Re: [PATCH v2 8/9] bfs: remove multiple assignments

Re: [PATCH v2 8/9] bfs: remove multiple assignments

[Dmitri Vorobiev]

Tigran Aivazian wrote:
> On Sat, 26 Jan 2008, Dmitri Vorobiev wrote:
>> -    inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME_SEC;
>> +    inode->i_mtime = CURRENT_TIME_SEC;
>> +    inode->i_atime = CURRENT_TIME_SEC;
>> +    inode->i_ctime = CURRENT_TIME_SEC;
> 
> multiple assignments like "x = y = z = value;" can potentially
> (depending on the compiler and arch) be faster than "x = value; y =
> value; z=value;"
> 
> I am surprized that this script complains about them as it is a
> perfectly valid thing to do in C.

I think it seems wise to ask the maintainers of checkpatch.pl to
comment on that. I'm Cc:ing them now.

Thanks,

Dmitri

Re: [PATCH v2 8/9] bfs: remove multiple assignments

[Joel Schopp]

>>> -    inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME_SEC;
>>> +    inode->i_mtime = CURRENT_TIME_SEC;
>>> +    inode->i_atime = CURRENT_TIME_SEC;
>>> +    inode->i_ctime = CURRENT_TIME_SEC;
>> multiple assignments like "x = y = z = value;" can potentially
>> (depending on the compiler and arch) be faster than "x = value; y =
>> value; z=value;"
>>
>> I am surprized that this script complains about them as it is a
>> perfectly valid thing to do in C.
> 
> I think it seems wise to ask the maintainers of checkpatch.pl to
> comment on that. I'm Cc:ing them now.
> 

There are plenty of things that are valid to do in C that don't make for 
maintainable code.  These scripts are designed to make your code easier for 
real people to review and maintain.

As for if this can be faster we don't deal in the realm of "can".  Please 
show a concrete example of gcc making Linux kernel code faster with 
multiple assignments per line.  If you can do that I'm willing to change my 
mind and I'll lead the charge for mutliple assignments per line.

Re: [PATCH v2 8/9] bfs: remove multiple assignments

[Al Viro]

On Mon, Jan 28, 2008 at 01:02:03AM -0600, Joel Schopp wrote:
> >>>-    inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME_SEC;
> >>>+    inode->i_mtime = CURRENT_TIME_SEC;
> >>>+    inode->i_atime = CURRENT_TIME_SEC;
> >>>+    inode->i_ctime = CURRENT_TIME_SEC;
> >>multiple assignments like "x = y = z = value;" can potentially
> >>(depending on the compiler and arch) be faster than "x = value; y =
> >>value; z=value;"
> >>
> >>I am surprized that this script complains about them as it is a
> >>perfectly valid thing to do in C.
> >
> >I think it seems wise to ask the maintainers of checkpatch.pl to
> >comment on that. I'm Cc:ing them now.
> >
> 
> There are plenty of things that are valid to do in C that don't make for 
> maintainable code.  These scripts are designed to make your code easier for 
> real people to review and maintain.

Except that in this case the new variant is not equivalent to the old one...

Re: [PATCH v2 8/9] bfs: remove multiple assignments

[Dmitri Vorobiev]

Al Viro wrote:
> On Mon, Jan 28, 2008 at 01:02:03AM -0600, Joel Schopp wrote:
>>>>> -    inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME_SEC;
>>>>> +    inode->i_mtime = CURRENT_TIME_SEC;
>>>>> +    inode->i_atime = CURRENT_TIME_SEC;
>>>>> +    inode->i_ctime = CURRENT_TIME_SEC;
>>>> multiple assignments like "x = y = z = value;" can potentially
>>>> (depending on the compiler and arch) be faster than "x = value; y =
>>>> value; z=value;"
>>>>
>>>> I am surprized that this script complains about them as it is a
>>>> perfectly valid thing to do in C.
>>> I think it seems wise to ask the maintainers of checkpatch.pl to
>>> comment on that. I'm Cc:ing them now.
>>>
>> There are plenty of things that are valid to do in C that don't make for 
>> maintainable code.  These scripts are designed to make your code easier for 
>> real people to review and maintain.
> 
> Except that in this case the new variant is not equivalent to the old one...

Yes, you're right. In fact, I felt like sending yet another version
of these patches, but this gets preempted all the time by "the other things".

Dmitri