Untitled :: Boost.OpenMethod

Performance

Open-methods can be as fast as ordinary virtual member functions when compiled with optimization.

First, let’s examine the code generated by clang for an ordinary virtual function call:

void call_virtual_function(const Node& node, std::ostream& os) {
    node.postfix(os);
}

Clang compiles this function to the following assembly on the x64 architecture:

mov	rax, qword ptr [rdi]
mov	rax, qword ptr [rax + 24]
jmp	rax                             # TAILCALL

llvm-mca estimates this code has a throughput of 1 cycle per dispatch.

Let’s look at a method call now:

void call_via_ref(const Node& node, std::ostream& os) {
    postfix(node, os);
}

This compiles to (variable names are shortened for readability):

	mov		rax, rdi
	mov		rcx, qword ptr [rdi]
	mov		rdi, qword ptr [rip + mult]
	imul 	rdi, qword ptr [rcx - 8]
	movzx	ecx, byte ptr [rip + shift]
	shr		rdi, cl
	mov		rdx, rsi
	mov		rcx, qword ptr [rip + vptr_vector_vptrs]
	mov		rdi, qword ptr [rcx + 8*rdi]
	mov		rcx, qword ptr [rip + postfix::fn+88]
	mov		rcx, qword ptr [rdi + 8*rcx]
	mov		rsi, rax
	jmp		rcx                             # TAILCALL

This is quite a few instructions more. Upon closer examination, we see that many are memory reads, independent of one another; they can thus be executed in parallel. For example, the first three instructions can execute simultaneously.

llvm-mca estimates a throughput of 4 cycles per dispatch. However, the difference is amortized by the time spent passing the arguments and returning from the function; plus, of course, executing the body of the function.

Micro- and RDTSC-based benchmarks suggest that dispatching an open-methods with a single virtual argument via a reference is between 30% and 50% slower than calling the equivalent virtual function, with an empty body and no other arguments. In most real programs, the overhead would be unnoticeable.

However, call_via_ref does two things: it constructs a virtual_ptr<Node> from a const Node&; and then it calls the method.

The construction of the virtual_ptr is the costly part. It performs a lookup in a perfect hash table, indexed by pointers to std::type_info, to find the correct vtable. Then it stores a pointer to it in the virtual_ptr object, along with a pointer to the object.^[1]

If we already have a virtual_ptr:

void call_via_virtual_ptr(virtual_ptr<const Node> node, std::ostream& os) {
    postfix(node, os);
}

A method call compiles to:

mov	rax, qword ptr [rip + postfix::fn+88]
mov	rax, qword ptr [rdi + 8*rax]
jmp	rax                             # TAILCALL

virtual_ptr arguments are passed through the method call, to the overrider, which can use them to make further method calls.

A program designed with open-methods in mind should use virtual_ptrs in place of plain pointers or references, as much as possible. Here is the Node example, rewritten to use virtual_ptrs thoughout:

#include <boost/openmethod.hpp>
using boost::openmethod::virtual_ptr;

struct Node {
    virtual ~Node() {}
    virtual int value() const = 0;
};

struct Variable : Node {
    Variable(int value) : v(value) {}
    int value() const override { return v; }
    int v;
};

struct Plus : Node {
    Plus(virtual_ptr<const Node> left, virtual_ptr<const Node> right)
        : left(left), right(right) {}
    int value() const override { return left->value() + right->value(); }
    virtual_ptr<const Node> left, right;
};

struct Times : Node {
    Times(virtual_ptr<const Node> left, virtual_ptr<const Node> right)
        : left(left), right(right) {}
    int value() const override { return left->value() * right->value(); }
    virtual_ptr<const Node> left, right;
};

#include <iostream>

using boost::openmethod::virtual_ptr;

BOOST_OPENMETHOD(postfix, (virtual_ptr<const Node> node, std::ostream& os), void);

BOOST_OPENMETHOD_OVERRIDE(
    postfix, (virtual_ptr<const Variable> var, std::ostream& os), void) {
    os << var->v;
}

BOOST_OPENMETHOD_OVERRIDE(
    postfix, (virtual_ptr<const Plus> plus, std::ostream& os), void) {
    postfix(plus->left, os);
    os << ' ';
    postfix(plus->right, os);
    os << " +";
}

BOOST_OPENMETHOD_OVERRIDE(
    postfix, (virtual_ptr<const Times> times, std::ostream& os), void) {
    postfix(times->left, os);
    os << ' ';
    postfix(times->right, os);
    os << " *";
}

BOOST_OPENMETHOD_CLASSES(Node, Variable, Plus, Times);

#include <boost/openmethod/initialize.hpp>

int main() {
    boost::openmethod::initialize();
    Variable a{2}, b{3}, c{4};
    Plus d{a, b}; Times e{d, c};
    postfix(e, std::cout);
    std::cout << " = " << e.value() << "\n"; // 2 3 + 4 * = 20
}

1. This is how Go and Rust implement dynamic dispatch.

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