Hello. I am a maintainer of busybox project. One of our goals is to optimize
our code for size. The following testcase was derived from busybox source:
struct client_config_t {
char foreground;
char quit_after_lease;
char release_on_quit;
char abort_if_no_lease;
char *interface;
char *script;
char *clientid;
char *fqdn;
int retries;
int timeout;
char arp[6];
};
struct client_config_t udhcp_client_config1;
struct client_config_t client_config2 = {
/* Default options. */
.abort_if_no_lease = 0,
.foreground = 0,
.quit_after_lease = 0,
.release_on_quit = 0,
.interface = "eth0",
.script = "/share/udhcpc/default.script",
.clientid = 0,
.fqdn = 0,
.retries = 3,
.timeout = 3,
.arp = "\0\0\0\0\0\0", /* appease gcc-3.0 */
};
int udhcpc_main(int argc, char *argv[])
{
udhcp_client_config1.interface = "eth0";
udhcp_client_config1.script = "/share/udhcpc/default.script";
udhcp_client_config1.retries = udhcp_client_config1.timeout = 3;
return 0;
}
Even when compiled with optimisation for size (gcc -Os -S -fomit-frame-pointer
dhcpc.c), both data- and bss-placed structures are aligned to 32 _bytes_.
The structs are itself only _36 bytes_ large! When a lot of structures from
different .o modules are combined into final executable, we waste a lot of
space (on average, 15.5 bytes per structure):
.file "dhcpc.c"
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "eth0"
.LC1:
.string "/share/udhcpc/default.script"
.text
.globl udhcpc_main
.type udhcpc_main, @function
udhcpc_main:
movl $.LC0, udhcp_client_config1+4
movl $.LC1, udhcp_client_config1+8
movl $3, udhcp_client_config1+24
movl $3, udhcp_client_config1+20
xorl %eax, %eax
ret
.size udhcpc_main, .-udhcpc_main
.globl client_config2
.data
.align 32 <---------------------HERE
.type client_config2, @object
.size client_config2, 36
client_config2:
.byte 0
.byte 0
.byte 0
.byte 0
.long .LC0
.long .LC1
.long 0
.long 0
.long 3
.long 3
.string ""
.string ""
.string ""
.string ""
.string ""
.string ""
.zero 2
.comm udhcp_client_config1,36,32 <---- HERE
.ident "GCC: (GNU) 4.1.1"
.section .note.GNU-stack,"",@progbits
Is it possible to instruct gcc to use smaller alignment?
BTW, is there any progress on bug 22158 (also related to data alignment)? I
have a patch there...
--
Summary: overzealous alignment of structure - 32 bytes (not
bits!)
Product: gcc
Version: 4.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: vda dot linux at googlemail dot com
GCC build triplet: i386-pc-linux-gnu
GCC host triplet: i386-pc-linux-gnu
GCC target triplet: i386-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30004
