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Lambda not working

Nav
New Contributor I
296 Views
Hi,
I've installed gcc version 4.4.2 on my Fedora system. I got the below code from this forum, but when I compile, I get:
g++ -ltbb -o lambda1 lambda1.cpp

lambda1.cpp: In function void par_ms(int, int, int*):
lambda1.cpp:43: error: expected primary-expression before [ token
lambda1.cpp:43: error: expected primary-expression before ] token
lambda1.cpp:44: error: expected primary-expression before [ token
lambda1.cpp:44: error: expected primary-expression before ] token

I thought lambda's were supported in the latest version of gcc. Please help.

[cpp]#include 
#include
#include

#define N 9999999

using namespace std;
using namespace tbb;

void merge(int beg, int mid, int end, int *A)
{
vector tmp;
int i = beg;
int j = mid;

while ( ( i < mid ) && ( j < end ) )
{
if ( A < A ) {tmp.push_back( A );i++;} else {tmp.push_back( A );j++;}
}

while ( i < mid )
{
tmp.push_back( A );
i++;
}

while ( j < end )
{
tmp.push_back( A );
j++;
}

for ( int t = 0; t < (int) tmp.size(); t++ ) {A[ beg + t ] = tmp;}
}

void par_ms(int beg, int end, int *A)
{
if ( beg + 1 == end ) {return;}

int mid = beg + (end - beg)/2;

parallel_invoke(
[&](){ par_ms(beg, mid, A); },
[&](){ par_ms(mid, end, A); }
);

merge( beg, mid, end, A);

return;
}

int main()
{
task_scheduler_init init(-1);

int A;

for ( int i = 0; i < N; i++ ) {A = N - i;}

par_ms(0, N, A);

for ( int i = 0; i < 10; i++ ) {cout << i << " " << A << endl;}
for ( int i = N-10; i < N; i++ ) {cout << i << " " << A << endl;}

return 0;
}//main
[/cpp]

0 Kudos
29 Replies
Dmitry_Vyukov
Valued Contributor I
265 Views
Quoting - Nav
Hi,
I've installed gcc version 4.4.2 on my Fedora system. I got the below code from this forum, but when I compile, I get:
g++ -ltbb -o lambda1 lambda1.cpp


Supply -std=c++0x key
http://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html
Nav
New Contributor I
265 Views
Thanks.
Tried g++ -std=c++0x -ltbb -o lambda1 lambda1.cpp

and got:

In file included from /home/username/TBB/tbbSource/include/tbb/_concurrent_queue_internal.h:37,
from /home/username/TBB/tbbSource/include/tbb/concurrent_queue.h:32,
from /home/username/TBB/tbbSource/include/tbb/tbb.h:47,
from lambda1.cpp:3:
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:278: error: exception_ptr in namespace std does not name a type
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:294: error: expected unqualified-id before & token
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:294: error: expected ) before & token
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:294: error: expected ; before & token
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:295: error: expected ; before tbb_exception_ptr
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h: In member function void tbb::internal::tbb_exception_ptr::throw_self():
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:291: error: rethrow_exception is not a member of std
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:291: error: my_ptr was not declared in this scope
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h: In constructor tbb::internal::tbb_exception_ptr::tbb_exception_ptr(const tbb::captured_exception&):
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:295: error: class tbb::internal::tbb_exception_ptr does not have any field named my_ptr
/home/username/TBB/tbbSource/include/tbb/tbb_exception.h:295: error: copy_exception is not a member of std
lambda1.cpp: In function void par_ms(int, int, int*):
lambda1.cpp:43: error: expected primary-expression before [ token
lambda1.cpp:43: error: expected primary-expression before ] token
lambda1.cpp:44: error: expected primary-expression before [ token
lambda1.cpp:44: error: expected primary-expression before ] token


Dmitry_Vyukov
Valued Contributor I
265 Views
Quoting - Nav
Thanks.
Tried g++ -std=c++0x -ltbb -o lambda1 lambda1.cpp

and got:



Humm... well, following table suggests that gcc 4.4 supports "Propogating exceptions" C++0x feature:
http://gcc.gnu.org/projects/cxx0x.html

I dunno. You may try to supply -std=gnu++0x
or update to the most latest gcc
or just wait for comments from TBB team

Btw, what documentation says on lambda support? what compilers are supported?

Nav
New Contributor I
265 Views
Quoting - Dmitriy Vyukov

Humm... well, following table suggests that gcc 4.4 supports "Propogating exceptions" C++0x feature:
http://gcc.gnu.org/projects/cxx0x.html

I dunno. You may try to supply -std=gnu++0x
or update to the most latest gcc
or just wait for comments from TBB team

Btw, what documentation says on lambda support? what compilers are supported?

Tried with -std=gnu++0x
Same errors showed up.

