Re: Add -Wsuggest-attribute=cold

[Jeff Law <law@redhat.com>]

On 07/24/2017 01:08 PM, Jan Hubicka wrote:
>> On Mon, Jul 24, 2017 at 2:56 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>>> Hi,
>>> this patch adds -Wsuggest-attribute=cold because we can now statically detect
>>> cold functions atuomatically.
>>>
>>> Bootstrapped/regtested x86_64-linux. Plan to commit it tomorrow if there are no
>>> complains.
>>>
>>> Honza
>>>
>>>         * invoke.texi (Wsuggest-attribute=cold): Document.
>>>         * common.opt (Wsuggest-attribute=cold): New
>>>         * ipa-pure-const.c (warn_function_cold): New function.
>>>         * predict.c (compute_function_frequency): Use it.
>>>         * predict.h (warn_function_cold): Declare.
>>>
>>>         * gcc.dg/cold-1.c: New testcase.
>>
>> Would it be possible to also do -Wsuggest-attribute=hot for symmetry's
>> sake? Just wondering.
> 
> It would be nice, but it is kind of impossible to detect hot spots of the
> program with reasonable certainity. (that is why profile feedback is useful :)
> This cold attribute detection looks really for very simple pattern where the
> function inavoidably calls other cold function or does something similarly
> unlikely (Eh or trap).  This is fairly limited pattern, but it is useful i.e.
> to detect which libstdc++ functions can have this annotation that further
> improve branch prediction.
So what's the advantage of a user adding the attribute if the compiler
can infer it?  Presumably by adding the attribute, it's known without
analysis and can be taken advantage of by its callers?
Jeff