[cfe-commits] Performance of OwningPtr/move()...

[steve naroff]

Hey Howard,

I've forwarded two simplistic modifications to your example.

For this contrived example, the "abstraction penalty" appears to be 2x  
when compiled with -O2 (see user time below).

Although this is a contrived example, the cost seems significant.

What's your perspective?

snaroff

[steve-naroffs-imac-2:llvm/tools/clang] snaroff% cat howard.cpp
#include <iostream>
#include "OwningPtr.h"

int* source(int i)
{
    llvm::OwningPtr<int> p(new int(i));
    return p.take();
}

void binOp(int* a, int *b)
{
    int c = *a + *b;
}

int main()
{
    for (int i = 0; i < 10000000; i++) {
      binOp(source(i), source(i+1));
    }
}

[steve-naroffs-imac-2:llvm/tools/clang] snaroff% c++ howard.cpp -o h1 - 
O2
[steve-naroffs-imac-2:llvm/tools/clang] snaroff% cat howard2.cpp
#include <iostream>
#include "OwningPtr.h"

llvm::OwningPtr<int> source(int i)
{
   llvm::OwningPtr<int> p(new int(i));
   return move(p);
}

void binOp(llvm::OwningPtr<int> a, llvm::OwningPtr<int> b)
{
   int c = *a + *b;
}

int main()
{
    for (int i = 0; i < 10000000; i++) {
      binOp(source(i), source(i+1));
    }
}

[steve-naroffs-imac-2:llvm/tools/clang] snaroff% c++ howard2.cpp -o h2  
-O2
[steve-naroffs-imac-2:llvm/tools/clang] snaroff% time ./h1
1.090u 0.290s 0:01.39 99.2%	0+0k 0+0io 0pf+0w
[steve-naroffs-imac-2:llvm/tools/clang] snaroff% time ./h2
2.143u 0.006s 0:02.15 99.5%	0+0k 0+0io 0pf+0w

On Dec 13, 2008, at 4:21 PM, Howard Hinnant wrote:

> In an effort to "de-mystify" the subject around this "smart pointer"  
> application I would like to take include/llvm/ADT/OwningPtr.h as  
> example code.  I've enclosed a modified OwningPtr.h just as an  
> example, not as a proposed change.  I encourage everyone to do a  
> diff between the enclosure and the current OwningPtr.h (the diff is  
> small & boilerplate).
>
> Consider this very simplistic sample code which is written to the  
> current OwningPtr.h interface:
>
> #include <iostream>
> #include "OwningPtr.h"
>
> int* source(int i)
> {
>    llvm::OwningPtr<int> p(new int(i));
>    std::cout << "source: " << *p << '\n';
>    return p.take();
> }
>
> void sink1(int* i)
> {
>    llvm::OwningPtr<int> p(i);
>    std::cout << "sink1: " << *p << '\n';
> }
>
> void sink2(int* i)
> {
>    std::cout << "sink2: " << *i << '\n';
> }
>
> int main()
> {
>    sink1(source(1));
>    sink2(source(2));
> }
>
> There is a "source" function which creates an OwningPtr, but does  
> not return an OwningPtr because OwningPtr isn't copy constructible.   
> It instead returns a pointer to the data (just an int here, likely  
> an Expr in clang).
>
> There are two clients: sink1 and sink2.  The clients have similar  
> signatures, but different semantics.  sink1 takes ownership of the  
> pointer.  sink2 does not.  sink2 may be this way by design, or this  
> could be coding error.
>
> The example above compiles, and prints out:
>
> source: 1
> sink1: 1
> source: 2
> sink2: 2
>
> Because I ran it with a memory-leak checker, it also printed out:
>
> Leaking memory (8 bytes) at 0xb010
> Total memory leaked is 8 bytes
>
> Now consider substituting in the modified OwningPtr.h:
>
> source: 1
> sink1: 1
> source: 2
> sink2: 2
> Leaking memory (8 bytes) at 0xb010
> Total memory leaked is 8 bytes
>
> well, same problem.
>
> Note:  This modification won't alter existing behavior.  However it  
> enables precautions to be taken to make the code safer.  The first  
> precaution to be made is to alter "source" to return an OwningPtr.   
> Before we couldn't do this.  Now we can:
>
> llvm::OwningPtr<int> source(int i)
> {
>    llvm::OwningPtr<int> p(new int(i));
>    std::cout << "source: " << *p << '\n';
>    return move(p);
> }
>
> Compile time errors:
>
> test.cpp: In function ‘int main()’:
> test.cpp:52: error: cannot convert ‘llvm::OwningPtr<int>’ to ‘int*’  
> for argument ‘1’ to ‘void sink1(int*)’
> test.cpp:53: error: cannot convert ‘llvm::OwningPtr<int>’ to ‘int*’  
> for argument ‘1’ to ‘void sink2(int*)’
>
> The compiler is now using the type system to alert the programmer to  
> possible memory leaks.  The first error is pointing to:
>
>    sink1(source(1));
>
> Ok, OwningPtr (correctly) will not implicitly convert to a raw  
> pointer.  We note that sink1 was designed to accept ownership of the  
> pointer so we modify it like so:
>
> void sink1(llvm::OwningPtr<int> p)
> {
>    std::cout << "sink1: " << *p << '\n';
> }
>
> Note that we can now use OwningPtr to pass by value to functions.   
> This is a pattern, like auto_ptr, that indicates to the client that  
> sink1 is accepting ownership of the referenced resource.
>
> The second error refers to:
>
>    sink2(source(2));
>
> Same problem.  We must now decide if sink2 is correct in not  
> claiming ownership of the resource, or if it is simply buggy.  If it  
> is a bug, we fix it the same way we fixed sink1.  So let's assume  
> that sink2 is correct as is.  We have to change our call site then:
>
>    sink2(source(2).get());
>
> Now the sample compiles and prints out:
>
> source: 1
> sink1: 1
> source: 2
> sink2: 2
>
> There are no memory leaks.  By eliminating the use of "take()", we  
> have greatly reduced our exposure to leaks because unowned raw  
> pointers are never passed around.  Yet we can still pass in (owned)  
> raw pointers (as in the sink2 example) when the API calls for it.
>
> Portability:  All compilers that enforce the rule that temporaries  
> can not bind to non-const references will compile this code.  And  
> there is a workaround for VC2005 (/Za off) which allows this example  
> to compile and behave identically.
>
> On VC2005 (/Za off), this modified OwningPtr will have approximately  
> the behavior of std::auto_ptr.  Everywhere else it will refuse to  
> move from lvalues with copy syntax:
>
>   OwningPtr<int> p(new int(1));
>   sink1(p);  // compile time error
>
> Summary:  This change allows us to use the type system to help  
> enforce resource ownership.  Exposure to unowned pointers (such as  
> what take() returns) is minimized but still allowed.
>
> For reference here is the complete altered example:
>
> #include <iostream>
> #include "OwningPtr.h"
>
> llvm::OwningPtr<int> source(int i)
> {
>    llvm::OwningPtr<int> p(new int(i));
>    std::cout << "source: " << *p << '\n';
>    return move(p);
> }
>
> void sink1(llvm::OwningPtr<int> p)  // owns int*
> {
>    std::cout << "sink1: " << *p << '\n';
> }
>
> void sink2(int* i)  // doesn't own int*
> {
>    std::cout << "sink2: " << *i << '\n';
> }
>
> int main()
> {
>    sink1(source(1));
>    sink2(source(2).get());
> }
>
> -Howard
> <OwningPtr.h>
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