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+# MLIR Reduce
+
+[TOC]
+
+A mlir module may contain unexpected behaviors after series of transformations.
+To root cause the problem or help verification after fixes, we may want to have
+a smaller case to work with. This document describes how we can narrow down a
+mlir module.
+
+## MLIR Reduce
+
+`mlir-reduce` supports reducing the module in several ways, e.g., rewrite
+complicated patterns into simpler form. To use it, the first thing you need to
+do is, provide a command which tells if a module is still the interesting case
+as the original module. Afterwards, select your reduction strategy then
+`mlir-reduce` will do the remining works for you. It's similar to
+[bugpoint](https://llvm.org/docs/CommandGuide/bugpoint.html).
+
+### Remove redundant operations
+
+`mlir-reduce` will try to remove a redundant operation by either eliminating it
+directly or applying some patterns. This is the most aggressive reduction way
+as it may result in an invalid module and we may end up retaining the error
+message. To avoid that, `mlir-reduce` always checks the validity and it expects
+the user will provide a valid module as well.
+
+### Reduce complicated patterns into simpler forms
+
+In some cases, you may want to rewrite an operation into a form you know which
+will not affect the problem you're debugging and it's simpler and smaller. For
+example, you may want to rewrite a tensor<?xindex> with unknown rank into a
+constant rank one like tensor<1xi32>. This may introduce further reduction
+chances because of precise type information.
+
+MLIR supports abundant dialects and `mlir-reduce` supports custom rewrite
+patterns for every dialect as well. To do that, you need to implement the
+`DialectReductionPatternInterface`. For example,
+
+```c++
+#include "mlir/Reducer/ReductionPatternInterface.h"
+
+struct MyReductionPatternInterface : public DialectReductionPatternInterface {
+  virtual void
+  populateReductionPatterns(RewritePatternSet &patterns) const final {
+    populateMyReductionPatterns(patterns);
+  }
+}
+```
+
+`mlir-reduce` will call `populateReductionPatterns` to collect the reduction
+rewrite patterns provided by each dialect. Here's a hint, if you use
+[DRR](../DeclarativeRewrites.md) to write the reduction patterns, you can
+leverage the method `populateWithGenerated` generated by `mlir-tblgen`.
+
+### Reduce with built-in passes
+
+MLIR provides amount of tranformation passes and some of them are useful for
+reducing the module size, e.g., Symbol-DCE. You can just use them to reduce the
+module.
+
+## How to Use it
+
+So far we provide two modes, the first one is reduction-tree, it will generate
+several reduced modules and try to do further reduction in between them. It will
+form a tree structure which tells how each intermediate module is reduced.
+According to different tree traversal mode, it may lead to different result. You
+can run with `-reduction-tree='traversal-mode=0'` for example.
+
+The second mode is opt-reduction-pass, it applies the given passes to reduce the
+module. Run it with arg `-opt-reduction-pass='opt-pass=symbol-dce'`.
+
+### Write the script for testing interesting
+
+As mentioned, you need to provide a command to hint `mlir-reduce` which case
+you're intersting. For each intermediate module generated during reduction,
+`mlir-reduce` will run the command over the intermediate module, return 1 for
+interesting case, 0 otherwise. The sample script,
+
+```shell
+mlir-opt -convert-vector-to-spirv $1 | grep "failed to materialize"
+if [[ $? -eq 0 ]]; then
+  exit 1
+else
+  exit 0
+fi
+```
+
+The sample command looks like `mlir-reduce $MLIR_MODULE
+-reduction-tree='traversal-mode=0 test=$TEST_SCRIPT'` note that the `test`
+argument is part of the mode argument.
+
+### Use with custom dialects
+
+To run with custom dialects, you need to build your own `mlir-reduce` and load
+the desired dialects. You need to link it with `MLIRReduceLib` and implement it
+like,
+
+```c++
+#include "mlir/Tools/mlir-reduce/MlirReduceMain.h"
+using namespace mlir;
+
+int main(int argc, char **argv) {
+  DialectRegistry registry;
+  registerMyDialects(registry);
+  MLIRContext context(registry);
+  return failed(mlirReduceMain(argc, argv, context));
+}
+
+```
+
+## Future works
+
+`mlir-reudce` is missing several features,
+* `-reduction-tree` now only supports `Single-Path` traversal mode, extends it
+  with different traveral strategies may reduce the module better.
+* Doesn't keep the intermediate optimal reduced module. The reduction process
+  may take a quite long time, it'll be better to interrupt it and we still can
+  get a reduced module so far.
+* Can't select the reduction patterns through command-line. For different cases,
+  we may only want to apply the part of reduction patterns.