Re: goto's ???
Mikulas Patocka (mikulas@artax.karlin.mff.cuni.cz)
Tue, 25 May 1999 23:01:19 +0200 (CEST)
>
> G'day all,
>
> I was just have a bit of a look at the fs code, and noticed something a
> little odd. There are alot of 'goto' statements in the code. From what I
> can see they are not needed.
>
> eg :
> line 632 of /usr/src/linux/fs/open.c :
>
> struct file *filp_open(const char * filename, int flags, int mode)
> {
> struct inode * inode;
> struct dentry * dentry;
> struct file * f;
> int flag,error;
>
> error = -ENFILE;
> f = get_empty_filp();
> if (!f)
> goto out;
> ....
> <SNIP>
>
> out:
> return ERR_PTR(error);
> }
>
> this seems like an extra jmp statement to me (unless the compiler
> optimises it)
>
> ie why not just do this :
> struct file *filp_open(const char * filename, int flags, int mode)
> {
> struct inode * inode;
> struct dentry * dentry;
> struct file * f;
> int flag,error;
>
> error = -ENFILE;
> f = get_empty_filp();
> if (!f)
> return ERR_PTR(error);
> ....
> <SNIP>
> }
>
> Cheers Mof.
Hi.
The first compiles as:
cmp f,0
jz ret
...
ret: mov eax,error
leave
ret
However the second is something like:
cmp f,0
jnz nz
mov eax,error
leave
ret
nz:
...
It's faster when the code goes straight without jumps. Errors do not
happen often, so the first is better. The compiler doesn't know which
branch is used more, but we know it, so we must suggest the compiler where
to put jumps.
Mikulas
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