[CIR] Backport support for GNUNullExpr (#1981)

Backport support for GNUNullExpr from the upstream

Author: AmrDeveloper

clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp

@@ -212,7 +212,7 @@ class ScalarExprEmitter : public StmtVisitor<ScalarExprEmitter, mlir::Value> {
     return emitNullValue(E->getType(), CGF.getLoc(E->getSourceRange()));
   }
   mlir::Value VisitGNUNullExpr(const GNUNullExpr *E) {
-    llvm_unreachable("NYI");
+    return emitNullValue(E->getType(), CGF.getLoc(E->getSourceRange()));
   }
   mlir::Value VisitOffsetOfExpr(OffsetOfExpr *E) {
     // Try folding the offsetof to a constant.

clang/test/CIR/CodeGen/gnu-null.cpp

@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -Wno-unused-value -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s -check-prefix=CIR
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -Wno-unused-value -fclangir -emit-llvm %s -o %t-cir.ll
+// RUN: FileCheck --input-file=%t-cir.ll %s -check-prefix=LLVM
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -Wno-unused-value -emit-llvm %s -o %t.ll
+// RUN: FileCheck --input-file=%t.ll %s -check-prefix=OGCG
+
+void gnu_null_expr() {
+  long a = __null;
+  int *b = __null;
+}
+
+// CIR: %[[A_ADDR:.*]] = cir.alloca !s64i, !cir.ptr<!s64i>, ["a", init]
+// CIR: %[[B_ADDR:.*]] = cir.alloca !cir.ptr<!s32i>, !cir.ptr<!cir.ptr<!s32i>>, ["b", init]
+// CIR: %[[CONST_0:.*]] = cir.const #cir.int<0> : !s64i
+// CIR: cir.store {{.*}} %[[CONST_0]], %[[A_ADDR]] : !s64i, !cir.ptr<!s64i>
+// CIR: %[[CONST_NULL:.*]] = cir.const #cir.ptr<null> : !cir.ptr<!s32i>
+// CIR: cir.store {{.*}} %[[CONST_NULL]], %[[B_ADDR]] : !cir.ptr<!s32i>, !cir.ptr<!cir.ptr<!s32i>>
+
+// LLVM: %[[A_ADDR:.*]] = alloca i64, i64 1, align 8
+// LLVM: %[[B_ADDR:.*]] = alloca ptr, i64 1, align 8
+// LLVM: store i64 0, ptr %[[A_ADDR]], align 8
+// LLVM: store ptr null, ptr %[[B_ADDR]], align 8
+
+// OGCG: %[[A_ADDR:.*]] = alloca i64, align 8
+// OGCG: %[[B_ADDR:.*]] = alloca ptr, align 8
+// OGCG: store i64 0, ptr %[[A_ADDR]], align 8
+// OGCG: store ptr null, ptr %[[B_ADDR]], align 8