[Bug c++/16405] New: Non optimized code when using default copy constructor

["guillaume dot melquiond at ens-lyon dot fr" <gcc-bugzilla@gcc.gnu.org>]

I have been compiling the following snippet with g++ version 3.3.4, 3.4.0,
3.5.0. They are Debian packages, so the 3.4.0 version is a bit old (and a
prerelease), sorry. So maybe the bug has already been fixed in g++ 3.4. However
it is still present in a recent snapshot of the trunk. In the following, the
function goes from using only one temporary in g++ 3.3 to using four temporaries
in g++ 3.5.

--------------------------------------------------------------------
struct T {
  int a[128];
  // T(T const &v);
  T &operator+=(T const &v);
  T operator+(T const &v) const { T t = *this; t += v; return t; }
};

extern T a, b, c;
void f() { a = b + c; }
--------------------------------------------------------------------

So the code just describes a big plain structure and its operator+ is defined
with its operator+=. The function f computes an addition and operator+ is
inlined into it.

When compiling with g++ 3.3, the generated assembly is optimal: there is only
one temporary, b is copied into this temporary, c is added to this temporary,
the result is then assigned to a.

When compiling with g++ 3.4, it is no more optimal: there are two temporaries, b
is copied into one, c is added to this temporary holding b, the result is copied
into the other temporary, and the temporary is then assigned to a. Maybe the
NRVO did not kick in?

When compiling with g++ 3.5, it is even worse: there are four temporaries, b is
copied into one, c is added to this temporary holding b, the result then goes
into two successive temporaries before being assigned to a. So there is one copy
that does not mean anything with respect to the code? And please note there is
one temporary that just lies on the stack and is never used (since only three
temporaries are used, not four).

If a copy constructor is added (just comment out the line), the three compilers
generate a similar code: only one temporary is ever used. It is the expected and
optimal code generation. So it seems g++ has some kind of problem with the
default copy constructor starting with 3.4.

As a side note, even with a non-default copy constructor, g++ 3.5 still
allocates a temporary that is never used. Maybe it is an unrelated bug?

The snippet is each time compiled with
$ g++-... -Wall -O3 -S test.cpp

Here are the exact versions used.

$ LANG=C g++-3.3 -v
Reading specs from /usr/lib/gcc-lib/i486-linux/3.3.4/specs
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared --with-system-zlib
--enable-nls --without-included-gettext --enable-__cxa_atexit
--enable-clocale=gnu --enable-debug --enable-java-gc=boehm
--enable-java-awt=xlib --enable-objc-gc i486-linux
Thread model: posix
gcc version 3.3.4 (Debian 1:3.3.4-2)

$ LANG=C g++-3.4 -v
Reading specs from /usr/lib/gcc/i486-linux/3.4.0/specs
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr
--libexecdir=/usr/lib --with-gxx-include-dir=/usr/include/c++/3.4
--enable-shared --with-system-zlib --enable-nls --enable-threads=posix
--without-included-gettext --program-suffix=-3.4 --enable-__cxa_atexit
--enable-clocale=gnu --enable-libstdcxx-debug --enable-java-gc=boehm
--enable-java-awt=gtk --disable-werror i486-linux
Thread model: posix
gcc version 3.4.0 20040403 (prerelease) (Debian)

$ LANG=C g++-snapshot -v
Reading specs from /usr/lib/gcc-snapshot/lib/gcc/i486-linux-gnu/3.5.0/specs
Configured with: ../src/configure -v --enable-languages=c,c++,java,f95,objc
--prefix=/usr/lib/gcc-snapshot --enable-shared --with-system-zlib --enable-nls
--enable-threads=posix --without-included-gettext --disable-werror
--enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug
--enable-java-gc=boehm --enable-java-awt=gtk i486-linux-gnu
Thread model: posix
gcc version 3.5.0 20040704 (experimental)

-- 
           Summary: Non optimized code when using default copy constructor
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: guillaume dot melquiond at ens-lyon dot fr
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16405