Re: [proto] The proper way to compose function returning expressions
On 4/25/2012 1:41 PM, Mathias Gaunard wrote: On 24/04/12 22:31, Eric Niebler wrote: On 4/23/2012 10:17 PM, Joel Falcou wrote: On 04/24/2012 12:15 AM, Eric Niebler wrote: I think this is an important issues to solve as far as Proto grokability does. Agreed. It would be very nice to have. But you still have to know when to use it. One of my coworker on NT2 tried to do just this (the norm2 thingy) and he get puzzled by the random crash. [...] The implicit_expr code lived in a detail namespace in past versions of proto. You can find it if you dig through subversion history. I'm not going to do that work for you because the code was broken in subtle ways having to do with the consistency of terminal handling. Repeated attempts to close the holes just opened new ones. It really should be left for dead. I'd rather see what you come up with on your own. The issue Joel had in NT2 was probably unrelated to this. In NT2 we hold all expressions by value unless the tag is boost::proto::tag::terminal. This was done by modifying as_child in our domain. I strongly recommend doing this for most proto-based DSLs. It makes auto foo = some_proto_expression work as expected, and allows expression rewriting of the style that was shown in the thread without any problem. There is probably a slight compile-time cost associated to it, though. Interesting. I avoided this design because I was uncertain whether the compiler would be able to optimize out all the copies of the intermediate nodes. You're saying NT2 does it this way and doesn't suffer performance problems? And you've hand-checked the generated code and found it to be optimal? That would certainly change things. -- Eric Niebler BoostPro Computing http://www.boostpro.com ___ proto mailing list proto@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/proto
Re: [proto] The proper way to compose function returning expressions
On 26/04/12 18:02, Eric Niebler wrote: Interesting. I avoided this design because I was uncertain whether the compiler would be able to optimize out all the copies of the intermediate nodes. You're saying NT2 does it this way and doesn't suffer performance problems? And you've hand-checked the generated code and found it to be optimal? That would certainly change things. NT2 treats large amounts of data per expression, so construction time is not very important. It's the time to evaluate the tree in a given position that matters (which only really depends on proto::value and proto::child_cN, which are always inlined now). We also have another domain that does register-level computation, where construction overhead could be a problem. The last tests we did with this was a while ago and was with the default Proto behaviour. That particular domain didn't get sufficient testing to give real conclusions about the Proto overhead. ___ proto mailing list proto@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/proto
Re: [proto] The proper way to compose function returning expressions
On 4/26/2012 9:35 AM, Mathias Gaunard wrote: On 26/04/12 18:02, Eric Niebler wrote: Interesting. I avoided this design because I was uncertain whether the compiler would be able to optimize out all the copies of the intermediate nodes. You're saying NT2 does it this way and doesn't suffer performance problems? And you've hand-checked the generated code and found it to be optimal? That would certainly change things. NT2 treats large amounts of data per expression, so construction time is not very important. It's the time to evaluate the tree in a given position that matters (which only really depends on proto::value and proto::child_cN, which are always inlined now). We also have another domain that does register-level computation, where construction overhead could be a problem. The last tests we did with this was a while ago and was with the default Proto behaviour. That particular domain didn't get sufficient testing to give real conclusions about the Proto overhead. In that case, I will hold off making any core changes to Proto until I have some evidence that it won't cause performance regressions. Thanks, -- Eric Niebler BoostPro Computing http://www.boostpro.com ___ proto mailing list proto@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/proto
Re: [proto] The proper way to compose function returning expressions
On 4/23/2012 10:17 PM, Joel Falcou wrote: On 04/24/2012 12:15 AM, Eric Niebler wrote: implicit_expr() returns an object that holds its argument and is convertible to any expression type. The conversion is implemented by trying to implicitly convert all the child expressions, recursively. It sort of worked, but I never worked out all the corner cases, and documenting it would have been a bitch. Perhaps I should take another look. Patches welcome. :-) I think this is an important issues to solve as far as Proto grokability does. Agreed. It would be very nice to have. But you still have to know when to use it. One of my coworker on NT2 tried to do just this (the norm2 thingy) and he get puzzled by the random crash. I think we should at least document the issues (I can write that and submit a patch for the doc) and maybe resurrect this implicit_expr. Do you have any remnant of code lying around so I don't start from scratch ? The implicit_expr code lived in a detail namespace in past versions of proto. You can find it if you dig through subversion history. I'm not going to do that work for you because the code was broken in subtle ways having to do with the consistency of terminal handling. Repeated attempts to close the holes just opened new ones. It really should be left for dead. I'd rather see what you come up with on your own. -- Eric Niebler BoostPro Computing http://www.boostpro.com ___ proto mailing list proto@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/proto
[proto] The proper way to compose function returning expressions
Let's say we have a bunch of functions like sum and sqr defined on a proto domain to return expression of tag sum_ and sqr_ in this domain. One day we want to make a norm2(x) function which is basically sum(sqr(x)). My feeling is that I should be able to write it using sqr and sum expressions. Alas it seems this results in dandling reference, crash and some sad pandas. Then I remember about proto::deep_copy but I have a worries. x is usually a terminal holding a huge matrix like value and I just don't want this huge matrix to be copied. What's the correct way to handle such a problem ? How can I build new function returning expressions built from expression composition without incurring a huge amount of copy ? ___ proto mailing list proto@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/proto
Re: [proto] The proper way to compose function returning expressions
On 4/23/2012 1:01 PM, Joel Falcou wrote:
Let's say we have a bunch of functions like sum and sqr defined on a
proto domain to return
expression of tag sum_ and sqr_ in this domain. One day we want to make
a norm2(x) function
which is basically sum(sqr(x)).
