[mlir][Transforms] GreedyPatternRewriteDriver: Add flag to control constant CSE'ing

By default, the greedy pattern rewrite driver CSE's constant ops. If an op is CSE'd with an op in a parent region, the op is effectively "hoisted". Over the last years, users have described situations where this is not desirable/necessary. This commit adds a new flag to `GreedyRewriteConfig` that controls CSE'ing of constants. For testing purposes, it is also exposed as a canonicalizer pass flag.

Author: matthias-springer

mlir/include/mlir/Transforms/GreedyPatternRewriteDriver.h

@@ -47,6 +47,10 @@ class GreedyRewriteConfig {
   /// Note: Only applicable when simplifying entire regions.
   bool enableRegionSimplification = true;
 
+  /// If set to "true", constants are CSE'd (even across multiple regions that
+  /// are in a parent-ancestor relationship).
+  bool cseConstants = true;
+
   /// This specifies the maximum number of times the rewriter will iterate
   /// between applying patterns and simplifying regions. Use `kNoLimit` to
   /// disable this iteration limit.

mlir/include/mlir/Transforms/Passes.td

@@ -35,6 +35,8 @@ def Canonicalizer : Pass<"canonicalize"> {
     Option<"enableRegionSimplification", "region-simplify", "bool",
            /*default=*/"true",
            "Perform control flow optimizations to the region tree">,
+    Option<"cseConstants", "cse-constants", "bool", /*default=*/"true",
+           "CSE constant operations">,
     Option<"maxIterations", "max-iterations", "int64_t",
            /*default=*/"10",
            "Max. iterations between applying patterns / simplifying regions">,

mlir/lib/Transforms/Canonicalizer.cpp

@@ -33,6 +33,7 @@ struct Canonicalizer : public impl::CanonicalizerBase<Canonicalizer> {
       : config(config) {
     this->topDownProcessingEnabled = config.useTopDownTraversal;
     this->enableRegionSimplification = config.enableRegionSimplification;
+    this->cseConstants = config.cseConstants;
     this->maxIterations = config.maxIterations;
     this->maxNumRewrites = config.maxNumRewrites;
     this->disabledPatterns = disabledPatterns;
@@ -45,6 +46,7 @@ struct Canonicalizer : public impl::CanonicalizerBase<Canonicalizer> {
     // Set the config from possible pass options set in the meantime.
     config.useTopDownTraversal = topDownProcessingEnabled;
     config.enableRegionSimplification = enableRegionSimplification;
+    config.cseConstants = cseConstants;
     config.maxIterations = maxIterations;
     config.maxNumRewrites = maxNumRewrites;
 

mlir/lib/Transforms/Utils/GreedyPatternRewriteDriver.cpp

@@ -848,13 +848,13 @@ LogicalResult RegionPatternRewriteDriver::simplify(bool *changed) && {
     if (!config.useTopDownTraversal) {
       // Add operations to the worklist in postorder.
       region.walk([&](Operation *op) {
-        if (!insertKnownConstant(op))
+        if (!config.cseConstants || !insertKnownConstant(op))
           addToWorklist(op);
       });
     } else {
       // Add all nested operations to the worklist in preorder.
       region.walk<WalkOrder::PreOrder>([&](Operation *op) {
-        if (!insertKnownConstant(op)) {
+        if (!config.cseConstants || !insertKnownConstant(op)) {
           addToWorklist(op);
           return WalkResult::advance();
         }

mlir/test/Pass/run-reproducer.mlir

@@ -14,7 +14,7 @@ func.func @bar() {
   external_resources: {
     mlir_reproducer: {
       verify_each: true,
-      // CHECK:  builtin.module(func.func(cse,canonicalize{ max-iterations=1 max-num-rewrites=-1 region-simplify=false test-convergence=false top-down=false}))
+      // CHECK:  builtin.module(func.func(cse,canonicalize{cse-constants=true max-iterations=1 max-num-rewrites=-1 region-simplify=false test-convergence=false top-down=false}))
       pipeline: "builtin.module(func.func(cse,canonicalize{max-iterations=1 max-num-rewrites=-1 region-simplify=false top-down=false}))",
       disable_threading: true
     }

mlir/test/Transforms/test-canonicalize.mlir

@@ -1,5 +1,6 @@
 // RUN: mlir-opt %s -pass-pipeline='builtin.module(func.func(canonicalize))' | FileCheck %s
 // RUN: mlir-opt %s -pass-pipeline='builtin.module(func.func(canonicalize{region-simplify=false}))' | FileCheck %s --check-prefixes=CHECK,NO-RS
+// RUN: mlir-opt %s -pass-pipeline='builtin.module(func.func(canonicalize{cse-constants=false}))' | FileCheck %s --check-prefixes=NO-CSE
 
 // CHECK-LABEL: func @remove_op_with_inner_ops_pattern
 func.func @remove_op_with_inner_ops_pattern() {
@@ -89,3 +90,16 @@ func.func @test_region_simplify() {
 ^bb1:
   return
 }
+
+// CHECK-LABEL: do_not_cse_constant
+// CHECK: %[[c0:.*]] = arith.constant 0 : index
+// CHECK: return %[[c0]], %[[c0]]
+// NO-CSE-LABEL: do_not_cse_constant
+// NO-CSE: %[[c0:.*]] = arith.constant 0 : index
+// NO-CSE: %[[c1:.*]] = arith.constant 0 : index
+// NO-CSE: return %[[c0]], %[[c1]]
+func.func @do_not_cse_constant() -> (index, index) {
+  %0 = arith.constant 0 : index
+  %1 = arith.constant 0 : index
+  return %0, %1 : index, index
+}