2015-07-03 16:56, Bruce Richardson: > On Fri, Jul 03, 2015 at 05:45:34PM +0200, Thomas Monjalon wrote: > > Hi Bruce, > > > > 2015-07-03 16:40, Bruce Richardson: > > > As well as the fast-path functions in the rxtx code, there are also > > > functions which set up and tear down the descriptor rings. Since these > > > are not performance critical functions, there is no need to have them > > > extensively optimized, so we add __attribute__((cold)) to their > > > definitions. This has the side-effect of making debugging them easier as > > > the compiler does not optimize them as heavily, so more variables are > > > accessible by default in gdb. > > > > What is the benefit, compared to -O0? > > First off, it's per function, rather than having to use -O0 globally. > Secondly, > it doesn't disable optimization, it just tells the compiler that the code is > not on the hotpath - whether or not the compiler optimizes it is up to the > compiler itself. From GCC documentation: > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes > > "The cold attribute on functions is used to inform the compiler that the > function is unlikely to be executed. The function is optimized for size > rather > than speed and on many targets it is placed into a special subsection of the > text section so all cold functions appear close together, improving code > locality of non-cold parts of program. The paths leading to calls of cold > functions within code are marked as unlikely by the branch prediction > mechanism. > It is thus useful to mark functions used to handle unlikely conditions, such > as > perror, as cold to improve optimization of hot functions that do call marked > functions in rare occasions."
I know it may provide some optimization of the hot path. I was asking compared to -O0 because you were justifying this change for debug. In other words, for debugging, -O0 is probably better. So the reason of this change should be the optimization. And it would be interesting to know if you have seen some performance improvement.
