Re: [PATCH v2] perf tool: fix endianness handling of u32 data in samples

[David Ahern <dsahern@gmail.com>]

On 09/02/2011 02:01 PM, Arnaldo Carvalho de Melo wrote:
> Look again:
> 
> #define WARN_ONCE(condition, format...) ({                      \
>         static bool __warned;                                   \
>         int __ret_warn_once = !!(condition);                    \
>                                                                 \
>         if (unlikely(__ret_warn_once))                          \
>                 if (WARN(!__warned, format))                    \
>                         __warned = true;                        \
>         unlikely(__ret_warn_once);                              \
> }) 
> 
> 
> See that ({ }) construct? It evaluates to what is in its last statement,
> which is...
> 
> 	unlikely(__ret_warn_once);
> 
> Forget about the unlikely, __ret_warn_once is:
> 
> 	!!condition
> 
> I.e. it always evaluates to what is passed as condition, so in fact it
> could be seen as:
> 
>              if (swapped) {
>                      /* undo swap of u64, then swap on individual
>                      u32s */
>                      u.val64 = bswap_64(u.val64);
>                      u.val32[0] = bswap_32(u.val32[0]);
>                      u.val32[1] = bswap_32(u.val32[1]);
>              }
> 
> The rest is the boilerplate needed to warn the user the first time
> condition is true.
> 
> - Arnaldo

Ok, I get it now.

David