Re: Being more careful about iospace accesses..

[David Woodhouse]

On Wed, 2004-09-15 at 16:26 -0700, Linus Torvalds wrote:
>  - if you want to go outside that bitwise type, you have to convert it 
>    properly first. For example, if you want to add a constant to a __le16 
>    type, you can do so, but you have to use the proper sequence:
> 
> 	__le16 sum, a, b;
> 
> 	sum = a + b;	/* INVALID! "warning: incompatible types for operation (+)" */
> 	sum = cpu_to_le16(le16_to_cpu(a) + le16_to_cpu(b));	/* Ok */
> 
> See? 

Yeah right, that latter case is _so_ much more readable, and makes it
_so_ easy for the compiler to optimise precisely when it wants to do the
byte-swapping, especially if the back end has load-and-swap or
store-and-swap instructions. :)

It's even nicer when it ends up as:

	sum = cpu_to_le16(le16_to_cpu(a) + le16_to_cpu(b));	/* Ok */
	sum |= c;
	sum = cpu_to_le16(le16_to_cpu(sum) + le16_to_cpu(d));

I'd really quite like to see the real compiler know about endianness,
too. I dare say I _could_ optimise the above (admittedly contrived but
not _so_ unlikely) case, but I don't _want_ to hand-optimise my code --
that's what I keep a compiler _for_.

-- 
dwmw2


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