[Bug c++/53488] New: Incorrect code generated when capturing a constant by reference in a lambda

[Bug c++/53488] New: Incorrect code generated when capturing a constant by reference in a lambda

[jpalecek at web dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

             Bug #: 53488
           Summary: Incorrect code generated when capturing a constant by
                    reference in a lambda
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jpalecek@web.de


Created attachment 27497
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27497
Preprocessed source

I have the following source:

jirka@debian:~/benchmark$ cat error.cpp 
#include <iostream>

template<class T>
void f()
{
  const int i=sizeof(T);
  auto fun = [&] { std::cout << i << std::endl; };
  fun();
}

int main()
{
  f<double>();
  return 0;
}

When I compile and run this program, it outputs garbage (instead of
sizeof(double)):

jirka@debian:~/benchmark$ g++-4.7 --std=c++0x error.cpp
jirka@debian:~/benchmark$ ./a.out 
134514706

Changing f to a nontemplate, i to be nondependent ot captured by value produces
correct output.

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-05-25
            Version|unknown                     |4.7.0
     Ever Confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-25 16:16:25 UTC ---
The bug rpeorting guidelines ask for the output of 'gcc -v' so we at least know
which version you're using, "g++-4.7" isn't very useful. I'm assuming 4.7.0

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[jpalecek at web dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

--- Comment #2 from Jiří Paleček <jpalecek at web dot de> 2012-05-25 17:54:00 UTC ---
The bug occurs with these compiler versions:

jirka@debian:~/benchmark$ g++-4.7 -v
Using built-in specs.
COLLECT_GCC=g++-4.7
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i486-linux-gnu/4.7/lto-wrapper
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.0-9'
--with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --enable-objc-gc --enable-targets=all --with-arch-32=i586
--with-tune=generic --enable-checking=release --build=i486-linux-gnu
--host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.7.0 (Debian 4.7.0-9) 

jirka@debian:~/benchmark$
LD_LIBRARY_PATH=/usr/lib/gcc-snapshot/lib:$LD_LIBRARY_PATH
PATH=/usr/lib/gcc-snapshot/bin:$PATH g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc-snapshot/libexec/gcc/i486-linux-gnu/4.8.0/lto-wrapper
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 20120501-1'
--with-bugurl=file:///usr/share/doc/gcc-snapshot/README.Bugs
--enable-languages=c,ada,c++,java,go,fortran,objc,obj-c++
--prefix=/usr/lib/gcc-snapshot --enable-shared --enable-linker-build-id
--with-system-zlib --disable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-snap/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-snap
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-snap
--with-arch-directory=i386 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-targets=all --with-arch-32=i586 --with-tune=generic
--disable-werror --enable-checking=yes --build=i486-linux-gnu
--host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.8.0 20120501 (experimental) [trunk revision 187013] (Debian
20120501-1)

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #3 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-05-25 20:09:20 UTC ---
An interesting aspect of this example is that when using gcc 4.8.0 20120520
(experimental) compiled in C++11 mode with -Wall it produces the warning

"7|warning: 'i' is used uninitialized in this function [-Wuninitialized]"

where line 7 is the line of the lambda-expression and producing an output of:

0

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[lunow at math dot hu-berlin.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

--- Comment #4 from Daniel Lunow <lunow at math dot hu-berlin.de> 2012-06-24 14:21:31 UTC ---
Created attachment 27693
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27693
small test case

Just wanted to report the same bug with gcc 4.7.1. 
I found out, that it occurs, if the initializer depends on some template
argument.

C:\dev\projects\compiler test>g++ -std=c++11 gcc_test5.cpp -Wall -o
gcc_test5.exe
gcc_test5.cpp: In lambda function:
gcc_test5.cpp:10:29: warning: 'i' is used uninitialized in this function
[-Wuninitialized]

C:\dev\projects\compiler test>gcc_test5.exe
0
which should have given 4.

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[lunow at math dot hu-berlin.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

--- Comment #5 from Daniel Lunow <lunow at math dot hu-berlin.de> 2012-06-24 14:30:19 UTC ---
Created attachment 27694
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27694
integral constant-expression not an integral constant-expression

This test case should compile, but it does not. Its not the same as the
constant not beeing captured, but I can imagine it is very close problem.

