[CLANG-BPF] change __builtin_preserve_access_index() signature

[yonghong-song]

Summary

The clang intrinsic __builtin_preserve_access_index() currently
has signature:

const void * __builtin_preserve_access_index(const void * ptr)

This may cause compiler warning when:

• parameter type is "volatile void *" or "const volatile void *", or
• the assign-to type of the intrinsic does not have "const" qualifier.

Further, this signature does not allow dereference of the
builtin result pointer as it is a "const void *" type, which
adds extra step for the user to do type casting.

Let us change the signature to:

PointerT __builtin_preserve_access_index(PointerT ptr)

such that the result and argument types are the same.
With this, directly dereferencing the builtin return value
becomes possible.

[yonghong-song]

does not change arg type and make result type the same as arg type

[ast]

that's indeed much better. Thanks for adding all the tests.

clang/docs/LanguageExtensions.rst

 
 ``__builtin_preserve_access_index``
 -----------------------------------
 
 ``__builtin_preserve_access_index`` specifies a code section where
 array subscript access and structure/union member access are relocatable
 under bpf compile-once run-everywhere framework. Debuginfo (typically
 with ``-g``) is needed, otherwise, the compiler will exit with an error.
+The return type for the intrinsic is the same as the type of the
+argument, and must be a pointer type.
 
 **Syntax**:
 
 .. code-block:: c
 
-  const void * __builtin_preserve_access_index(const void * ptr)
+  PointerT __builtin_preserve_access_index(PointerT ptr)
 
 **Example of Use**:
 
 .. code-block:: c
 
   struct t {
     int i;
     int j;

clang/include/clang/Basic/Builtins.def

 BUILTIN(__builtin_smulll_overflow, "bSLLiCSLLiCSLLi*", "n")
 
 // Clang builtins (not available in GCC).
 BUILTIN(__builtin_addressof, "v*v&", "nct")
 BUILTIN(__builtin_operator_new, "v*z", "tc")
 BUILTIN(__builtin_operator_delete, "vv*", "tn")
 BUILTIN(__builtin_char_memchr, "c*cC*iz", "n")
 BUILTIN(__builtin_dump_struct, "ivC*v*", "tn")
-BUILTIN(__builtin_preserve_access_index, "vC*vC*", "nU")
+BUILTIN(__builtin_preserve_access_index, "v.", "t")
 
 // Safestack builtins
 BUILTIN(__builtin___get_unsafe_stack_start, "v*", "Fn")
 BUILTIN(__builtin___get_unsafe_stack_bottom, "v*", "Fn")
 BUILTIN(__builtin___get_unsafe_stack_top, "v*", "Fn")
 BUILTIN(__builtin___get_unsafe_stack_ptr, "v*", "Fn")
 
 // Nontemporal loads/stores builtins

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_builtin_launder_invalid_arg : Error<
   "%select{non-pointer|function pointer|void pointer}0 argument to "
   "'__builtin_launder' is not allowed">;
 
 def err_bit_cast_non_trivially_copyable : Error<
   "__builtin_bit_cast %select{source|destination}0 type must be trivially copyable">;
 def err_bit_cast_type_size_mismatch : Error<
   "__builtin_bit_cast source size does not equal destination size (%0 vs %1)">;
+
+def err_builtin_preserve_access_index_invalid_arg : Error<
+  "__builtin_preserve_access_index argument must a pointer type instead of %0">;
 } // end of sema component.

clang/lib/Sema/SemaChecking.cpp

static bool SemaBuiltinAddressof(Sema &S, CallExpr *TheCall) {
   if (ResultType.isNull())
     return true;
 
   TheCall->setArg(0, Arg.get());
   TheCall->setType(ResultType);
   return false;
 }
 
-/// Check the number of arguments, and set the result type to
+/// Check the number of arguments and arg type, and set the result type to
 /// the argument type.
 static bool SemaBuiltinPreserveAI(Sema &S, CallExpr *TheCall) {
   if (checkArgCount(S, TheCall, 1))
     return true;
 
+  // The argument type must be a pointer
+  ExprResult Arg = TheCall->getArg(0);
+  QualType Ty = Arg.get()->getType();
+  if (!Ty->isPointerType()) {
+    S.Diag(Arg.get()->getBeginLoc(),
+           diag::err_builtin_preserve_access_index_invalid_arg)
+        << Ty << Arg.get()->getSourceRange();
+    return true;
+  }
+
   TheCall->setType(TheCall->getArg(0)->getType());
   return false;
 }
 
 static bool SemaBuiltinOverflow(Sema &S, CallExpr *TheCall) {
   if (checkArgCount(S, TheCall, 3))
     return true;
 

clang/test/Sema/builtin-preserve-access-index.c

 // RUN: %clang_cc1 -x c -triple x86_64-pc-linux-gnu -dwarf-version=4 -fsyntax-only -verify %s
 
 const void *invalid1(const int *arg) {
   return __builtin_preserve_access_index(&arg[1], 1); // expected-error {{too many arguments to function call, expected 1, have 2}}
 }
 
-void *invalid2(const int *arg) {
-  return __builtin_preserve_access_index(&arg[1]); // expected-warning {{returning 'const void *' from a function with result type 'void *' discards qualifiers}}
+const void *invalid2(const int *arg) {
+  return __builtin_preserve_access_index(1); // expected-error {{__builtin_preserve_access_index argument must a pointer type instead of 'int'}}
 }
 
-const void *invalid3(const int *arg) {
-  return __builtin_preserve_access_index(1); // expected-warning {{incompatible integer to pointer conversion passing 'int' to parameter of type 'const void *'}}
+void *invalid3(const int *arg) {
+  return __builtin_preserve_access_index(&arg[1]); // expected-warning {{returning 'const int *' from a function with result type 'void *' discards qualifiers}}
+}
+
+const void *invalid4(volatile const int *arg) {
+  return __builtin_preserve_access_index(arg); // expected-warning {{returning 'const volatile int *' from a function with result type 'const void *' discards qualifiers}}
+}
+
+int *valid5(int *arg) {
+  return __builtin_preserve_access_index(arg);
+}
+
+int valid6(const volatile int *arg) {
+  return *__builtin_preserve_access_index(arg);
+}
+
+struct s { int a; int b; };
+
+int valid7(struct s *arg) {
+  return *__builtin_preserve_access_index(&arg->b);
 }