| Topic: |
DEVELOP > c-Plus-Plus |
| User: |
"Jakob Bieling" |
| Date: |
18 Apr 2004 07:57:20 AM |
| Object: |
Q: Return value optimization thru several function calls? |
Hi!
I am using VC++ 7.1 and have a question about return value optimization.
Consider the following code:
#include <list>
#include <string>
struct test
{
std::list <std::string> m_strings;
};
test func1 (int r)
{
if (r == 0)
return test ();
test t = func1 (r - 1);
t.m_strings.push_back ("lala");
return t;
}
int main ()
{
test t = func1 (3);
}
Now, stepping thru the code, one would assume that 'test' objects are
copy constructed and destroyed often. My question is, would a decent
compiler, like VC++ 7.1, be able to optimize this in such a way that only a
single 'test' object is used throughout the code? To make this post more
on-topic, would the Standard even allow such an optimization, as it is a
pretty great change to the original code ..
Thanks in advance!
--
jb
(replace y with x if you want to reply by e-mail)
.
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| User: "Denis Remezov" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 07:18:22 AM |
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Jakob Bieling wrote:
Hi!
I am using VC++ 7.1 and have a question about return value optimization.
Consider the following code:
#include <list>
#include <string>
struct test
{
std::list <std::string> m_strings;
};
test func1 (int r)
{
if (r == 0)
return test ();
test t = func1 (r - 1);
t.m_strings.push_back ("lala");
return t;
}
int main ()
{
test t = func1 (3);
}
As Jeff said, in the above example the compiler is allowed to optimise
away unnecessary locals (in func1()), temporaries and the corresponding
copy constructor calls. If func1() were defined like this:
test func1 (int r)
{
test t;
t.m_strings.push_back ("lala");
return t;
}
with the same main(), I would definitely expect just one construction and
destruction from a decent compiler. The recursion, however, may complicate
things a bit. You can add a couple of ctors to test (default and copy) and
a dtor that cout a message and see.
Consider this too:
int main()
{
test t;
//do something to t ...
t = func1 (3);
}
Not much luck in this case: t will be constructed first, then at least one
object of class test will be created as a result of func1(), then operator =
will be called.
When I care about performance, I usually define func1() as
void func1 (int r, test& t) {...}
If the functional notation were important, I would consider using proxies
(and accomodate the "actual" objects acordingly), but I would not return
really big objects from a function.
Denis
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| User: "Jakob Bieling" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 11:30:47 AM |
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"Denis Remezov" <denis_remezovREMOVETHIS@yahoo.ca.removethis> wrote in
message news:4082720E.E59BEA0D@yahoo.ca.removethis...
When I care about performance, I usually define func1() as
void func1 (int r, test& t) {...}
If the functional notation were important,
Well, it is not important, but I do care about design and the above
seems like a rather ugly solution; tho it would serve the purpose pretty
well.
I would consider using proxies
(and accomodate the "actual" objects acordingly), but I would not return
really big objects from a function.
It is not that the object is big, but it contains a linked list of
non-POD objects, so creating copies of it where it is not necessary, seems
like a bad idea.
I assume the proxy would take care of doing a fast swap rather than a
deep-copy of the objects, correct?
Thanks!
--
jb
(replace y with x if you want to reply by e-mail)
.
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| User: "Denis Remezov" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 09:45:09 AM |
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Jakob Bieling wrote:
"Denis Remezov" <denis_remezovREMOVETHIS@yahoo.ca.removethis> wrote in
message news:4082720E.E59BEA0D@yahoo.ca.removethis...
When I care about performance, I usually define func1() as
void func1 (int r, test& t) {...}
If the functional notation were important,
Well, it is not important, but I do care about design and the above
seems like a rather ugly solution;
[snip]
It depends. In some respect, the syntax is inferior, I agree; func1() now
also needs to be careful to handle or dispose of the existing contents
of t properly (that could be a bad thing). Potential advantages are
(performance issues aside):
0) Class test does not need to be constructible in the given scope.
1) It is further guaranteed and made explicit that no construction
or copying of objects of class test will take place.
I would consider using proxies
(and accomodate the "actual" objects acordingly), but I would not return
really big objects from a function.
It is not that the object is big, but it contains a linked list of
non-POD objects,
[snip]
Sorry I didn't make it clear, but I meant to include that into the
definition of a "big object".
I assume the proxy would take care of doing a fast swap rather than a
deep-copy of the objects, correct?
