[flang][CodeGen] Transform `fir.unreachable` to `llvm.unreachable`

[awarzynski]

Summary

This patch extends the FIRToLLVMLowering pass in Flang by adding a
hook to transform fir.unreachable to llvm.unreachable.

This is part of the upstreaming effort from the fir-dev branch in [1].

[1] https://github.com/flang-compiler/f18-llvm-project

Co-authored-by: Eric Schweitz <[e-mail]>

[clementval]

LGTM thanks

flang/lib/Optimizer/CodeGen/CodeGen.cpp

struct UndefOpConversion : public FIROpConversion<fir::UndefOp> {
   matchAndRewrite(fir::UndefOp undef, OpAdaptor,
                   mlir::ConversionPatternRewriter &rewriter) const override {
     rewriter.replaceOpWithNewOp<mlir::LLVM::UndefOp>(
         undef, convertType(undef.getType()));
     return success();
   }
 };
 
+// convert to LLVM IR dialect `unreachable`
+struct UnreachableOpConversion : public FIROpConversion<fir::UnreachableOp> {
+  using FIROpConversion::FIROpConversion;
+
+  mlir::LogicalResult
+  matchAndRewrite(fir::UnreachableOp unreach, OpAdaptor adaptor,
+                  mlir::ConversionPatternRewriter &rewriter) const override {
+    rewriter.replaceOpWithNewOp<mlir::LLVM::UnreachableOp>(unreach);
+    return success();
+  }
+};
+
 struct ZeroOpConversion : public FIROpConversion<fir::ZeroOp> {
   using FIROpConversion::FIROpConversion;
 
   mlir::LogicalResult
   matchAndRewrite(fir::ZeroOp zero, OpAdaptor,
                   mlir::ConversionPatternRewriter &rewriter) const override {
     auto ty = convertType(zero.getType());
     if (ty.isa<mlir::LLVM::LLVMPointerType>()) {
       rewriter.replaceOpWithNewOp<mlir::LLVM::NullOp>(zero, ty);
     } else if (ty.isa<mlir::IntegerType>()) {
       rewriter.replaceOpWithNewOp<mlir::LLVM::ConstantOp>(
           zero, ty, mlir::IntegerAttr::get(zero.getType(), 0));
     } else if (mlir::LLVM::isCompatibleFloatingPointType(ty)) {
       rewriter.replaceOpWithNewOp<mlir::LLVM::ConstantOp>(
           zero, ty, mlir::FloatAttr::get(zero.getType(), 0.0));
     } else {
       // TODO: create ConstantAggregateZero for FIR aggregate/array types.
       return zero.emitOpError(
           "conversion of fir.zero with aggregate type not implemented yet");
     }
     return success();
   }
 };
+
 } // namespace
 
 namespace {
 /// Convert FIR dialect to LLVM dialect
 ///
 /// This pass lowers all FIR dialect operations to LLVM IR dialect. An
 /// MLIR pass is used to lower residual Std dialect to LLVM IR dialect.
 ///
 /// This pass is not complete yet. We are upstreaming it in small patches.
 class FIRToLLVMLowering : public fir::FIRToLLVMLoweringBase<FIRToLLVMLowering> {
 public:
   mlir::ModuleOp getModule() { return getOperation(); }
 
   void runOnOperation() override final {
     auto *context = getModule().getContext();
     fir::LLVMTypeConverter typeConverter{getModule()};
     mlir::OwningRewritePatternList pattern(context);
-    pattern.insert<AddrOfOpConversion, HasValueOpConversion, GlobalOpConversion,
-                   UndefOpConversion, ZeroOpConversion>(typeConverter);
+    pattern
+        .insert<AddrOfOpConversion, HasValueOpConversion, GlobalOpConversion,
+                UndefOpConversion, UnreachableOpConversion, ZeroOpConversion>(
+            typeConverter);
     mlir::populateStdToLLVMConversionPatterns(typeConverter, pattern);
     mlir::arith::populateArithmeticToLLVMConversionPatterns(typeConverter,
                                                             pattern);
     mlir::ConversionTarget target{*context};
     target.addLegalDialect<mlir::LLVM::LLVMDialect>();
 
     // required NOPs for applying a full conversion
     target.addLegalOp<mlir::ModuleOp>();

flang/test/Fir/convert-to-llvm.fir

 
 // CHECK: %{{.*}} = llvm.mlir.constant(0.000000e+00 : f16) : f16
 // CHECK: %{{.*}} = llvm.mlir.constant(0.000000e+00 : bf16) : bf16
 // CHECK: %{{.*}} = llvm.mlir.constant(0.000000e+00 : f32) : f32
 // CHECK: %{{.*}} = llvm.mlir.constant(0.000000e+00 : f64) : f64
 // CHECK: %{{.*}} = llvm.mlir.constant(0.000000e+00 : f80) : f80
 // CHECK: %{{.*}} = llvm.mlir.constant(0.000000e+00 : f128) : f128
 // CHECK-NOT: fir.zero_bits
+
+// -----
+
+// Verify that fir.unreachable is transformed to llvm.unreachable
+
+// CHECK:  llvm.func @test_unreachable() {
+// CHECK-NEXT:    llvm.unreachable
+// CHECK-NEXT:  }
+
+func @test_unreachable() {
+  fir.unreachable
+}