llvm-project

8c810641 - [MLIR][Arith] Fix index_cast/index_castui chain folding to check intermediate width (#189042)

[MLIR][Arith] Fix index_cast/index_castui chain folding to check intermediate width (#189042) The patterns `IndexCastOfIndexCast` and `IndexCastUIOfIndexCastUI` in ArithCanonicalization.td incorrectly eliminated a pair of index casts whenever the outer result type equalled the original source type, without verifying that the intermediate cast was lossless. For example, the following was wrong folded to `%arg0`: %0 = index_castui %arg0 : i64 to index %1 = index_castui %0 : index to i8 ← truncates to 8 bits %2 = index_castui %1 : i8 to index ← incorrectly removed The pattern matched `%1`/`%2` because `i8.to(index)` has the same result type as `i64.to(index)`, even though the i8 intermediate silently drops 56 bits. The same bug existed for the signed `index_cast` variant. Fix: move the optimization into the `fold` methods of `IndexCastOp` and `IndexCastUIOp` with an explicit check that the intermediate type is at least as wide as the source type (using `IndexType::kInternalStorageBitWidth` as the representative width for `index`). Only then is the round-trip guaranteed lossless and the chain can be collapsed. Fixes #90238 Fixes #90296 Assisted-by: Claude Code