Messages in this thread | | | Date | Fri, 02 Sep 2011 14:08:24 -0600 | From | David Ahern <> | Subject | Re: [PATCH v2] perf tool: fix endianness handling of u32 data in samples |
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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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