On Saturday, February 4, 2017 at 6:32:52 PM UTC+8, Axel Wagner wrote: > > The spec says this: > https://golang.org/ref/spec#Size_and_alignment_guarantees > Now, I can't really read from this whether this applies to fields or not. > If it does, it would seem to me, that, indeed, WaitGroup wouldn't need to > use the trick it does. > > For the example you name: It isn't particularly telling, as on nacl (at > least on x86{,-64}, don't know a lot about ARM) pointers have 32 bits and > so does int, meaning both rddeadline and wrdeadline are indeed 64-bit > aligned. >
Yes, there are 6 4-byte fields before the two 8-byte fields. Ok, then another question, is it safe to embed expvar.Float type in 32bit OSes? https://golang.org/src/expvar/expvar.go?s=1577:1608#L55 > > On Sat, Feb 4, 2017 at 11:12 AM, T L <tapi...@gmail.com <javascript:>> > wrote: > >> >> >> On Saturday, February 4, 2017 at 5:56:30 PM UTC+8, T L wrote: >>> >>> >>> >>> 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. >>> >> >> >> If this is true, why not define WaitGroup.state as uint64 directly? >> >> The following is the code in go src which thinks 8-byte sized fields in >> structs are also 64-bit aligned. >> >> // syscall/net_nacl.go >> >> type netFile struct { >> defaultFileImpl >> proto *netproto >> sotype int >> listener *msgq >> packet *msgq >> rd *byteq >> wr *byteq >> >> * rddeadline int64 wrdeadline int64* >> addr Sockaddr >> raddr Sockaddr >> } >> ... >> >> func SetReadDeadline(fd int, t int64) error { >> f, err := fdToNetFile(fd) >> if err != nil { >> return err >> } >> *atomic.StoreInt64(&f.rddeadline, t)* >> return nil >> } >> ... >> >> func SetWriteDeadline(fd int, t int64) error { >> f, err := fdToNetFile(fd) >> if err != nil { >> return err >> } >> * atomic.StoreInt64(&f.wrdeadline, t)* >> return nil >> } >> >> >> >> >>> >>> >>>> >>>> >>> -- >> You received this message because you are subscribed to the Google Groups >> "golang-nuts" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to golang-nuts...@googlegroups.com <javascript:>. >> For more options, visit https://groups.google.com/d/optout. >> > > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
