Re: is portable aliasing possible in C++?

[Hei Chan <structurechart at yahoo dot com>]

On Monday, September 15, 2014 7:22 PM, Andrew Haley <aph@redhat.com> wrote:
On 09/15/2014 12:07 PM, Hei Chan wrote:
> 
>> On Monday, September 15, 2014 4:36 PM, Andrew Haley <aph@redhat.com> wrote:
>> On 15/09/14 03:36, Hei Chan wrote:
>>>
>>> This is an interesting thread.
>>>
>>> I think it is very common that people try to avoid making a copy
>>> from the buffer filled by recv() (or alike) to achieve lowest
>>> latency.
>>>
>>> Given that
>>> 1. The "union trick" has always worked with GCC, and is now hallowed
>>> by the standard.  So it sounds like GCC might change in the future.
>> 
>> Why?
> 
> Your statement that the trick "is now hallowed by the standard"
> makes it sounds like at some point GCC won't guarantee it work
> anymore.

I disagree.  It does not say that.  GCC will not change this
behaviour.

>>> 2. Somewhere in the code that might manipulate the buffer via
>>> somehow casted packed C struct.  Hence, any compiler is unlikely
>>> able to avoid making call if memcpy() is used.
>> 
>> I don't understand what you mean by this.  You can always write a
>> function which takes a pointer to a character type and calls memcpy()
>> to copy it into any scalar type, and it won't unnecessarily call
>> anything; or if it does that's a missed-optimization bug.
> 
> 
> Sorry, it is a typo -- I mean "compiler is unlikely able to avoid
> making *a copy* if memcpy() is used".

The compiler is likely to be able to avoid making a copy if memcpy() is
used.

> Using the unsafe reinterpret_cast (C fashion cast), it won't have an
> extra copy.

The alignment requirement is a property of the hardware.  It is not a
property of the software.  If the type needs aligning, it'll have to
be aligned somehow.  Using reinterpret_cast does not help.

> Using memcpy(), the compiler will have to make a copy
> because it sees that few lines, for example, down, the program tries
> to manipulate the copy.

So, don't manipulate the copy, then.  Use it once, then throw it away.


Sometimes, due to the endianness, I am forced to manipulate the copy...

>>> Then, I have the following questions:
>>> A. I use GCC and portability isn't an issue.  What is the best type
>>> punning method to achieve lowest latency?
> 
>> A union.  You need a union to guarantee alignment.
> 
> So I guess there is no way to avoid a copy if the code manipulates
> the member of the union, right?

There is no need for a copy.  I already produced an example which
proves that.

> I understand that union and memcpy() would guarantee alignment.  I
> was just hoping that there is a way of guaranteeing alignment
> without an extra copy.  Sounds like there is no way?

It depends on the processor; on x86 and some ARMs and others yes.  On
some others no.




Andrew.