Matt Mackall wrote:
>
>
>> Yes, but:
>>
>> if (expr1 && expr2)
>> var = true;
>> else
>> var = false;
>>
>>is usually better turned into something that avoids jumps
>>when it's safe to evaluate both parts unconditionally:
>>
>> var = (expr1 != 0) & (expr2 != 0);
>>
>>or (if you can stand it):
>>
>> var = !!expr1 & !!expr2;
>>
>>
>
>Such ugly transformations are a job for compiler writers and may
>occassionally be acceptable in some critical paths. The IO path, which
>is literally dozens of function calls deep from read()/write() to
>driver methods, does not qualify.
>
What's ugly about them? If I were a compiler developer, I would look
for "!!" (which I'm sure many compilers do) and deal with it properly.
I have seen, however, that gcc produces the same machine code for { if
(x) {} } as for { if (x != 0) {} }. Additionally, I would put "!!" in C
programming books so that people understand what it means when they come
across it. In my mind, it's the "make-it-a-bool" operator.
I certainly don't advocate optimizations that completely obfuscate the
meaning of the code, but for ones which are relatively innocuous and
make sense, why not? When not to do that is when you know what the
compiler is going to do with it. If you can add more characters so that
it makes it more understandable without impacting what the compiler
produces, then by all means, do it. Another way to "add more characters
to make it readable without impacting code size" is to add comments. :)
Not to say that I'm any saint in that area.
But I do appreciate it when people take the time to write good,
explanatory comments. I'm not saying that you should comment every line
(do you comment your comments? :), but putting something before the
function which explans it is always a good thing, IMHO. Even when faced
with the most readable code, I have some sort of mental block. I like
it when I get to read long english textual descriptions of the POINT
behind a function before I read the code so I have an abstract framework
into which I fit the details. I have a love-hate relationship with details.
Also, It seems that not all compilers perform these "obvious"
optimizations. But if any of the gcc contributors are watching some of
the recent lkml discussions, I have faith that they'll add some of those
optimizations.
Anyhow, I have no emotional attachment to my opinions about comments. I
do it my way, you do it yours. I see the merit in all sides of it. The
only problem is that if I have trouble reading your code, I will feel
less inclined to read it. Oh well.
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This archive was generated by hypermail 2b29 : Wed Apr 23 2003 - 22:00:21 EST