Lunderberg opened a new pull request, #16067:
URL: https://github.com/apache/tvm/pull/16067
Some Relax operators have requirements regarding their AST that are stronger
than are checked by the C++ types being used. These are similar to checks that
are present in the `tvm::relax::WellFormed` utility, such as checks forbidding
the use of undefined variables, which are also stronger than required by the
underlying C++ types. However, because every operator may have unique
requirements, it would be unreasonable to expect a writer of a
`relax::ExprMutator` to be aware of and to maintain all such requirements.
This PR introduces an operation operator attribute `FNormalize`. If
defined, this function is used to apply an operator-specific normalization.
The implementation of `FNormalize` has the following design decisions.
* If no change is required, `FNormalize` should return the input argument
unmodified.
* `FNormalize` is only responsible for normalization of the operator itself.
The expression it returns may be unnormalized (e.g. contain nested
expressions).
* `FNormalize` receives the `BlockBuilder` as an argument, to allow
context-dependent normalization.
For example, an operator whose normalization requires in-line expressions
may use `BlockBuilder::LookupBinding` to perform variable replacement.
* `FNormalize` is applied after `FInferStructInfo`. `FNormalize` may assume
that the `relax::Call` passed to `FNormalize` has well-defined struct info.
* Corollary: `FInferStructInfo` may not assume that its `relax::Call`
argument has been passed through `FNormalize`.
This is a reasonable requirement, because (1) shape inference should
depend only on the struct info of arguments and not the values themselves, and
(2) this only impacts operators that use `FNormalize`.
* `FNormalize` should not be used to apply simplifications, and should be
limited to cases where the same computation may be expressed in multiple
manners.
For example, replacing a by-variable tuple with an in-line tuple in
`R.call_tir` is a form of normalization, but replacing `R.add(arg, R.const(0))`
with `arg` is a form of simplification.
This separation is to ensure that `FNormalize` has minimal overhead, as
some simplifications may have large computational costs, and `FNormalize` is
applied as part of all `ExprMutator` usage. A later PR will introduce an
attribute `FSimplify`, along with a dedicated pass to apply simplifications.
* Use of `FNormalize` is suppressed while parsing TVMScript. TVMScript must
be able to generate test cases that trigger specific failure modes, and that
may include producing un-normalized relax IR. In addition, TVMScript must be
stable when passed through a round-trip from IR to text to IR.
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