[rfc] a few kern.mk and bsd.sys.mk related changes

[Garrett Cooper]

On May 31, 2011, at 3:46 AM, Alexander Best <arundel at freebsd.org> wrote:

> On Tue May 31 11, Dimitry Andric wrote:
>> On 2011-05-31 11:57, Alexander Best wrote:
>> ...
>>>>> however i've often read messages - mostly by bruce evans - claiming that
>>>>> anything greater than -O will in fact decrease a kernel's ability to be
>>>>> debugged just as well as a kernel with -O.
>>>> The critical option when -O2 is used is -fno-omit-frame-pointers, since 
>>>> removing
>>>> frame pointers makes debugging impossible (on i386). With -O2 code is 
>>>> moved around and
>>>> removed, so debugging is more difficult, but can still provide useful
>>>> information.
>>> any reason we cannot use -O2 -fno-omit-frame-pointers -fno-strict-aliasing 
>>> as
>>> standard COPTFLAGS with debugging enabled for *all* archs?
>> 
>> Most likely, the performance gain from -O2 is rather small, except for
>> special cases, but the pain during debugging is increased a great deal.
>> 
>> Even if you add frame pointers, with -O2 large pieces of code can be
>> transformed, variables or even entire functions can be completely
>> eliminated, and so on, making debugging much more difficult.
> 
> *lol* we're moving in circles. so back to the beginning: why not use -O
> for all archs, if debugging was enabled? for amd64 -O2 is always set, no
> matter, if debugging is enabled or disabled.

I don't know, but I've run into cases where gcc has inlined or shuffled around code on amd64 with -O2 -fno-strict-aliasing, so I changed my make.conf to use -O0 when DEBUG_FLAGS was defined.
Thanks,
-Garrett