Something of the kind. I was thinking about proxy objects sharing pointers
to the "real" objects and reference-counting them, deferring the
actual creation and copying until such point in time that a write-operation
is requested.
Denis
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| User: "David Harmon" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 02:12:17 PM |
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On Sun, 18 Apr 2004 18:30:47 +0200 in comp.lang.c++, "Jakob Bieling"
<netsurf@gmy.net> wrote,
When I care about performance, I usually define func1() as
void func1 (int r, test& t) {...}
If the functional notation were important,
Well, it is not important, but I do care about design and the above
seems like a rather ugly solution; tho it would serve the purpose pretty
well.
I am sure your example is simplified and that there would be other
concerns in a complex application. Nevertheless, for the purpose of the
simple example I would consider constructing a single object and using a
member function with the implied 'this' to make it perhaps a bit less
ugly than the explicit reference argument. In my view, this gets closer
to the implementation originally desired, albeit at the cost of a member
function that you might prefer as a non-menber.
#include <iostream>
using std::cerr;
#include <list>
#include <string>
struct test {
std::list<std::string> m_strings;
test & func1(int r);
// instrumentation:
test() { cerr << "Construct!\n"; }
test(const test & x)
: m_strings(x.m_strings)
{ cerr << "Copy construct " << m_strings.size() << "!\n"; }
~test() { cerr << "Destruct " << m_strings.size() << "!\n"; }
//
};
test & test::func1(int r)
{
if (r == 0)
return *this;
func1(r - 1);
m_strings.push_back("lala");
return *this;
}
int main()
{
test t;
t.func1(3);
}
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| User: "Jeff Schwab" |
|
| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 08:57:36 AM |
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Jakob Bieling wrote:
Hi!
I am using VC++ 7.1 and have a question about return value optimization.
Consider the following code:
#include <list>
#include <string>
struct test
{
std::list <std::string> m_strings;
};
test func1 (int r)
{
if (r == 0)
return test ();
test t = func1 (r - 1);
t.m_strings.push_back ("lala");
return t;
}
int main ()
{
test t = func1 (3);
}
Now, stepping thru the code, one would assume that 'test' objects are
copy constructed and destroyed often. My question is, would a decent
compiler, like VC++ 7.1, be able to optimize this in such a way that only a
single 'test' object is used throughout the code? To make this post more
on-topic, would the Standard even allow such an optimization, as it is a
pretty great change to the original code ..
The standard allows the optimization (6.3.3).
.
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| User: "Jakob Bieling" |
|
| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 09:45:43 AM |
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"Jeff Schwab" <jeffplus@comcast.net> wrote in message
news:btednZVn855RFR_dRVn-sQ@comcast.com...
The standard allows the optimization (6.3.3).
In my copy, 6.3 is "Compound statement or block" and only has a single
paragraph. IOW, there is no 6.3.3?
regards
--
jb
(replace y with x if you want to reply by e-mail)
.
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| User: "Victor Bazarov" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 11:52:19 AM |
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"Jakob Bieling" <netsurf@gmy.net> wrote...
"Jeff Schwab" <jeffplus@comcast.net> wrote in message
news:btednZVn855RFR_dRVn-sQ@comcast.com...
The standard allows the optimization (6.3.3).
In my copy, 6.3 is "Compound statement or block" and only has a single
paragraph. IOW, there is no 6.3.3?
Jeff probably meant 6.6.3 "The return statement".
V
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| User: "Jakob Bieling" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 12:38:26 PM |
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"Victor Bazarov" <v.Abazarov@comAcast.net> wrote in message
news:7nygc.20953$ru4.18150@attbi_s52...
"Jakob Bieling" <netsurf@gmy.net> wrote...
"Jeff Schwab" <jeffplus@comcast.net> wrote in message
news:btednZVn855RFR_dRVn-sQ@comcast.com...
The standard allows the optimization (6.3.3).
In my copy, 6.3 is "Compound statement or block" and only has a
single
paragraph. IOW, there is no 6.3.3?
Jeff probably meant 6.6.3 "The return statement".
Thanks .. there is a reference to 12.2 which explicitly allows the
optimization of constructing the return value directly in place of the
variable that is used to hold the return value, thus eliminating the
temporary copy.
Guess I will go for Dennis' proxy-class approach, since it does not look
like a decent compiler will do the optimization I am after.
Thanks for the comments!
--
jb
(replace y with x if you want to reply by e-mail)
.
