I tried the following testcase with various GCC versions available as Debian
packages. With 3.3.5, the copy from *a to the stack frame of g is done
word-by-word with rep movsl. With 3.4.4, it is done by memcpy. Both previous
methods are fine. With 3.4.6, the copy is done byte-by-byte without string
opcodes. With 4.0.3 and 4.1.0, it is done byte-by-byte and out-of-line: there
are two jumps for each copied byte! So argument copy got broken for x86 during
GCC 3.4 cycle and it did not get any better with GCC 4.
typedef struct A { int a[1000]; } A;
void g(A);
void f(A *a) { g(*a); }
Assembly output with GCC4 and -O3 optimization:
pushl %ebp
xorl %edx, %edx
movl %esp, %ebp
subl $4008, %esp
movl 8(%ebp), %ecx
.L3:
cmpl $4000, %edx
jb .L2
call g
leave
ret
.p2align 4,,7
.L2:
movzbl (%ecx,%edx), %eax
movb %al, (%esp,%edx)
incl %edx
.p2align 4,,3
jmp .L3
$ LANG=C gcc-3.4 -v
Reading specs from /usr/lib/gcc/i486-linux-gnu/3.4.6/specs
Configured with: ../src/configure -v --enable-languages=c,c++,f77,pascal
--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-clocale=gnu
--enable-libstdcxx-debug --with-tune=i686 i486-linux-gnu
Thread model: posix
gcc version 3.4.6 (Debian 3.4.6-1)
$ LANG=C gcc-4.1 -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,java,fortran,objc,obj-c++,ada,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu
--enable-libstdcxx-debug --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-4.1-1.4.2.0/jre --enable-mpfr
--with-tune=i686 --enable-checking=release i486-linux-gnu
Thread model: posix
gcc version 4.1.0 (Debian 4.1.0-1)
--
Summary: Structures are copied byte by byte into function
arguments
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: guillaume dot melquiond at ens-lyon dot fr
GCC target triplet: i486-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27055
