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. state1 [12]byte sema uint32 } func (wg *WaitGroup) state() *uint64 { if uintptr(unsafe.Pointer(&wg.state1))%8 == 0 { return (*uint64)(unsafe.Pointer(&wg.state1)) } else { return (*uint64)(unsafe.Pointer(&wg.state1[4])) } } On Thursday, February 2, 2017 at 2:18:33 AM UTC+8, T L wrote: > > > > On Thursday, February 2, 2017 at 12:11:39 AM UTC+8, Jan Mercl wrote: >> >> On Wed, Feb 1, 2017 at 5:04 PM T L <tapi...@gmail.com> wrote: >> >> > But what does an allocated struct or slice means? A struct or slice >> allocated on heap, not stack? >> >> There's no heap nor stack from the POV of the language spec. Allocated >> means appropriate amount of storage is now used by the instance. Everything >> else is an implementation detail and has no influence on the semantics. >> > > So allocated means addressable? > > >> >> -- >> >> -j >> > -- 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.
