On Monday, September 15, 2014 7:22 PM, Andrew Haley <aph@xxxxxxxxxx> wrote: On 09/15/2014 12:07 PM, Hei Chan wrote: > >> On Monday, September 15, 2014 4:36 PM, Andrew Haley <aph@xxxxxxxxxx> 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.
