------- Additional Comments From guillaume dot melquiond at ens-lyon dot fr
2005-08-26 17:38 -------
> all of 3.4 and 4.1 produce exactly two temporaries.
Yet I said that g++ 3.4 did not produce any temporary, and I still think so. No
temporaries, only g's stack frame. See the following assembly code for the C
testcase (the generated assembly is the same as for C++, but easier to read
since there is no name mangling nor local labels).
h:
pushl %ebp
movl %esp, %ebp
subl $4008, %esp
movl %esp, %eax
subl $12, %esp
pushl %eax
call f
addl $12, %esp
call g
leave
ret
For the sake of completeness, I'm also writing the assembly output for GCC 4.0,
so that the regression with respect to GCC 3.4 is clearly visible. Two
temporaries and two memory copies:
h:
pushl %ebp
movl %esp, %ebp
pushl %esi
pushl %ebx
subl $8012, %esp
leal -8008(%ebp), %ebx
pushl %ebx
call f
leal -4008(%ebp), %esi
subl $8, %esp
pushl $4000
pushl %ebx
pushl %esi
call memcpy
subl $3968, %esp
movl %esp, %eax
pushl %edx
pushl $4000
pushl %esi
pushl %eax
call memcpy
addl $16, %esp
call g
addl $4000, %esp
leal -8(%ebp), %esp
popl %ebx
popl %esi
popl %ebp
ret
The C testcase is almost identical to the C++ testcase:
typedef struct A { int a[1000]; } A;
A f();
void g(A);
void h() { g(f()); }
And this is my version of GCC 3.4:
$ LANG=C gcc-3.4 -v
Reading specs from /usr/lib/gcc/i486-linux-gnu/3.4.5/specs
Configured with: ../src/configure -v
--enable-languages=c,c++,f77,pascal,objc,ada --prefix=/usr --libexecdir=/usr/lib
--with-gxx-include-dir=/usr/include/c++/3.4 --enable-shared --with-system-zlib
--enable-nls --without-included-gettext --program-suffix=-3.4
--enable-__cxa_atexit --enable-libstdcxx-allocator=mt --enable-clocale=gnu
--enable-libstdcxx-debug i486-linux-gnu
Thread model: posix
gcc version 3.4.5 20050821 (prerelease) (Debian 3.4.4-8)
Hope it helps.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23372
