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