My feeling is that I should be able to write it using sqr and sum
expressions.
Alas it seems this results in dandling reference, crash and some sad pandas.
Then I remember about proto::deep_copy but I have a worries. x is
usually a terminal
holding a huge matrix like value and I just don't want this huge matrix
to be copied.
What's the correct way to handle such a problem ? How can I build new
function returning
expressions built from expression composition without incurring a huge
amount of copy ?
Right. The canonical way of doing this is as follows:
#include boost/proto/proto.hpp
namespace proto = boost::proto;
struct sum_ {};
struct sqr_ {};
namespace result_of
{
templatetypename T
struct sum
: proto::result_of::make_exprsum_, T
{};
templatetypename T
struct sqr
: proto::result_of::make_exprsqr_, T
{};
templatetypename T
struct norm2
: sumtypename sqrT::type
{};
}
templatetypename T
typename result_of::sumT ::type const
sum(T t)
{
return proto::make_exprsum_(boost::ref(t));
}
templatetypename T
typename result_of::sqrT ::type const
sqr(T t)
{
return proto::make_exprsqr_(boost::ref(t));
}
templatetypename T
typename result_of::norm2T ::type const
norm2(T t)
{
return
proto::make_exprsum_(proto::make_exprsqr_(boost::ref(t)));
}
int main()
{
sum(proto::lit(1));
sqr(proto::lit(1));
norm2(proto::lit(1));
}
As you can see, the norm2 is not implemented in terms of the sum and sqr
functions. That's not really ideal, but it's the only way I know of to
get fine grained control over which parts are stored by reference and
which by value.
You always need to use make_expr to build expression trees that you
intend to return from a function. That's true even for the built-in
operators. You can't ever return the result of expressions like a+b*42
... because of the lifetime issues.
You can't use deep_copy for the reason you mentioned.
I once had a function proto::implicit_expr, which you could have used
like this:
templatetypename T
typename result_of::norm2T ::type const
norm2(T t)
{
return proto::implicit_expr(sum(sqr(x)));
}
implicit_expr() returns an object that holds its argument and is
convertible to any expression type. The conversion is implemented by
trying to implicitly convert all the child expressions, recursively. It
sort of worked, but I never worked out all the corner cases, and
documenting it would have been a bitch. Perhaps I should take another
look. Patches welcome. :-)
--
Eric Niebler
BoostPro Computing
http://www.boostpro.com
___
proto mailing list
proto@lists.boost.org
http://lists.boost.org/mailman/listinfo.cgi/proto
Re: [proto] The proper way to compose function returning expressions
On 04/24/2012 12:15 AM, Eric Niebler wrote: implicit_expr() returns an object that holds its argument and is convertible to any expression type. The conversion is implemented by trying to implicitly convert all the child expressions, recursively. It sort of worked, but I never worked out all the corner cases, and documenting it would have been a bitch. Perhaps I should take another look. Patches welcome. :-) I think this is an important issues to solve as far as Proto grokability does. One of my coworker on NT2 tried to do just this (the norm2 thingy) and he get puzzled by the random crash. I think we should at least document the issues (I can write that and submit a patch for the doc) and maybe resurrect this implicit_expr. Do you have any remnant of code lying around so I don't start from scratch ? ___ proto mailing list proto@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/proto
