched[B].transform_layout(transform, pad_value=lambda io,ii: tir.arbitrary(dtype="float32"))
+
+# Padding is introduced, and wraps to the beginning of the array.
+sched[B].transform_layout(transform, pad_value=lambda io,ii: B[0, (io-14)%4])
+```
+
+The `Buffer` object stores a predicate to identify which indices
+contain padding, along with the expression given in `pad_value`. This
+expression may only contain constants and the transformed buffer
+itself, and may not introduce dependencies on another buffer.
+
+For a producer of the transformed buffer, if `pad_value` is defined,
+the padding value must be written to the padding prior to the
+completion of the operator. Effectively, the producer must have a
+postlude as follows:
+
+```python
+for transformed_indices in T.grid(*transformed_shape):
+ if padding_predicate(*transformed_indices):
+ B[transformed_indices] = pad_value(*transformed_indices)
+```
+
+For a consumer of the transformed buffer, these padding values are
+initially unused, but may be used in later simplifications.
+
+## Overcompute vs Branching
+
+Depending on the computation being performed and the value stored in
+the padding, there can be trade-offs between branching and
+overcompute. For example, consider the following `PrimFunc`, which
+computes the sum over each row of the input data.
+
+```python
+@T.prim_func
+def row_summation(a: T.handle, b: T.handle):
+ A = T.match_buffer(shape=(16, 14), dtype="float32")
+ B = T.match_buffer(shape=(16,), dtype="float32")
+ for i in T.serial(16):
+ B[i] = 0.0
+ for j in T.serial(14):
+ B[i] = B[i] + A[i, j]
+```
+
+We'd like to transform the layout of buffer `A` from `[i, j]` to `[i,
+j//4, j%4]`, along with the loop iteration. By default, after using
+the `transform_layout` and `split` metaschedule primitives, we have
+the following function.
+
+```python
+@T.prim_func
+def row_summation(a: T.handle, b: T.handle):
+ A = T.match_buffer(shape=(16, 4, 4), dtype="float32")
+ B = T.match_buffer(shape=(16,), dtype="float32")
+ for i in T.serial(16):
+ B[i] = 0.0
+ for j_outer, j_inner in T.grid(4, 4):
+ if 4*j_outer + j_inner < 14:
+ B[i] = B[i] + A[i, j_outer, j_inner]
+```
+
+If the conditional can be removed, this function would be much more
+amenable for later vectorization, or to reduce branch divergence when
+bound to a thread index. If the padding in `A` is pre-filled with
+zero, then `B[i] = B[i] + 0.0` is a no-op, and can be performed
+without changing the final computation.
+
+```python
+@T.prim_func
+def row_summation(a: T.handle, b: T.handle):
+ A = T.match_buffer(shape=(16, 4, 4), dtype="float32")
+ B = T.match_buffer(shape=(16,), dtype="float32")
+ for i in T.serial(16):
+ B[i] = 0.0
+ for j_outer, j_inner in T.grid(4, 4):
+ B[i] = B[i] + A[i, j_outer, j_inner]
+```
+
+By annotating the layout transformation with the value stored in the
+padding, this condition can be proven, allowing this conditional to
+automatically be removed. Since the tradeoff between branching and
+overcompute may or may not be beneficial dependent on the schedule,
+these options are exposed as two additional transformations,
+`tir.transform.RemoveBranchingThroughOvercompute` and
+`tir.transform.RemoveOvercomputeThroughBranching`.
+
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+## TIR Changes
+
+### Buffer Annotation of Padding Predicate/Constraint Pairs
Review Comment:
i agree it's helpful to think through alternatives here. could we consider some example transformations we may want to make (e.g. eliding or moving the operations which write to the padding) or pattern-matching on such operations and reducing them to hardware intrinsics (e.g. perhaps there is a way to tell the hardware how much padding to include when the value is always constant and a particular operation is in use).
on the one hand, modeling the padding computation explicitly in TIR is a more logical reuse of existing TIR. on the other hand, it may be more expensive to match this and the compiler may be slower.
i'm not necessarily in favor fo any one solution, but i think this is the sort of thing we should discuss to try and inform that decision.
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