https://play.golang.org/p/VWysBwbPCu
The above program prints 64-bit aligned addresses on the playground, but that's a 64 bit platform (with 32 bit pointers). On a 32 bit platform x.j is not guaranteed to be 64-bit aligned. The same would be true if we substituted a WaitGroup for int64 in that example. On Fri, Feb 3, 2017 at 8:49 PM 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? > > > > 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.
