Separate CHLO transforms for expanding compositions and lowering broadcasts.

* The former is typically invariant regardless of backend.
* The latter may need to be done differently depending on capabilities of the lowering target.

PiperOrigin-RevId: 374492924

include/mlir-hlo/Dialect/mhlo/transforms/mhlo_passes.td

@@ -19,8 +19,10 @@ def ChloLegalizeToHloPass : FunctionPass<"chlo-legalize-to-hlo"> {
   let summary = "Legalize CHLO to HLO.";
   let constructor = "createChloLegalizeToHloPass()";
   let options = [
-    Option<"broadcast_only_", "broadcast-only", "bool",
+    Option<"legalize_broadcasts_", "legalize-broadcasts", "bool",
-           /*default=*/"false", "Only lower broadcasting chlo to non-broadcasting equivalents">,
+           /*default=*/"true", "Legalize implicit broadcasts to explicit HLO broadcasting forms">,
+    Option<"expand_compositions_", "expand-compositions", "bool",
+           /*default=*/"true", "Expands client-centric compositions to HLO primitives">,
   ];
 }
 

include/mlir-hlo/Dialect/mhlo/transforms/passes.h

@@ -46,7 +46,7 @@ std::unique_ptr<OperationPass<FuncOp>> createLegalizeToStdPass();
 
 /// Lowers from the CHLO dialect to the HLO dialect.
 std::unique_ptr<FunctionPass> createChloLegalizeToHloPass(
-    bool broadcast_only = false);
+    bool legalize_broadcasts = true, bool expand_compositions = true);
 
 /// Lowers from HLO dialect to LHLO dialect allocating/deallocating temporary
 /// buffers if necessary.

include/mlir-hlo/Dialect/mhlo/transforms/rewriters.h

@@ -116,9 +116,10 @@ void PopulateChloBroadcastingPatterns(MLIRContext *context,
                                       OwningRewritePatternList *patterns);
 
 // Populates a collection of conversion patterns for legalizing client-HLO to
-// HLO. Includes decomposition of operations and inserting of explicit
+// HLO by decomposing client-operations to corresponding sequences of more
-// broadcasts.
+// primitive operations. This does not include the
-void PopulateLegalizeChloToHloPatterns(MLIRContext *context,
+// PopulateChloBroadcastingPatterns above.
+void PopulateDecomposeChloPatterns(MLIRContext *context,
                                    OwningRewritePatternList *patterns);
 
 }  // namespace chlo

lib/Dialect/mhlo/transforms/chlo_legalize_to_hlo.cc

@@ -1234,17 +1234,16 @@ void PopulateChloBroadcastingPatterns(MLIRContext *context,
   PopulateForBroadcastingBinaryOp<ConvertRankedDynamicBroadcastBinaryOp>(
       context, patterns, 5);
   patterns->insert<ConvertSelectOp>(context);
+  patterns->insert<ConvertConstantLikeOp>(context);
 }
 
-void PopulateLegalizeChloToHloPatterns(MLIRContext *context,
+void PopulateDecomposeChloPatterns(MLIRContext *context,
                                    OwningRewritePatternList *patterns) {
   populateWithGenerated(*patterns);
-  PopulateChloBroadcastingPatterns(context, patterns);
 
   // Other patterns.
   // clang-format off
-  patterns->insert<ConvertConstantLikeOp,
+  patterns->insert<ConvertDigammaOp,
-                   ConvertDigammaOp,
                    ConvertErfOp,
                    ConvertErfcOp,
                    ConvertLgammaOp,

lib/Dialect/mhlo/transforms/chlo_legalize_to_hlo_pass.cc

@@ -31,10 +31,12 @@ namespace {
 
 struct ChloLegalizeToHloPass
     : public ChloLegalizeToHloPassBase<ChloLegalizeToHloPass> {
-  explicit ChloLegalizeToHloPass(bool broadcast_only)
+  explicit ChloLegalizeToHloPass(bool legalize_broadcasts,
+                                 bool expand_compositions)
       : ChloLegalizeToHloPassBase<
             ChloLegalizeToHloPass>::ChloLegalizeToHloPassBase() {
-    this->broadcast_only_ = broadcast_only;
+    this->legalize_broadcasts_ = legalize_broadcasts;
+    this->expand_compositions_ = expand_compositions;
   }
 
   void getDependentDialects(DialectRegistry &registry) const override {
@@ -53,13 +55,15 @@ struct ChloLegalizeToHloPass
         mlir::shape::ShapeDialect, mlir::scf::SCFDialect>();
     conversionTarget.addLegalOp<chlo::MinimumBroadcastShapesOp>();
 
-    if (broadcast_only_) {
+    if (legalize_broadcasts_) {
       chlo::PopulateChloBroadcastingPatterns(&getContext(),
                                              &conversionPatterns);
-      conversionTarget.addLegalOp<chlo::ZetaOp, chlo::PolygammaOp>();
+    }
+
+    if (expand_compositions_) {
+      chlo::PopulateDecomposeChloPatterns(&getContext(), &conversionPatterns);
     } else {
-      chlo::PopulateLegalizeChloToHloPatterns(&getContext(),
+      conversionTarget.addLegalOp<chlo::ZetaOp, chlo::PolygammaOp>();
-                                              &conversionPatterns);
     }
 
     if (failed(applyPartialConversion(getOperation(), conversionTarget,
@@ -71,8 +75,10 @@ struct ChloLegalizeToHloPass
 
 }  // namespace
 
-std::unique_ptr<FunctionPass> createChloLegalizeToHloPass(bool broadcast_only) {
+std::unique_ptr<FunctionPass> createChloLegalizeToHloPass(
-  return std::make_unique<ChloLegalizeToHloPass>(broadcast_only);
+    bool legalize_broadcasts, bool expand_compositions) {
+  return std::make_unique<ChloLegalizeToHloPass>(legalize_broadcasts,
+                                                 expand_compositions);
 }
 
 }  // namespace mhlo

lib/Dialect/mhlo/transforms/chlo_legalize_to_hlo_patterns.td

@@ -14,6 +14,10 @@ limitations under the License.
 ==============================================================================*/
 
 // This is the legalization pattern definition file for CHLO to MHLO.
+// These are included in the PopulateDecomposeChloPatterns factory
+// and should only include canonical expansions which are not actually
+// ambiguous/different for various backends. Avoid patterns that are actually
+// lowering to non-canonical forms.
 
 include "mlir/IR/OpBase.td"
 include "mlir-hlo/Dialect/mhlo/IR/hlo_ops.td"

tests/chlo_legalize_to_hlo_broadcasts.mlir

@@ -1,4 +1,4 @@
-// RUN: mlir-hlo-opt -chlo-legalize-to-hlo="broadcast-only=true" -cse -canonicalize -split-input-file -verify-diagnostics %s -o - | FileCheck %s
+// RUN: mlir-hlo-opt -chlo-legalize-to-hlo="legalize-broadcasts=true expand-compositions=false" -cse -canonicalize -split-input-file -verify-diagnostics %s -o - | FileCheck %s
 
 // Check the non-broadcast case for each registered op, then just check a
 // representative op for detailed broadcast semantics.