C:\dev\projects\compiler test>g++ -std=c++11 gcc_test6.cpp
gcc_test6.cpp: In instantiation of 'foo() [with T = void]::<lambda()>':
gcc_test6.cpp:8:4:   required from 'struct foo() [with T = void]::<lambda()>'
gcc_test6.cpp:8:3:   required from 'void foo() [with T = void]'
gcc_test6.cpp:13:13:   required from here
gcc_test6.cpp:8:15: error: size of array is not an integral constant-expression

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[jpalecek at web dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

Jiří Paleček <jpalecek at web dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lunow at math dot
                   |                            |hu-berlin.de

--- Comment #6 from Jiří Paleček <jpalecek at web dot de> 2012-06-24 20:12:43 UTC ---
Daniel, sorry, but I don't think your second ex

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[jpalecek at web dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

--- Comment #7 from Jiří Paleček <jpalecek at web dot de> 2012-06-24 20:23:38 UTC ---
Daniel, sorry, but I don't think your second example is relevant. First this
bug occurs only when capturing by reference, not by value, but your second
example uses capture by value. Moreover, while reading the standard, I can't
find anything that mandates the usage like yours to work. AFAIK it allows the
compiler to treat the lambda from your second example akin to this:

int foo() {
  const int i = 1;
  struct Lambda {
    const int i;
    Lambda(int i_) : i(i_) {}
    void operator()()const {
      int x[i];
    }
  };
  Lambda l(i);
}

which wouldn't work, and quite rightly. It couldn't even be modified to work,
as i must be static if it has to be both compile time constant and member of a
class.

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[lunow at math dot hu-berlin.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

Daniel Lunow <lunow at math dot hu-berlin.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lunow at math dot
                   |                            |hu-berlin.de

--- Comment #8 from Daniel Lunow <lunow at math dot hu-berlin.de> 2012-06-25 00:32:00 UTC ---
Then you must have missed the example in 5.1.2 [expr.prim.lambda] p. 12.
[Example:
void f1(int i) {
  int const N = 20;
  auto m1 = [=]{
    int const M = 30;
    auto m2 = [i]{
      int x[N][M]; // OK: N and M are not odr-used
      // ...
    };
  };
  // ...
}
- end example]
Sure its non-normative, but this is what has always worked through a
combination of
8.3.4 [dcl.array], 3.4.1p1,6 [basic.lookup.unqual], 3.3.1p1
[basic.scope.declarative], 3.3.3p1,3 [basic.scope.local] and 5.19p2
[expr.const].

By the way the bug is independent of capturing by value/reference and in my
second example nothing is captured at all. It is not necessary to capture,
because i is not odr-used (5.1.2p11, 3.2p2).

I just put it here because, it occurs under the same circumstances 
 * constant integral local variable
 * initialized by an expression dependent on a template parameter, but not if
the initializer is the name of a non-template type parameter
 * used (but not odr-used) inside lambda.
and of course gcc has no problems with both examples, if i's initialization
does not depend on a template parameter.

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[ai.azuma at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

Ai Azuma <ai.azuma at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ai.azuma at gmail dot com

--- Comment #9 from Ai Azuma <ai.azuma at gmail dot com> 2012-06-25 14:28:00 UTC ---
I don't know if that helps, but there exists a bug report similar to one
reported in Comment 0.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50939

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[jpalecek at web dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

Jiří Paleček <jpalecek at web dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |DUPLICATE

--- Comment #10 from Jiří Paleček <jpalecek at web dot de> 2012-08-19 14:04:34 UTC ---
I found a similar preceding bugreport 52026. Therefore, I'm marking as
duplicate.

*** This bug has been marked as a duplicate of bug 52026 ***

[Bug c++/53488] Incorrect code generated when capturing a constant by reference in a lambda

[jpalecek at web dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53488

--- Comment #11 from Jiří Paleček <jpalecek at web dot de> 2012-08-19 14:11:22 UTC ---
BTW I have proposed a patch that fixes this problem here:
http://gcc.gnu.org/ml/gcc-patches/2012-08/msg01252.html. Please take a look at
it.