C99: Suggestions for style(9)

[Christoph Mallon]

M. Warner Losh schrieb:
> In message: <49FA8D73.6040207 at gmx.de>
>             Christoph Mallon <christoph.mallon at gmx.de> writes:
> : M. Warner Losh schrieb:
> : > In message: <20090428114754.GB89235 at server.vk2pj.dyndns.org>
> : >             Peter Jeremy <peterjeremy at optushome.com.au> writes:
> : > : >> +.Sh LOCAL VARIABLES
> : > : 
> : > : >Last, but definitely not least, I added this paragraph about the use of 
> : > : >local variables. This is to clarify, how today's compilers handle 
> : > : >unaliased local variables.
> : > : 
> : > : Every version of gcc that FreeBSD has ever used would do this for
> : > : primitive types when optimisation was enabled.  This approach can
> : > : become expensive in stack (and this is an issue for kernel code) when
> : > : using non-primitive types or when optimisation is not enabled (though
> : > : I'm not sure if this is supported).  Assuming that gcc (and icc and
> : > : clang) behaves as stated in all supported optimisation modes, this
> : > : change would appear to be quite safe to make.
> : > 
> : > Agreed, in general.  We don't want to optimize our code style based on
> : > what one compiler does, perhaps on x86.
> : 
> : I'm not sure whether I understand this - in particular the last three words.
> 
> I'm saying we shouldn't tune our coding standard to the optimizations
> that the compiler of the hour gives, especially if those optimizations
> to the style are tuned to one architecture.  Since there's little
> evidence presented on how these style changes will help any
> architecture, it is hard to judge if this is the case or not.

The main goal of the proposed change is not about optimisation, but 
about clarity of the code: It is better to declare multiple variables 
with meaningful names instead of having one generic "int k;" used in 
several different contexts. I just also mentioned, that re-using 
variables in different contexts when taking its address is involved, 
leads to worse machine code, but this is a minor point. It's just that 
clarity for a reader and quality of the generated code nicely correlate 
here.

	Christoph