------- Comment #17 from etienne_lorrain at yahoo dot fr 2009-02-03 16:38
-------
(In reply to comment #15)
> The advantage of such a RTL pass (or just adding such optimization to another
> RTL pass) would be that it would handle also say: ...
Why only limit that pass to constants, and only to writes:
etie...@cygne:~$ gcc --version
gcc (Debian 4.3.3-3) 4.3.3
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
etie...@cygne:~$ cat tmp.c
struct S
{
char a;
char b;
char c;
char d;
} u, v;
void fct (void) {
u.a = v.a;
u.b = v.b;
u.c = v.c;
u.d = v.d;
}
etie...@cygne:~$ gcc -Os -S tmp.c -o tmp.s
etie...@cygne:~$ cat tmp.s
.file "tmp.c"
.text
.globl fct
.type fct, @function
fct:
movb v, %al
pushl %ebp
movl %esp, %ebp
popl %ebp
movb %al, u
movb v+1, %al
movb %al, u+1
movb v+2, %al
movb %al, u+2
movb v+3, %al
movb %al, u+3
ret
.size fct, .-fct
.comm u,4,4
.comm v,4,4
.ident "GCC: (Debian 4.3.3-3) 4.3.3"
.section .note.GNU-stack,"",@progbits
etie...@cygne:~$
A single 32 bits read, and a single 32 bits write should be sufficient,
when none of the fields of the structure is declared volatile.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22141
