[flang][fir]: Add conversion of fir.if to scf.if.

flang/lib/Optimizer/Transforms/FIRToSCF.cpp

@@ -87,13 +87,52 @@ struct DoLoopConversion : public OpRewritePattern<fir::DoLoopOp> {
     return success();
   }
 };
+
+void copyBlockAndTransformResult(PatternRewriter &rewriter, Block &srcBlock,
+                                 Block &dstBlock) {
+  Operation *srcTerminator = srcBlock.getTerminator();
+  auto resultOp = cast<fir::ResultOp>(srcTerminator);
+
+  dstBlock.getOperations().splice(dstBlock.begin(), srcBlock.getOperations(),
+                                  srcBlock.begin(), std::prev(srcBlock.end()));
+
+  if (!resultOp->getOperands().empty()) {
+    rewriter.setInsertionPointToEnd(&dstBlock);
+    scf::YieldOp::create(rewriter, resultOp->getLoc(), resultOp->getOperands());
+  }
+
+  rewriter.eraseOp(srcTerminator);
+}
+
+struct IfConversion : public OpRewritePattern<fir::IfOp> {
+  using OpRewritePattern<fir::IfOp>::OpRewritePattern;
+  LogicalResult matchAndRewrite(fir::IfOp ifOp,
+                                PatternRewriter &rewriter) const override {
+    bool hasElse = !ifOp.getElseRegion().empty();
+    auto scfIfOp =
+        scf::IfOp::create(rewriter, ifOp.getLoc(), ifOp.getResultTypes(),
+                          ifOp.getCondition(), hasElse);
+
+    copyBlockAndTransformResult(rewriter, ifOp.getThenRegion().front(),
+                                scfIfOp.getThenRegion().front());
+
+    if (hasElse) {
+      copyBlockAndTransformResult(rewriter, ifOp.getElseRegion().front(),
+                                  scfIfOp.getElseRegion().front());
+    }
+
+    scfIfOp->setAttrs(ifOp->getAttrs());
+    rewriter.replaceOp(ifOp, scfIfOp);
+    return success();
+  }
+};
 } // namespace
 
 void FIRToSCFPass::runOnOperation() {
   RewritePatternSet patterns(&getContext());
-  patterns.add<DoLoopConversion>(patterns.getContext());
+  patterns.add<DoLoopConversion, IfConversion>(patterns.getContext());
   ConversionTarget target(getContext());
-  target.addIllegalOp<fir::DoLoopOp>();
+  target.addIllegalOp<fir::DoLoopOp, fir::IfOp>();
   target.markUnknownOpDynamicallyLegal([](Operation *) { return true; });
   if (failed(
           applyPartialConversion(getOperation(), target, std::move(patterns))))

flang/test/Fir/FirToSCF/if.fir

@@ -0,0 +1,57 @@
+// RUN: fir-opt %s --fir-to-scf | FileCheck %s
+
+// CHECK-LABEL:   func.func @test_only(
+// CHECK-SAME:      %[[ARG0:.*]]: i1, %[[ARG1:.*]]: i32) {
+// CHECK:           scf.if %[[ARG0]] {
+// CHECK:             %[[VAL_1:.*]] = arith.addi %[[ARG1]], %[[ARG1]] : i32
+// CHECK:           }
+// CHECK:           return
+// CHECK:         }
+func.func @test_only(%arg0 : i1, %arg1 : i32) {
+  fir.if %arg0 {
+    %0 = arith.addi %arg1, %arg1 : i32
+  }
+  return
+}
+
+// CHECK-LABEL:   func.func @test_else()   {
+// CHECK:           %[[VAL_1:.*]] = arith.constant false
+// CHECK:           %[[VAL_2:.*]] = arith.constant 2 : i32
+// CHECK:           scf.if %[[VAL_1]] {
+// CHECK:             %[[VAL_3:.*]] = arith.constant 3 : i32
+// CHECK:           } else {
+// CHECK:             %[[VAL_3:.*]] = arith.constant 3 : i32
+// CHECK:           }
+// CHECK:           return
+// CHECK:         }
+func.func @test_else()   {
+  %false = arith.constant false
+  %1 = arith.constant 2 : i32
+  fir.if %false {
+    %2 = arith.constant 3 : i32
+  } else {
+    %3 = arith.constant 3 : i32
+  }
+  return
+}
+
+// CHECK-LABEL:   func.func @test_two_result()  {
+// CHECK:           %[[VAL_1:.*]] = arith.constant 2.000000e+00 : f32
+// CHECK:           %[[VAL_2:.*]] = arith.constant false
+// CHECK:           %[[RES:[0-9]+]]:2 = scf.if %[[VAL_2]] -> (f32, f32) {
+// CHECK:             scf.yield %[[VAL_1]], %[[VAL_1]] : f32, f32
+// CHECK:           } else {
+// CHECK:             scf.yield %[[VAL_1]], %[[VAL_1]] : f32, f32
+// CHECK:           }
+// CHECK:           return
+// CHECK:         }
+func.func @test_two_result() {
+  %1 = arith.constant 2.0 : f32
+  %cmp = arith.constant false
+  %x, %y = fir.if %cmp -> (f32, f32) {
+    fir.result %1, %1 : f32, f32
+  } else {
+    fir.result %1, %1 : f32, f32
+  }
+  return
+}