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| User: "David Harmon" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 01:16:55 PM |
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On Sun, 18 Apr 2004 19:38:26 +0200 in comp.lang.c++, "Jakob Bieling"
<netsurf@gmy.net> wrote,
Guess I will go for Dennis' proxy-class approach, since it does not look
like a decent compiler will do the optimization I am after.
A decent compiler will certainly do that optimization!
Now the question remains, whether there is one yet that does? Perhaps
not.
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| User: "Jakob Bieling" |
|
| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 01:21:44 PM |
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"David Harmon" <source@netcom.com> wrote in message
news:40cec582.225276188@news.west.earthlink.net...
On Sun, 18 Apr 2004 19:38:26 +0200 in comp.lang.c++, "Jakob Bieling"
<netsurf@gmy.net> wrote,
Guess I will go for Dennis' proxy-class approach, since it does not
look
like a decent compiler will do the optimization I am after.
A decent compiler will certainly do that optimization!
Now the question remains, whether there is one yet that does? Perhaps
not.
;)
--
jb
(replace y with x if you want to reply by e-mail)
.
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| User: "Jeff Schwab" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 03:28:00 PM |
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Victor Bazarov wrote:
"Jakob Bieling" <netsurf@gmy.net> wrote...
"Jeff Schwab" <jeffplus@comcast.net> wrote
The standard allows the optimization (6.3.3).
In my copy, 6.3 is "Compound statement or block" and only has a single
paragraph. IOW, there is no 6.3.3?
Jeff probably meant 6.6.3 "The return statement".
Thanks, Victor. You're right, I did mean 6.6.3.
.
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| User: "Petec" |
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| Title: Re: Q: Return value optimization thru several function calls? |
18 Apr 2004 11:57:26 AM |
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Jakob Bieling wrote:
"Jeff Schwab" <jeffplus@comcast.net> wrote in message
news:btednZVn855RFR_dRVn-sQ@comcast.com...
The standard allows the optimization (6.3.3).
In my copy, 6.3 is "Compound statement or block" and only has a
single paragraph. IOW, there is no 6.3.3?
regards
It's not in mine (2003 revision) either...
- Pete
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| User: "tom_usenet" |
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| Title: Re: Q: Return value optimization thru several function calls? |
19 Apr 2004 07:38:06 AM |
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On Sun, 18 Apr 2004 14:57:20 +0200, "Jakob Bieling" <netsurf@gmy.net>
wrote:
Hi!
I am using VC++ 7.1 and have a question about return value optimization.
Consider the following code:
#include <list>
#include <string>
struct test
{
std::list <std::string> m_strings;
};
test func1 (int r)
{
if (r == 0)
return test ();
The line above disables the optimization for most compilers.
Generally, for NRVO to work, all return statements must return the
same object (although this isn't a requirement of 12.8/15, just common
practice).
test t = func1 (r - 1);
t.m_strings.push_back ("lala");
return t;
So the above probably won't be optimized.
}
int main ()
{
test t = func1 (3);
}
Now, stepping thru the code, one would assume that 'test' objects are
copy constructed and destroyed often. My question is, would a decent
compiler, like VC++ 7.1, be able to optimize this in such a way that only a
single 'test' object is used throughout the code? To make this post more
on-topic, would the Standard even allow such an optimization, as it is a
pretty great change to the original code ..
Both GCC and VC7.1 failed to optimize the original code (I added
instrumentation). Here is a version that on GCC 3.3 prints:
Default
Destroy
which is what you were after. VC++ 7.1 doesn't implement NRVO, so that
prints:
Default
Copy
Destroy
Copy
Destroy
Copy
Destroy
Copy
Destroy
Copy
Destroy
Copy
Destroy
Copy
Destroy
Copy
Destroy
Destroy
Oh dear! Here's the code:
#include <list>
#include <string>
#include <iostream>
using namespace std;
struct test
{
test()
{
cout << "Default\n";
}
test(test const& t)
:m_strings(t.m_strings)
{
cout << "Copy\n";
}
~test()
{
cout << "Destroy\n";
}
test& operator=(test const& t)
{
m_strings = t.m_strings;
cout << "Assign\n";
return *this;
}
std::list <std::string> m_strings;
};
test func1 (int r)
{
test t(r == 0? test() : func1 (r - 1));
if (r != 0)
t.m_strings.push_back ("lala");
return t;
}
int main ()
{
test t = func1 (3);
}
And what's the moral of this? Well, to enable NRVO, make sure you:
1. Ensure all return statements return the same local object.
2. Avoid assignments.
3. Use GCC.
I assume MS will implement NRVO eventually...
Tom
--
C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
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