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
}
>
>
>>
>>
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