On Saturday, February 4, 2017 at 5:40:25 PM UTC+8, T L wrote: > > > > On Friday, February 3, 2017 at 10:44:26 PM UTC+8, Ian Lance Taylor wrote: >> >> On Fri, Feb 3, 2017 at 5:38 AM, T L <tapi...@gmail.com> wrote: >> > Why does WaitGroup.state method check 64-bit alignment at run time? >> > Why not make the state field as the first word of WaitGroup struct? >> > >> > // https://golang.org/src/sync/waitgroup.go?s=1857:1892#L20 >> > >> > type WaitGroup struct { >> > >> > noCopy noCopy >> > >> > // 64-bit value: high 32 bits are counter, low 32 bits are waiter >> count. >> > >> > // 64-bit atomic operations require 64-bit alignment, but 32-bit >> > >> > // compilers do not ensure it. So we allocate 12 bytes and then use >> > >> > // the aligned 8 bytes in them as state. >> >> Doesn't this comment explain the problem? >> >> Ian >> > > Part of. Ok I get it now. > > BTW, I have another question, can an allocated local int64 value be relied > upon to be 64-bit aligned? > > By search the usages of 64bit functions in atomic package in go source, I think the answer should be yes. And the go source also imply any 8-byte sized fields, and the fields followed 8-byte sized fields, in any struct are also relied upon to be 64-bit aligned.
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