On Sat, Feb 4, 2017 at 5:49 AM, T L <tapir....@gmail.com> wrote: > On Saturday, February 4, 2017 at 11:03:11 AM UTC+8, Matt Harden wrote: >> >> Never mind; I just realized that a WaitGroup is not necessarily at the >> start of an allocation or global variable. >> > > Not very get it. > Do you mean WaitGroup will be embedded in other types? > So we can't use the 64bit atomic functions for all exported types in > libraries? >
You can, but must make sure that they are aligned properly. Either like WaitGroup does, or, for example, by using the common idiom of factory functions that do the allocation (or by just putting it on the user to make sure). > > >> >> On Fri, Feb 3, 2017 at 7:01 PM Matt Harden <matt....@gmail.com> wrote: >> >>> Doesn't the statement "32-bit compilers to not ensure [64 bit alignment >>> at the start of an allocation]" contradict the sync/atomic statement "The >>> first word in a global variable or in an allocated struct or slice can be >>> relied upon to be 64-bit aligned."? >>> >>> On Fri, Feb 3, 2017 at 6:44 AM Ian Lance Taylor <ia...@golang.org> >>> 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 >>>> >>>> -- >>>> 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. >>>> 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. > -- 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.
