[modular_aot] Code generation for NullCheck

Issue: https://github.com/dart-lang/sdk/issues/61635
Change-Id: I0a3ba9dae62514ed9330366e9e4e33b38f44e4c8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/504760
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

pkg/native_compiler/lib/back_end/arm64/assembler.dart

@@ -818,6 +818,11 @@ final class Arm64Assembler extends Assembler with Uint32OutputBuffer {
     callRuntime(RuntimeEntry.FatalError, 1);
   }
 
+  @override
+  void breakpoint() {
+    brk(0);
+  }
+
   /// Generate code for inline object allocation.
   void inlineAllocation(
     Register resultReg,
@@ -1898,6 +1903,11 @@ final class Arm64Assembler extends Assembler with Uint32OutputBuffer {
     );
   }
 
+  void brk(int imm) {
+    assert(_isUint(16, imm));
+    emit(B31 | B30 | B28 | B26 | B21 | (imm << 5));
+  }
+
   void scvtf(
     FPRegister rd,
     Register rn, [

pkg/native_compiler/lib/back_end/arm64/code_generator.dart

@@ -901,7 +901,17 @@ final class Arm64CodeGenerator extends CodeGenerator {
 
   @override
   void visitNullCheck(NullCheck instr) {
-    _asm.unimplemented('Unimplemented: code generation for NullCheck');
+    final operandReg = inputReg(instr, 0);
+    final resultReg = outputReg(instr);
+    if (operandReg != resultReg) {
+      _asm.mov(resultReg, operandReg);
+    }
+    final Label slowPath = addSlowPath(() {
+      _asm.callRuntime(RuntimeEntry.NullCastError, 0);
+      _asm.breakpoint();
+    });
+    _asm.cmp(resultReg, nullReg);
+    _asm.b(slowPath, .equal);
   }
 
   int _getNumberOfInputsForSubtypeTestCache(

pkg/native_compiler/lib/back_end/assembler.dart

@@ -202,6 +202,7 @@ abstract base class Assembler {
   void callStub(Code stub);
 
   void unimplemented(String message);
+  void breakpoint();
 }
 
 /// Assembler output buffer holding 32-bit instructions.

pkg/native_compiler/test/back_end/arm64/assembler_test.dart

@@ -398,6 +398,10 @@ void main() {
         'blr lr\n',
       );
     });
+    test('breakpoint', () {
+      asm.breakpoint();
+      expectDisassembly('brk #0x0\n');
+    });
     test('inlineAllocation - object size 16', () {
       final slowPath = Label();
       asm.inlineAllocation(
@@ -1637,7 +1641,16 @@ void main() {
         'ret r1\n',
       );
     });
-
+    test('brk', () {
+      asm.brk(0x1234);
+      expectDisassembly('brk #0x1234\n');
+      expectThrows(() {
+        asm.brk(-1);
+      });
+      expectThrows(() {
+        asm.brk(0x10000);
+      });
+    });
     test('scvtf', () {
       asm.scvtf(V0, R0);
       asm.scvtf(V1, R2, .s32);