[Bug c++/51385] New: Unnecessary instantiation converting to pointer to template class instance

[Bug c++/51385] New: Unnecessary instantiation converting to pointer to template class instance

[hstong at ca dot ibm.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51385

             Bug #: 51385
           Summary: Unnecessary instantiation converting to pointer to
                    template class instance
    Classification: Unclassified
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: hstong@ca.ibm.com
              Host: powerpc64-unknown-linux-gnu
            Target: powerpc64-unknown-linux-gnu


The test case below uses SFINAE to check whether pointers to certain types
can be implicitly converted.

When a template class which is expensive to instantiate is used as the type
pointed-to by the target of the conversion, GCC takes a long time to compile.

I do not believe that the instantiation of the type in question is necessary to
determine the validity of the conversion (notwithstanding the non-normative
note
in 3.2 [basic.def.odr] paragraph 4). See Core Issue 50 regarding that note.


### Self-contained source:$ cat bb2.C
#ifndef SLOW
enum { Depth = 3 };
#else
enum { Depth = 15 };
#endif

template <unsigned N> struct NTmpl;
template <typename T, typename U, typename V = NTmpl<sizeof(T *)> >
struct PtrConvs {
   enum { bad = 1 };
};

template <typename Target, typename Source>
struct PtrConvs<Target, Source, NTmpl<sizeof (*(Target **)0 = (Source *)0)> >;

template <unsigned N, typename T> struct FastGrowingTemplate;

template <typename T>
struct FastGrowingTemplate<0, T> { };

template <unsigned N, typename T>
struct FastGrowingTemplate :
   FastGrowingTemplate<N - 1, const T *>,
   FastGrowingTemplate<N - 1, volatile T *>
{ };

struct B { };

typedef char chk[1];
typedef char chk[PtrConvs<FastGrowingTemplate<Depth, short>, B>::bad];


### Time without -DSLOW:$ time /data/gcc/bin/g++-4.6.0 bb2.C -c

real    0m0.236s
user    0m0.007s
sys     0m0.006s


### Time with -DSLOW:$ time /data/gcc/bin/g++-4.6.0 bb2.C -c -DSLOW

real    1m45.826s
user    1m42.505s
sys     0m0.312s


### g++ -v output:$ /data/gcc/bin/g++-4.6.0 -v
Using built-in specs.
COLLECT_GCC=/data/gcc/bin/g++-4.6.0
COLLECT_LTO_WRAPPER=/data/gcc/libexec/gcc/powerpc64-unknown-linux-gnu/4.6.0/lto-wrapper
Target: powerpc64-unknown-linux-gnu
Configured with: ./configure --prefix=/data/gcc --program-suffix=-4.6.0
--disable-libssp --disable-libgcj --enable-version-specific-runtime-libs
--with-cpu=default32 --enable-secureplt --with-long-double-128 --enable-shared
--enable-__cxa_atexit --enable-threads=posix --enable-languages=c,c++,fortran
--with-gmp=/usr/local
Thread model: posix
gcc version 4.6.0 (GCC)

[Bug c++/51385] Unnecessary instantiation converting to pointer to template class instance

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51385

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-02 10:36:14 UTC ---
Here's a version which isn't a compile-time-hog but demonstrates the
unnecessary instantiation with a static_assert (so needs -std=c++11)


template <unsigned N> struct NTmpl;
template <typename T, typename U, typename V = NTmpl<sizeof(T *)> >
struct PtrConvs {
   enum { bad = 1 };
};

template <typename Target, typename Source>
struct PtrConvs<Target, Source, NTmpl<sizeof (*(Target **)0 = (Source *)0)> >;

template<typename T> struct test { static const bool value = true; };
template<> struct test<short> { static const bool value = false; };

template <typename T>
struct FussyTemplate
{
    static_assert( test<T>::value, "not short" );
};

struct B { };

typedef char chk[1];
typedef char chk[PtrConvs<FussyTemplate<short>, B>::bad];


EFG and Clang compile this happily, without instantiating FussyTemplate<short>

[Bug c++/51385] Unnecessary instantiation converting to pointer to template class instance

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51385

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011-12-02
                 CC|                            |jason at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-12-02 10:42:28 UTC ---
Let's add Jason in CC, definitely not a regression, but the issue seems pretty
interesting, eh.

[Bug c++/51385] Unnecessary instantiation converting to pointer to template class instance

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51385

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-02 10:47:45 UTC ---
Interestingly G++ is happy to not instantiate the template when it's not
possible to, i.e. making this change and defining -DINCOMPLETE allows it to
compile:

template <typename T>
struct FussyTemplate
#ifndef INCOMPLETE
{
    static_assert( test<T>::value, "not short" );
}
#endif
;

But if the class template definition is available it gets instantiated.

[Bug c++/51385] Unnecessary instantiation converting to pointer to template class instance

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51385

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
This is fixed in 4.9.0, I'm adding the testcase and closing the bug.

[Bug c++/51385] Unnecessary instantiation converting to pointer to template class instance

[paolo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51385

--- Comment #5 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Mon Sep 29 09:06:31 2014
New Revision: 215680

URL: https://gcc.gnu.org/viewcvs?rev=215680&root=gcc&view=rev
Log:
2014-09-29  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/51385
    * g++.dg/template/pr51385.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/template/pr51385.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/51385] Unnecessary instantiation converting to pointer to template class instance

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51385

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.9.0

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Done.