Date: Wed, 25 May 2011 12:02:43 -0400 From: Arnaud Lacombe <lacombar@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-current@freebsd.org, Dimitry Andric <dim@freebsd.org> Subject: Re: [PATCH] Fix CFLAGS overwrite by Makefile Message-ID: <BANLkTimgSdATAUyMbFyCPNigRF2vP8%2BA-A@mail.gmail.com> In-Reply-To: <201105250943.15362.jhb@freebsd.org> References: <1306267772-31084-1-git-send-email-lacombar@gmail.com> <4DDC17E5.2020700@FreeBSD.org> <BANLkTi=wbrvXHDiKix8DYYi9kjk5N-_vBw@mail.gmail.com> <201105250943.15362.jhb@freebsd.org>
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Hi, On Wed, May 25, 2011 at 9:43 AM, John Baldwin <jhb@freebsd.org> wrote: > On Tuesday, May 24, 2011 5:30:37 pm Arnaud Lacombe wrote: >> Hi, >> >> On Tue, May 24, 2011 at 4:41 PM, Dimitry Andric <dim@freebsd.org> wrote: >> > On 2011-05-24 22:09, Arnaud Lacombe wrote: >> >> >> >> Many Makefile (espectially under sys/boot/) overwrite the value of > CFLAGS. >> >> This is an issue if you want to generate code for a specific CPU as > before >> >> the >> >> Makefile is interpreted, CFLAGS might already have been set with CPU >> >> specific >> >> settings by<bsd.cpu.mk>, which is source from sys.mk. >> > >> > ... >> >> >> >> --- a/sys/boot/i386/boot2/Makefile >> >> +++ b/sys/boot/i386/boot2/Makefile >> > >> > ... >> > >> > The problem with this patch is that for some of the things you fixed, >> > stuff like boot-time programs, you NEVER want any CPU specific setting= s! >> > You must use the default, lowest common denominator setting instead, o= r >> > there is no guarantee the boot program will be correct. >> > >> To use your argument against you: with the default, the boot program >> is not correct (see below). > > Thousands of machines succesfully booting FreeBSD would seem to contradic= t > your assertion. > my assertion is backed by experiment, so I'm fine with the contradiction. >> > So that is why these Makefiles purposefully overwrite CFLAGS. it is no= t >> > by accident. >> You just might be right, but unless the code say the overwrite is >> _on_purpose_, I would not assume the state of mind of the author, one >> way or another. > > The boot code is certainly intended to be something that works across the > board. =A0Also, I doubt you will see any user-visible performance differe= nce > from changing the optimization options for the boot code. > Saving a few kilobytes of space might be more important than speed in some environment. >> > Besides, for space-constrained things like boot2, you >> > might not even be able to compile it when using non-standard settings, >> > since the code might grow too large. >> > >> or can shrink by using more optimized instructions. > > Well, your test in a later e-mail is a bit flawed. =A0GCC tends to insert > a lot of padding for newer CPUs to align things on more optimal boundarie= s. > That does not explain at all why the default setting always produces the worst code, size-wise. > We run 'sed' over the assembly version of boot2 to strip all that out. > ... to save 4 bytes. > However, the more important point for the boot code is that it needs to > just work. > and it does not, because gcc defaults are _bad_. - Arnaud
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