Constant byteorder macros.

[H. Peter Anvin]

Followup to:  <E12AyZR-0002gU-00 en devel2.axiom.internal>
By author:    David Woodhouse <dwmw2 en infradead.org>
In newsgroup: linux.dev.kernel
>
> 
> In order to use cpu_to_le16() et al. in case statements, for example:
> 
>  switch (status) {
> 	case cpu_to_le16(CONSTANT_XYZ):
> 
> 
> ... I need to define new macros __constant_cpu_to_le16(), etc.
> 
> Ideally, gcc would be able to tell that it's constant, and I would be able to 
> use the 'normal' cpu_to_le16 macro. But as someone's evidently already given 
> up on that with __constant_htonl() etc., I'm going to assume that it's not 
> currently possible.
> 

Given that gcc has a __builtin_constant_p(x), you ought to be able to
do:

#define cpu_to_le16(x) ( __builtin_constant_p(x) ? __constant_cpu_to_le16(x) : __cpu_to_le16(x) )

	-hpa
-- 
<hpa en transmeta.com> at work, <hpa en zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."

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