Re: [dpdk-dev] [PATCH 1/2] ixgbe: add "cold" attribute to setup/teardown fns

[Bruce Richardson <bruce.richardson@intel.com>]

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."

/Bruce