Hi,
I noticed the apparently simple entry in the TODO list saying:
From: Jeff Garzik
1) The string form
[const] char *foo = "blah";
creates two variables in the final assembly output, a
static string, and a char pointer to the static string.
The alternate string form
[const] char foo[] = "blah";
is better because it declares a single variable.
For variables marked __initdata, the "*foo" form causes only
the pointer, not the string itself, to be dropped from the
kernel image, which is a bug. Using the "foo[]" form with
regular 'ole local variables also makes the assembly shorter.
A quick grep found some targets - but I'm not sure that there is
really a benefit; I was comparing the size of the .o's before and
after the change and noticed that on x86-64 gcc 4.4.1
(ubuntu karmic alpha 4:4.4.1-1ubuntu2)
the two different forms cause wildly different optimisation, and
the [] form causes gcc to build the string using instructions.
e.g. with a little standalone test:
int main(int argc, char* argv[]){
/*char *frob="hello";*/
char frob[]="hello";
printf("%s\n", frob);
}
produces code that looks like:
movl $1819043176, (%rsp)
movw $111, 4(%rsp)
call __printf_chk
Doing the same trick to some of the error messages in ntfs/inode.c
I can see the same thing is happening for fairly large strings - e.g.
0000000000000280 <ntfs_truncate>:
280: 55 push %rbp
281: 49 ba 20 20 4c 65 61 mov $0x6e697661654c2020,%r10
288: 76 69 6e
28b: 48 89 e5 mov %rsp,%rbp
28e: 49 b9 67 20 66 69 6c mov $0x6c20656c69662067,%r9
295: 65 20 6c
298: 41 57 push %r15
29a: 49 b8 65 6e 67 74 68 mov $0x756f206874676e65,%r8
2a1: 20 6f 75
2a4: 41 56 push %r14
2a6: 48 be 74 20 6f 66 20 mov $0x6e797320666f2074,%rsi
2ad: 73 79 6e
2b0: 41 55 push %r13
2b2: 48 b9 63 20 77 69 74 mov $0x6920687469772063,%rcx
2b9: 68 20 69
2bc: 41 54 push %r12
2be: 53 push %rbx
2bf: 48 89 fb mov %rdi,%rbx
2c2: 48 81 ec d8 00 00 00 sub $0xd8,%rsp
2c9: 48 81 eb 08 01 00 00 sub $0x108,%rbx
2d0: 48 89 bd 38 ff ff ff mov %rdi,-0xc8(%rbp)
2d7: 65 48 8b 04 25 28 00 mov %gs:0x28,%rax
2de: 00 00
2e0: 48 89 45 c8 mov %rax,-0x38(%rbp)
2e4: 31 c0 xor %eax,%eax
2e6: 48 8b 97 28 ff ff ff mov -0xd8(%rdi),%rdx
2ed: 48 89 75 a8 mov %rsi,-0x58(%rbp)
2f1: 48 89 95 48 ff ff ff mov %rdx,-0xb8(%rbp)
2f8: 48 89 4d b0 mov %rcx,-0x50(%rbp)
2fc: 4c 89 55 90 mov %r10,-0x70(%rbp)
300: 4c 89 4d 98 mov %r9,-0x68(%rbp)
304: 4c 89 45 a0 mov %r8,-0x60(%rbp)
308: c7 45 b8 5f 73 69 7a movl $0x7a69735f,-0x48(%rbp)
30f: 66 c7 45 bc 65 2e movw $0x2e65,-0x44(%rbp)
315: c6 45 be 00 movb $0x0,-0x42(%rbp)
319: 48 c7 c1 00 00 00 00 mov $0x0,%rcx
320: 4c 8b 47 40 mov 0x40(%rdi),%r8
324: 48 c7 c2 00 00 00 00 mov $0x0,%rdx
32b: be 43 09 00 00 mov $0x943,%esi
330: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
337: e8 00 00 00 00 callq 33c <ntfs_truncate+0xbc>
as opposed to the old:
0000000000000280 <ntfs_truncate>:
280: 55 push %rbp
281: 48 c7 c1 00 00 00 00 mov $0x0,%rcx
288: 48 89 e5 mov %rsp,%rbp
28b: 48 c7 c2 00 00 00 00 mov $0x0,%rdx
292: 41 57 push %r15
294: be 43 09 00 00 mov $0x943,%esi
299: 41 56 push %r14
29b: 41 55 push %r13
29d: 41 54 push %r12
29f: 53 push %rbx
2a0: 48 89 fb mov %rdi,%rbx
2a3: 48 81 ec 98 00 00 00 sub $0x98,%rsp
2aa: 48 81 eb 08 01 00 00 sub $0x108,%rbx
2b1: 48 89 bd 78 ff ff ff mov %rdi,-0x88(%rbp)
2b8: 48 8b 87 28 ff ff ff mov -0xd8(%rdi),%rax
2bf: 48 89 45 88 mov %rax,-0x78(%rbp)
2c3: 31 c0 xor %eax,%eax
2c5: 4c 8b 47 40 mov 0x40(%rdi),%r8
2c9: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
2d0: e8 00 00 00 00 callq 2d5 <ntfs_truncate+0x55>
This seems pretty mad - is there a good reason gcc is doing this?
Because from that it would seem that changing to char*'s to char[] is
currently growing code rather than shrinking it. Is gcc just being overly
enthusiastic for instructions with large constants?
On related questions, looking through some of the constant strings
in the code does find some oddities:
1) fs/ntfs/inode.c:2331
static const char *es = " Leaving inconsistent metadata. Unmount and run "
"chkdsk.";
is a bit weird (especially since some functions declare a local es).
2) security/tomoyo/common.c:tomoyo_print_double_path_acl
I'm missing why the atmark1 and atmark2 exist at all.
Dave
--
-----Open up your eyes, open up your mind, open up your code -------
/ Dr. David Alan Gilbert | Running GNU/Linux on Alpha,68K| Happy \
\ gro.gilbert @ treblig.org | MIPS,x86,ARM,SPARC,PPC & HPPA | In Hex /
\ _________________________|_____ http://www.treblig.org |_______/
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