[all-commits] [llvm/llvm-project] 6093c2: [mlir][Transforms] Dialect conversion: Align handl...

[Matthias Springer via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 6093c26ac9e1f6ba919c0c3e18c6425ba1a625f8
      https://github.com/llvm/llvm-project/commit/6093c26ac9e1f6ba919c0c3e18c6425ba1a625f8
  Author: Matthias Springer <me at m-sp.org>
  Date:   2024-09-12 (Thu, 12 Sep 2024)

  Changed paths:
    M mlir/lib/Transforms/Utils/DialectConversion.cpp
    M mlir/test/Transforms/test-legalize-erased-op-with-uses.mlir

  Log Message:
  -----------
  [mlir][Transforms] Dialect conversion: Align handling of dropped values (#106760)

Handle dropped block arguments and dropped op results in the same way:
build a source materialization (that may fold away if unused). This
simplifies the code base a bit and makes it possible to merge
`legalizeConvertedArgumentTypes` and `legalizeConvertedOpResultTypes` in
a future commit. These two functions are almost doing the same thing
now.

As a side effect, this commit also changes the dialect conversion such
that temporary circular cast ops are no longer generated. (There was a
workaround in #107109 that can now be removed again.) Example:
```
%0 = "builtin.unrealized_conversion_cast"(%1) : (!a) -> !b
%1 = "builtin.unrealized_conversion_cast"(%0) : (!b) -> !a
// No further uses of %0, %1.
```

This happened when:
1. An op was erased. (No replacement values provided.)
2. A conversion pattern for another op builds a replacement value for
the erased op's results (first cast op) during `remapValues`, but that
SSA value is not used during the pattern application.
3. During the finalization phase, `legalizeConvertedOpResultTypes`
thinks that the erased op is alive because of the cast op that was built
in Step 2. It builds a cast from that replacement value to the original
type.
4. During the commit phase, all uses of the original op are replaced
with the casted value produced in Step 3. We have generated circular IR.

This problem can be avoided by making sure that source materializations
are generated for all dropped results. This ensures that we always have
some replacement SSA value in the mapping. Previously, we sometimes had
a value mapped and sometimes not. (No more special casing is needed
anymore to distinguish between "value dropped" or "value replaced with
SSA value".)



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications