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

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Date:	Fri, 02 Sep 2011 14:08:24 -0600
From:	David Ahern <dsahern@...il.com>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
CC:	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu, peterz@...radead.org, fweisbec@...il.com,
	paulus@...ba.org, tglx@...utronix.de, anton@...ba.org
Subject: Re: [PATCH v2] perf tool: fix endianness handling of u32 data in
 samples



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
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