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Date: Sat, 13 Aug 2005 08:03:00 -0000
From: "guillaume dot melquiond at ens-lyon dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20050813080257.23372.guillaume.melquiond@ens-lyon.fr>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug c++/23372] New: Temporary aggregate copy not elided when passing parameters by value
X-Bugzilla-Reason: CC
X-SW-Source: 2005-08/txt/msg01483.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

This bug is similar to bug 16405, but although 16405 was fixed in 4.0, this one
is still present and is a regression from GCC 3.4 (not from 3.3 as was the
previous one). So I prefer opening a new bug-report.

The testcase simply calls a function f by passing the parameter by value:

struct A {
  int a[1000];
  //A(A const &);
};
void f(A);
void g(A *a) { f(*a); }

When compiled with gcc 3.3 and 3.4, the generated code for g is optimal: the
value *a is directly copied in the stack frame that will be used by f. With gcc
4.0, there is first a temporary copy in the stack frame of g, before copying the
value in the stack frame of f (two memcpys instead of one).

When putting a dummy copy constructor, both memcpys disappear: the code is
optimal. So the problem seems to be with the default trivial copy constructor.

The testcase is compiled with "g++-4.0 -O3", Debian package:

Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f95,objc,ada,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib --enable-nls
--without-included-gettext --enable-threads=posix --program-suffix=-4.0
--enable-__cxa_atexit --enable-libstdcxx-allocator=mt --enable-clocale=gnu
--enable-libstdcxx-debug --enable-java-gc=boehm --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-4.0-1.4.2.0/jre
--enable-mpfr --disable-werror --enable-checking=release i486-linux-gnu
Thread model: posix
gcc version 4.0.2 20050806 (prerelease) (Debian 4.0.1-4)

-- 
           Summary: Temporary aggregate copy not elided when passing
                    parameters by value
           Product: gcc
           Version: 4.0.2
            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=23372