BPF: fix enum value 0 issue for __builtin_preserve_enum_value()

[yonghong-song]

Summary

Lorenz Bauer reported that the following code will have
compilation error for bpf target:

enum e { TWO };
bpf_core_enum_value_exists(enum e, TWO);

The clang emitted the following error message:

__builtin_preserve_enum_value argument 1 invalid

In SemaChecking, an expression like "*(enum NAME)1" will have
cast kind CK_IntegralToPointer, but "*(enum NAME)0" will have
cast kind CK_NullToPointer. Current implementation only permits
CK_IntegralToPointer, missing enum value 0 case.

This patch permits CK_NullToPointer cast kind and
the above test case can pass now.

clang/lib/Sema/SemaChecking.cpp

static bool isValidBPFPreserveEnumValueArg(Expr *Arg) {
   // format:
   //   __builtin_preserve_enum_value(*(<enum_type> *)<enum_value>,
   //                                 flag);
   const auto *UO = dyn_cast<UnaryOperator>(Arg->IgnoreParens());
   if (!UO)
     return false;
 
   const auto *CE = dyn_cast<CStyleCastExpr>(UO->getSubExpr());
-  if (!CE || CE->getCastKind() != CK_IntegralToPointer)
+  if (!CE)
+    return false;
+  if (CE->getCastKind() != CK_IntegralToPointer &&
+      CE->getCastKind() != CK_NullToPointer)
     return false;
 
   // The integer must be from an EnumConstantDecl.
   const auto *DR = dyn_cast<DeclRefExpr>(CE->getSubExpr());
   if (!DR)
     return false;
 
   const EnumConstantDecl *Enumerator =

clang/test/CodeGen/builtins-bpf-preserve-field-info-4.c

 // REQUIRES: bpf-registered-target
 // RUN: %clang -target bpf -emit-llvm -S -g -Xclang -disable-llvm-passes %s -o - | FileCheck %s
 
 #define _(x, y) (__builtin_preserve_enum_value((x), (y)))
 
 enum AA {
+  VAL0 = 0,
   VAL1 = 2,
   VAL2 = 0xffffffff80000000UL,
 };
-typedef enum { VAL10 = -2, VAL11 = 0xffff8000, }  __BB;
+typedef enum { VAL00, VAL10 = -2, VAL11 = 0xffff8000, }  __BB;
 
 unsigned unit1() {
   return _(*(enum AA *)VAL1, 0) + _(*(__BB *)VAL10, 1);
 }
 
 unsigned unit2() {
   return _(*(enum AA *)VAL2, 0) + _(*(__BB *)VAL11, 1);
 }
 
+unsigned unit3() {
+  return _(*(enum AA *)VAL0, 0) + _(*(__BB *)VAL00, 1);
+}
+
 // CHECK: @0 = private unnamed_addr constant [7 x i8] c"VAL1:2\00", align 1
 // CHECK: @1 = private unnamed_addr constant [9 x i8] c"VAL10:-2\00", align 1
 // CHECK: @2 = private unnamed_addr constant [17 x i8] c"VAL2:-2147483648\00", align 1
 // CHECK: @3 = private unnamed_addr constant [17 x i8] c"VAL11:4294934528\00", align 1
+// CHECK: @4 = private unnamed_addr constant [7 x i8] c"VAL0:0\00", align 1
+// CHECK: @5 = private unnamed_addr constant [8 x i8] c"VAL00:0\00", align 1
 
 // CHECK: call i64 @llvm.bpf.preserve.enum.value(i32 0, i8* getelementptr inbounds ([7 x i8], [7 x i8]* @0, i32 0, i32 0), i64 0), !dbg !{{[0-9]+}}, !llvm.preserve.access.index ![[ENUM_AA:[0-9]+]]
 // CHECK: call i64 @llvm.bpf.preserve.enum.value(i32 1, i8* getelementptr inbounds ([9 x i8], [9 x i8]* @1, i32 0, i32 0), i64 1), !dbg !{{[0-9]+}}, !llvm.preserve.access.index ![[TYPEDEF_ENUM:[0-9]+]]
 
 // CHECK: call i64 @llvm.bpf.preserve.enum.value(i32 2, i8* getelementptr inbounds ([17 x i8], [17 x i8]* @2, i32 0, i32 0), i64 0), !dbg !{{[0-9]+}}, !llvm.preserve.access.index ![[ENUM_AA]]
 // CHECK: call i64 @llvm.bpf.preserve.enum.value(i32 3, i8* getelementptr inbounds ([17 x i8], [17 x i8]* @3, i32 0, i32 0), i64 1), !dbg !{{[0-9]+}}, !llvm.preserve.access.index ![[TYPEDEF_ENUM]]
 
+// CHECK: call i64 @llvm.bpf.preserve.enum.value(i32 4, i8* getelementptr inbounds ([7 x i8], [7 x i8]* @4, i32 0, i32 0), i64 0), !dbg !{{[0-9]+}}, !llvm.preserve.access.index ![[ENUM_AA]]
+// CHECK: call i64 @llvm.bpf.preserve.enum.value(i32 5, i8* getelementptr inbounds ([8 x i8], [8 x i8]* @5, i32 0, i32 0), i64 1), !dbg !{{[0-9]+}}, !llvm.preserve.access.index ![[TYPEDEF_ENUM]]
+
 // CHECK: ![[ENUM_AA]] = !DICompositeType(tag: DW_TAG_enumeration_type, name: "AA"
 // CHECK: ![[TYPEDEF_ENUM]] = !DIDerivedType(tag: DW_TAG_typedef, name: "__BB"