[mlir][scf] Canonicalize nested scf.if's to scf.if + arith.and

Differential Revision: https://reviews.llvm.org/D115930

mlir/lib/Dialect/SCF/SCF.cpp

@@ -1596,14 +1596,60 @@ struct RemoveEmptyElseBranch : public OpRewritePattern<IfOp> {
   }
 };
 
+/// Convert nested `if`s into `arith.andi` + single `if`.
+///
+///    scf.if %arg0 {
+///      scf.if %arg1 {
+///        ...
+///        scf.yield
+///      }
+///      scf.yield
+///    }
+///  becomes
+///
+///    %0 = arith.andi %arg0, %arg1
+///    scf.if %0 {
+///      ...
+///      scf.yield
+///    }
+struct CombineNestedIfs : public OpRewritePattern<IfOp> {
+  using OpRewritePattern<IfOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(IfOp op,
+                                PatternRewriter &rewriter) const override {
+    // Both `if` ops must not yield results and have only `then` block.
+    if (op->getNumResults() != 0 || op.elseBlock())
+      return failure();
+
+    auto nestedOps = op.thenBlock()->without_terminator();
+    // Nested `if` must be the only op in block.
+    if (!llvm::hasSingleElement(nestedOps))
+      return failure();
+
+    auto nestedIf = dyn_cast<IfOp>(*nestedOps.begin());
+    if (!nestedIf || nestedIf->getNumResults() != 0 || nestedIf.elseBlock())
+      return failure();
+
+    Location loc = op.getLoc();
+    Value newCondition = rewriter.create<arith::AndIOp>(loc, op.condition(),
+                                                        nestedIf.condition());
+    auto newIf = rewriter.create<IfOp>(loc, newCondition);
+    Block *newIfBlock = newIf.thenBlock();
+    rewriter.eraseOp(newIfBlock->getTerminator());
+    rewriter.mergeBlocks(nestedIf.thenBlock(), newIfBlock);
+    rewriter.eraseOp(op);
+    return success();
+  }
+};
+
 } // namespace
 
 void IfOp::getCanonicalizationPatterns(RewritePatternSet &results,
                                        MLIRContext *context) {
-  results
-      .add<RemoveUnusedResults, RemoveStaticCondition, ConvertTrivialIfToSelect,
-           ConditionPropagation, ReplaceIfYieldWithConditionOrValue, CombineIfs,
-           RemoveEmptyElseBranch>(context);
+  results.add<CombineIfs, CombineNestedIfs, ConditionPropagation,
+              ConvertTrivialIfToSelect, RemoveEmptyElseBranch,
+              RemoveStaticCondition, RemoveUnusedResults,
+              ReplaceIfYieldWithConditionOrValue>(context);
 }
 
 Block *IfOp::thenBlock() { return &getThenRegion().back(); }

mlir/test/Dialect/SCF/canonicalize.mlir

@@ -429,6 +429,24 @@ func @replace_false_if_with_values() {
 
 // -----
 
+// CHECK-LABEL: @merge_nested_if
+// CHECK-SAME: (%[[ARG0:.*]]: i1, %[[ARG1:.*]]: i1)
+func @merge_nested_if(%arg0: i1, %arg1: i1) {
+// CHECK: %[[COND:.*]] = arith.andi %[[ARG0]], %[[ARG1]]
+// CHECK: scf.if %[[COND]] {
+// CHECK-NEXT: "test.op"()
+  scf.if %arg0 {
+    scf.if %arg1 {
+      "test.op"() : () -> ()
+      scf.yield
+    }
+    scf.yield
+  }
+  return
+}
+
+// -----
+
 // CHECK-LABEL: @remove_zero_iteration_loop
 func @remove_zero_iteration_loop() {
   %c42 = arith.constant 42 : index