The version of gcc I have is the latest. I just configured, built and installed it myself a few days back. Version 4.4.2.
The gcc 4.4.2 documentation does not even have the word 'lambda' in it.
But basically, gcc 4.4.x series is supposed to support lambda.
RafSchietekat
Black Belt
265 Views
"But basically, gcc 4.4.x series is supposed to support lambda."
Really?
Nav
New Contributor I
265 Views
Quoting - Raf Schietekat
"But basically, gcc 4.4.x series is supposed to support lambda."
Really?
Yes, that's what a person at LinuxQuestions.org had to say about it.
Do you mean to say that only version 4.5 of gcc supports lambda? I saw that page earlier too, but there appears to be no solid evidence which says anything about lambda support.
Besides, the current stable release of gcc is 4.4.2. http://gcc.gnu.org/

So how do I get the program to work? Anyone?
robert-reed
Valued Contributor II
265 Views
Quoting - Nav
Yes, that's what a person at LinuxQuestions.org had to say about it.
Do you mean to say that only version 4.5 of gcc supports lambda? I saw that page earlier too, but there appears to be no solid evidence which says anything about lambda support.
Besides, the current stable release of gcc is 4.4.2. http://gcc.gnu.org/

So how do I get the program to work? Anyone?

Switch to the Intel V11 compiler, which does support lambdas? :-) (Just a thought!) (Or at least switch to the Intel Compiler forum, where you might hit more people with gcc expertise?)
Alexey_K_Intel3
Employee
265 Views
Quoting - Nav
So how do I get the program to work? Anyone?
Rewrite to use explicit function objects (quite tedious but should work).
Maybe argument binding can help, though I don't know how to do it right.
Nav
New Contributor I
265 Views
@Alexy and Robert:
Thanks, but using gcc is a 'have to' for me right now.
I need to see lambdas working for me on gcc.
Will be posting in the Intel Compiler forum. If anyone here knows how to solve the problem, then please help out coz I'll be referring this thread too.
If I get any pointers I'll contribute back to this thread.
RafSchietekat
Black Belt
265 Views
"I need to see lambdas working for me on gcc."
Can you explain that? For TBB, lambdas are just sugar (you don't need them), so how is this going to affect your decision about TBB if the decision to use g++ has already been made?
Nav
New Contributor I
265 Views
Quoting - Raf Schietekat
"I need to see lambdas working for me on gcc."
Can you explain that? For TBB, lambdas are just sugar (you don't need them), so how is this going to affect your decision about TBB if the decision to use g++ has already been made?
Dear Raf, let me assure you that I'm not playing a game here. There are certain constraints I'm working with, and would be really grateful if the lambda problem is answered.
RafSchietekat
Black Belt
265 Views
"Dear Raf, let me assure you that I'm not playing a game here. There are certain constraints I'm working with, and would be really grateful if the lambda problem is answered."
You can both motivate and help us to help you by providing an answer to my question.
Nav
New Contributor I
265 Views
Quoting - Raf Schietekat
"Dear Raf, let me assure you that I'm not playing a game here. There are certain constraints I'm working with, and would be really grateful if the lambda problem is answered."
You can both motivate and help us to help you by providing an answer to my question.
Okay, I trust you have a reason for asking so:

Taking the reference of one of my previous posts:
http://software.intel.com/en-us/forums/showthread.php?t=70511

Firstly, I was very surprised that the parallel_for was made in such a way that it always required a function object (the obvious comparison which went with OpenMP, that if it's just a simple pragma there, then why not something simple here too).
This brings forward the coding complexity and amount of code that has to be written for developing a large application. Obviously the use of lambda's is going to simplify and reduce the time taken for me to do my work.

When Intel says that the amount of coding to be done using TBB is much lesser, they were comparing it with ordinary threading. Apparently, the amount of coding would be lesser than or equal to, if I use OpenMP.

Secondly, there are plenty of programmers who need to work with gcc. Learning complex TBB would not be much of an incentive to a programmer if lambda's (or any other feature that simplifies TBB) did not work in gcc (am mentioning this knowing that lambda's are feature of c++0x and not TBB).

Thirdly, I'm well aware that Intel encourages the use of OpenMP for specific type of programming needs. But for now, I need to use lambdas in place of function objects in my TBB program which has to be compiled with gcc 4.4.2. Could anyone help out?
Alexey_K_Intel3
Employee
265 Views
Quoting - Nav
@Alexy and Robert:
Thanks, but using gcc is a 'have to' for me right now.
I need to see lambdas working for me on gcc.

As Raf said, lambdas are just syntax sugar for more convenient use of TBB. Lambda is just an unnamed functor class written by the compiler. You can achieve the same effectby writing an explicit function class. The arguments passed to the function should be captured by the instance of the class (i.e. passed to its constructor and saved in the instance, either by value or by reference). The class should have a operator() method with no parameters; inside that method, you call the function passing the arguments captured earlier.

If for some reason you have to use both g++ and lambdas, then you have to wait for GCC 4.5.
Alexey_K_Intel3
Employee
265 Views
Quoting - Nav
Firstly, I was very surprised that the parallel_for was made in such a way that it always required a function object (the obvious comparison which went with OpenMP, that if it's just a simple pragma there, then why not something simple here too).
This brings forward the coding complexity and amount of code that has to be written for developing a large application. Obviously the use of lambda's is going to simplify and reduce the time taken for me to do my work.

