CODEWITHSUNDEEP
c++classthisrestrict-qualifier
cmwt
cmwt

Reputation: 349

How to apply restrict qualifier on this pointer

How can I apply GCC's/Clang's __restrict__ qualifier to the this pointer of a class?
This question was inspired by Richard Powell's CppCon 2018 talk, "How to Argue(ment)." I saw a similar question "restrict qualifier on member functions (restrict this pointer)." (All code can be found on Compiler Explorer)

void bar();

class Foo {
 public:
  int this_example() const {
    if (value > 0) {
      bar();
      return value;
    } else {
      return value;
    }
  }

 private:
  int value;
};

The above code generates the following assembly. In it, we can see that value has to be loaded twice, via the this pointer. This makes sense, it is a consequence that C++ inherited from C and the restrict qualifier allows the programmer to turn off the behavior. I can find no way to enable the restrict functionality for the this pointer.

Foo::this_example() const:               # @Foo::this_example() const
        push    rbx
        mov     eax, dword ptr [rdi]
        test    eax, eax
        jle     .LBB2_2
        mov     rbx, rdi
        call    bar()
        mov     eax, dword ptr [rbx]
.LBB2_2:
        pop     rbx
        ret

On the Compiler Explorer page, I show examples of method arguments using __restrict__ to elide the second load. There is also an example of passing a struct reference to a function and using __restrict__ to elide the second load.

I can imagine a world where the compiler would allow the programmer to mention the implicit this pointer in the arguments of a method. The compiler could then allow application of qualifiers to the this pointer. See code below for an example.

class Foo {
 public:
  int unrestricted(Foo *this);
  int restricted(Foo *__restrict__ this);
};

As a follow up question, is there something in the C++ standard or a C++ guideline that would make it so that this could never have the restrict qualifier?

Upvotes: 3

Views: 717

Answers (1)

Acorn
Acorn

Reputation: 26186

GCC's documentation for __restrict__ (as well as the linked question) mentions that you can actually restrict this:

You may also specify whether a member function’s this pointer is unaliased by using __restrict__ as a member function qualifier.

void T::fn () __restrict__
{
    /* … */
}

Within the body of T::fn, this has the effective definition T *__restrict__ const this. Notice that the interpretation of a __restrict__ member function qualifier is different to that of const or volatile qualifier, in that it is applied to the pointer rather than the object. This is consistent with other compilers that implement restricted pointers.

Note, however, that marking the this pointer as such does not prevent the second load.

Upvotes: 3

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