Smart pointers are often passed by const references. C++ experts, Andrei Alexandrescu, Scott Meyers and Herb Sutter, speculate on this topic during C++ and Beyond 2011 ([04:34] On shared_ptr performance and correctness).
Basically, a smart pointer that is passed-in by const reference already lives in the current scope, somewhere at the call site. It may be stored in a class member and you may do something that clears that member. But this is not the problem of passing by reference, it is the problem of your architecture and ownership policy.
However, this post is not about correctness. It is about performance and what we actually can gain by switching to const references. The first impression may be that the only thing we will get is avoidance of atomic increments/decrements in copy constructor and destructor. Let’s take a closer look and write some code to understand what is going on behind the scenes.
First, some helper functions:
const size_t NUM_CALLS = 10000000;
double GetSeconds()
{
return ( double )clock() / CLOCKS_PER_SEC;
}
void PrintElapsedTime( double ElapsedTime )
{
printf( "%f s/Mcalls\n", float( ElapsedTime / double( NUM_CALLS / 10000000 ) ) );
}
Then an intrusive counter:
class iIntrusiveCounter
{
public:
iIntrusiveCounter():FRefCounter(0) {};
virtual ~iIntrusiveCounter() {}
void IncRefCount() { FRefCounter++; }
void DecRefCount() { if ( --FRefCounter == 0 ) { delete this; } }
private:
std::atomic<int> FRefCounter;
};
And an ad hoc intrusive smart pointer:
template <class T> class clPtr
{
public:
clPtr(): FObject( 0 ) {}
clPtr( const clPtr& Ptr ): FObject( Ptr.FObject ) { FObject->IncRefCount(); }
clPtr( T* const Object ): FObject( Object ) { FObject->IncRefCount(); }
~clPtr() { FObject->DecRefCount(); }
clPtr& operator = ( const clPtr& Ptr )
{
T* Temp = FObject;
FObject = Ptr.FObject;
Ptr.FObject->IncRefCount();
Temp->DecRefCount();
return *this;
}
inline T* operator -> () const { return FObject; }
private:
T* FObject;
};
Pretty simple, right?
Let’s now declare a simple class, a smart pointer to an instance of which will be passed, first, by value and then by const reference to a function:
class clTestObject: public iIntrusiveCounter
{
public:
clTestObject():FPayload(32167) {}
// do some dummy work here
void Do()
{
FPayload++;
}
private:
int FPayload;
};
Everything is now ready to write the actual benchmarking code:
void ProcessByValue( clPtr<clTestObject> O ) { O->Do(); }
void ProcessByConstRef( const clPtr<clTestObject>& O ) { O->Do(); }
int main()
{
clPtr<clTestObject> Obj = new clTestObject;
for ( size_t j = 0; j != 3; j++ )
{
double StartTime = GetSeconds();
for ( size_t i = 0; i != NUM_CALLS; i++ ) { ProcessByValue( Obj ); }
PrintElapsedTime( GetSeconds() - StartTime );
}
for ( size_t j = 0; j != 3; j++ )
{
double StartTime = GetSeconds();
for ( size_t i = 0; i != NUM_CALLS; i++ ) { ProcessByConstRef( Obj ); }
PrintElapsedTime( GetSeconds() - StartTime );
}
return 0;
}
Let’s build it and see what happens. First, we will start with a completely unoptimized debug version (I use gcc.EXE (GCC) 4.10.0 20140420 (experimental)):
gcc -O0 main.cpp -lstdc++ -std=c++11
The run time is 0.375 s/Mcalls for the pass by value version versus 0.124 s/Mcalls for the pass by const reference version. A persuasive 3x performance difference in the debug build. That is good. Let’s take a look at the underlying assembly. The by-value version:
L25: leal -60(%ebp), %eax leal -64(%ebp), %edx movl %edx, (%esp) movl %eax, %ecx call __ZN5clPtrI12clTestObjectEC1ERKS1_ // call copy ctor subl $4, %esp leal -60(%ebp), %eax movl %eax, (%esp) call __Z14ProcessByValue5clPtrI12clTestObjectE leal -60(%ebp), %eax movl %eax, %ecx call __ZN5clPtrI12clTestObjectED1Ev // call dtor addl $1, -32(%ebp) L24: cmpl $10000000, -32(%ebp) jne L25
The by-const-reference version. Notice how clean it is even in a debug build:
L29: leal -64(%ebp), %eax movl %eax, (%esp) call __Z17ProcessByConstRefRK5clPtrI12clTestObjectE // just a single call addl $1, -40(%ebp) L28: cmpl $10000000, -40(%ebp) jne L29
All the calls are in their places and what we only save here are two expensive atomic operations.
But debug builds are not what we actually want, right? Let’s optimize it and see what happens:
gcc -O3 main.cpp -lstdc++ -std=c++11
The by-value time is now 0.168 seconds per Mcalls. The by-const-reference time is ZERO. I mean it. No matter how many iterations you have, the elapsed time in this simple test sample will be zero. Let’s see the assembly to check if we are not mistaken somewhere. This is the optimized by-value version:
L25: call _clock movl %eax, 36(%esp) fildl 36(%esp) movl $10000000, 36(%esp) fdivs LC0 fstpl 24(%esp) .p2align 4,,10 L24: movl 32(%esp), %eax lock addl $1, (%eax) // this is our inlined IncRefCount()... movl 40(%esp), %ecx addl $1, 8(%ecx) // bodies of ProcessByValue() and Do() - 2 instructions lock subl $1, (%eax) // .. and this is DecRefCount(). Quite impressive. jne L23 movl (%ecx), %eax call *4(%eax) L23: subl $1, 36(%esp) jne L24 call _clock
Ok, but why the by-const-reference version is so much faster we cannot measure it? Here it is:
call _clock movl %eax, 36(%esp) movl 40(%esp), %eax addl $10000000, 8(%eax) // here is the final result, no loops, no nothing call _clock movl %eax, 32(%esp) movl $20, 4(%esp) fildl 32(%esp) movl $LC2, (%esp) movl $1, 48(%esp) flds LC0 fdivr %st, %st(1) fildl 36(%esp) fdivp %st, %st(1) fsubrp %st, %st(1) fstpl 8(%esp) call _printf
Just Wow! The complete benchmark is actually in this assembly lines. The absence of atomic hassle lets the optimizer kick in and unroll everything into a single precalculated value. Of course, this is a very trivial code sample. However, it clearly makes 2 points why passing smart pointers by const reference is not a premature optimization but a serious performance improvement:
1) elimination of atomic operations is a large benefit in itself
2) elimination of atomic ops allows the optimizer to jump in and do its magic
Here is the full source code.
Results with your compiler may vary 🙂
P.S. Herb Sutter has a very elaborate essay on the topic, covering the C++ side in great detail.