When Intel says that the amount of coding to be done using TBB is much lesser, they were comparing it with ordinary threading. Apparently, the amount of coding would be lesser than or equal to, if I use OpenMP.

Secondly, there are plenty of programmers who need to work with gcc. Learning complex TBB would not be much of an incentive to a programmer if lambda's (or any other feature that simplifies TBB) did not work in gcc (am mentioning this knowing that lambda's are feature of c++0x and not TBB).

Using function objects is the usual practice for template C++ libraries such as STL and TBB. I doubt parallel_for could be written in a generic way without using a function object. Unfortunately, with a pure library we could not achieve the same simplicity that is possible with compiler support (e.g. in OpenMP or Cilk++). So you are right that TBB requires more coding than OpenMP. Meanwhile, TBB is not positioned as OpenMP replacement; if the latter works for you, then just use it.

I agree with the rest of what you said. In fact, lambda support in Intel Compiler was in a great deal driven by the desire to simplify use ofTBB.

So TBB does support GCC and does work with lambdas; but GCC needs to understand lambdas before you can start using both (no matter with TBB or not).
RafSchietekat
Black Belt
265 Views
#13 "Okay, I trust you have a reason for asking so:"
Thanks for the extra information.

I like that TBB works with currently-standard C++, but it would indeed be nice to be able to dispense with some of the boilerplate, although, if I understand correctly, it's not "need to" so much as "want to". What alternatives are you contemplating?

Perhaps you could write the code both without and with lambdas, and use a preprocessor switch to activate one or the other. When the time comes to throw the switch, it will be easier to only have to verify that the program still works, and then you can remove the old version if desired.

This may look like a lot of overhead if you zoom in on it, but Amdahl (reversed) sometimes also applies to programming effort: writing function objects is a fairly straightforward translation from lambda's, with roughly linear cost over part of the code, and then there's still testing etc., so it may not amount to that much in the big picture, as opposed to something like rearchitecting. Your decision, of course. :-)
ARCH_R_Intel
Employee
265 Views
Note that Microsoft, in their design of their PPL library for parallel programming, came to the same conclusion that function objects are fundamental to writing a parallel programming library for C++. VS 2010 will support the lambda syntactic sugar. The alternative would have been pragmas like OpenMP or keywords like Cilk. But those would require special compilers, and one of TBB's goals is portability. (On the other hand, Cilk can do some powerful stuff with its compiler support.)

In another year, you'll wonder how you ever got along without lambdas.

robert-reed
Valued Contributor II
265 Views
Quoting - Raf Schietekat
writing function objects is a fairly straightforward translation from lambda's, with roughly linear cost over part of the code, and then there's still testing etc.,
Yes, roughly linear, though the linear scale factor may vary depending on the local context around the kernel. I personally hope I never have to use the non-lambda approach again (not realistic in my job), having had my fill of it with pre-lambda compilers. The biggest challenge comes when the kernel you're trying to run parallel has a lot of local context (variables it mostly needs to read during execution) because making those variables available requires an explicit linkage, such as through a class initialization function that can have a hairy number of arguments you'll need to get right in both the declaration and the call. With the lambda construct, most of this can be handled with the simple [&] context syntax in the lambda definition. But as Alexey has said repeatedly,there's nothing you can do with lambdas functionally that you cannot do with an explicit function object class.
jimdempseyatthecove
Black Belt
265 Views

Robert,

While I agree that Lambda's ease the programming it does come at an expense. From my (limited) experience Lambda's have two issues:

a) theyhave a little higher overhead than an explicit functor with args or ->context, therefor the body of the Lambda function must perform more work in order for the extra cost to be amortized.

b) When using [&] and when using objects with reference counters you cannot turn off the IncReferences()/DecReferences() meaning these must nowinclude locks (runs slower). Pointers passed from outside the scope of theLambda to inside the scope of thelambdamight be more suitable. IOW create the additional reference(s) and pointers to these references outside the scope of the parallel_xxx with [&]Lambda and using pointer inside the Lambda functon.

Jim Dempsey
Nav
New Contributor I
120 Views
@Alexey:
Okay, so it's either gcc 4.5 (sometime in the future) or the trial version of Intel C++ compiler that I can try out for lambdas.

@Raf:
"if I understand correctly, it's not "need to" so much as "want to". What alternatives are you contemplating?"
I appreciate the help, but I wouldn't want to go into details.

Were you trying to say that "lambda" is almost the reverse of "amdahl"? That's cool :)
Yes, the cost perspective is also presented by Jim. There are other factors I'd mention too, but that's out of the scope of this discussion.

@Arch:
"In another year, you'll wonder how you ever got along without lambdas"
So true :)
I had read the part about how function objects are required when new threads need to use a copy constructor. It's interesting to hear about the Microsoft perspective too.
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