Date: Thu, 24 Mar 2005 12:03:19 -0800 From: "Vinod Kashyap" <vkashyap@amcc.com> To: Bruce Evans <bde@zeta.org.au> Cc: freebsd-amd64@freebsd.org Subject: RE: undefined reference to `memset' Message-ID: <IDVH3802.ESF@hadar.amcc.com>
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> This version makes the pessimizations and potential bugs clear:
> - clearing 100 bytes on every entry to the function is =
> wasteful. C90's
> auto initializers hide pessimizations like this. They should be
> used very rarely, especially in kernels. But they are =
> often misused,
> even in kernels, even for read-only data that should be =
> static. gcc
> doesn't optimize even "auto const x[100] =3D { 0 };" to a static
> initialization -- the programmer must declare the object =
> as static to
> prevent gcc laboriously clearing it on every entry to the function.
> - 100 bytes may be too much to put on the kernel stack. Objects just
> a little larger than this must be dynamically allocated unless they
> can be read-only.
> =
A statement like this (auto and not static) is necessary if you
are dealing with re-entrancy. Whatever the issues with wastage or
bad performance, a programmer should definitely be able to do it,
if he so desires. Also, the line I mentioned earlier was only
an example. Something like this also fails (your response already
explains this):
-----
struct x_type x =3D {0};
-----
> > Adding CFLAGS+=3D-fbuiltin, or CFLAGS+=3D-fno-builtin to =
> /sys/conf/Makefile.amd64
> > does not help.
> =
> -fno-builtin is already in CFLAGS, and if it has any effect =
> on this then
> it should be to cause gcc to generate a call to memset() =
> instead of doing
> the memory clearing inline. I think gcc has a builtin =
> memset() which is
> turned off by -fno-builtin, but -fno-builtin doesn't affect =
> cases where
> memset() is not referenced in the source code.
> =
> -ffreestanding should prevent gcc generating calls to library =
> functions
> like memset(). However, -ffreestanding is already in CFLAGS too, and
> there is a problem: certain initializations like the one in =
> your example
> need to use an interface like memset(), and struct copies =
> need to use and
> interface like memcpy(), so what is gcc to do when =
> -fno-builtin tells it
> to turn off its builtins and -ffreestanding tells it that the relevant
> interfaces might not exist in the library?
> =
> > Anyone knows what's happening?
> =
> gcc is expecting that memset() is in the library, but the =
> FreeBSD kernel
> is freestanding and happens not to have memset() in its library.
> =
How is it then, that an explicit call to memset (like in my example) works?=
As about other questions in the thread:
1. Yes, the example code is within a function,
2. I should have mentioned that I don't see the problem if I am
building only the kernel module. It happens only when I am building
the kernel integrating the module containing the example code.
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