Re: [RFC PATCH 1/2] Replace memset(0) with static initialization where possible

From: Brandon Casey <casey@nrlssc.navy.mil>
Date: Oct 9, 2008, 20:56 UTC
Message-ID: <CqZt8k7g-Uov2ItkgRw6K65RC0ee37ZAckIrbwPLNwDiaHtigtevnA@cipher.nrlssc.navy.mil>
In-Reply-To: <alpine.LFD.2.00.0810091534430.26244@xanadu.home>

Nicolas Pitre wrote:

Show 18 quoted lines

> On Thu, 9 Oct 2008, Brandon Casey wrote:
> 
>> "Possible" benefits:
>>
>>   1) more concise, so it improves readability in most cases
>>   2) gives compiler more flexibility when optimizing
> 
> Beware beware.  At some point this wasn't a gain at all with some gcc 
> versions as it was stupid enough to construct a temporary object on the 
> stack corresponding to "{ 0, }" and afterward do the assignment by 
> *copying* this object to a different stack slot corresponding to the 
> actual variable instead of constructing the initial value in place.
> 
> Also note that, on the other hand, gcc is smart enough to optimize a 
> memset() using a constant size and value already by doing appropriate 
> code replacement inline.  I've also seen cases where gcc did the 
> opposite and replaced an explicit assignment like your patch does with 
> an actual call to memset() when optimizing for size.

I'd call that flexibility.

> So when claiming flexibility for the compiler to better optimize things, 
> please make sure this is really what is happening through assembly dump 
> inspection.

I didn't claim the compiler _would_ do a better job at optimizing if memset wasn't used (though my expectation is that it would do no worse, and I don't have assembly dumps to back that up). I suggested it could give the compiler more flexibility. In some strange way you seem to agree with me and have given 3 examples of ways that compilers may optimize static initialization. :)

Anyway, this is a nothing patch. There is no functional change. If readability is not improved, it is not worth applying. Of course, I don't plan on scanning through and converting all of the existing assignments which use { 0, } to use memset. I find a single step declaration/initialization simpler. Not sure why that seems to be the case in the git source for simple variables but not structures.

-brandon