86771d0b65
This patch takes the first step towards a more principled modeling of undefined behavior in MLIR as discussed in the following discourse threads: 1. https://discourse.llvm.org/t/semantics-modeling-undefined-behavior-and-side-effects/4812 2. https://discourse.llvm.org/t/rfc-mark-tensor-dim-and-memref-dim-as-side-effecting/65729 This patch in particular does the following: 1. Introduces a ConditionallySpeculatable OpInterface that dynamically determines whether an Operation can be speculated. 2. Re-defines `NoSideEffect` to allow undefined behavior, making it necessary but not sufficient for speculation. Also renames it to `NoMemoryEffect`. 3. Makes LICM respect the above semantics. 4. Changes all ops tagged with `NoSideEffect` today to additionally implement ConditionallySpeculatable and mark themselves as always speculatable. This combined trait is named `Pure`. This makes this change NFC. For out of tree dialects: 1. Replace `NoSideEffect` with `Pure` if the operation does not have any memory effects, undefined behavior or infinite loops. 2. Replace `NoSideEffect` with `NoSideEffect` otherwise. The next steps in this process are (I'm proposing to do these in upcoming patches): 1. Update operations like `tensor.dim`, `memref.dim`, `scf.for`, `affine.for` to implement a correct hook for `ConditionallySpeculatable`. I'm also happy to update ops in other dialects if the respective dialect owners would like to and can give me some pointers. 2. Update other passes that speculate operations to consult `ConditionallySpeculatable` in addition to `NoMemoryEffect`. I could not find any other than LICM on a quick skim, but I could have missed some. 3. Add some documentation / FAQs detailing the differences between side effects, undefined behavior, speculatabilty. Reviewed By: rriddle, mehdi_amini Differential Revision: https://reviews.llvm.org/D135505
An out-of-tree MLIR dialect
This is an example of an out-of-tree MLIR dialect along with a standalone opt-like tool to operate on that dialect.
Building
This setup assumes that you have built LLVM and MLIR in $BUILD_DIR and installed them to $PREFIX. To build and launch the tests, run
mkdir build && cd build
cmake -G Ninja .. -DMLIR_DIR=$PREFIX/lib/cmake/mlir -DLLVM_EXTERNAL_LIT=$BUILD_DIR/bin/llvm-lit
cmake --build . --target check-standalone
To build the documentation from the TableGen description of the dialect operations, run
cmake --build . --target mlir-doc
Note: Make sure to pass -DLLVM_INSTALL_UTILS=ON when building LLVM with CMake in order to install FileCheck to the chosen installation prefix.