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Date: Tue, 31 May 2011 12:37:00 +0200
From: Dimitry Andric <dim@FreeBSD.org>
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To: Alexander Best <arundel@freebsd.org>
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Cc: Bruce Cran <bruce@cran.org.uk>, freebsd-hackers@FreeBSD.ORG,
	freebsd-toolchain@FreeBSD.ORG, Pan Tsu <inyaoo@gmail.com>
Subject: Re: [rfc] a few kern.mk and bsd.sys.mk related changes
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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.