Date: Sat, 21 Jul 2012 14:45:32 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Dimitry Andric <dim@freebsd.org> Cc: Kim Culhan <w8hdkim@gmail.com>, freebsd-current@freebsd.org, David Chisnall <theraven@freebsd.org> Subject: Re: -current build failure Message-ID: <20120721114532.GZ2676@deviant.kiev.zoral.com.ua> In-Reply-To: <20120721114316.GY2676@deviant.kiev.zoral.com.ua> References: <CAKZxVQV5xhFDN_WbTk-EMoQ18N8u1f4YhqKSJQFUzbX4NZxhUA@mail.gmail.com> <50097BF0.9010103@FreeBSD.org> <20120720163352.GS2676@deviant.kiev.zoral.com.ua> <9EBB4101-3117-4FE0-AD08-1053423BECD6@FreeBSD.org> <20120720231604.GT2676@deviant.kiev.zoral.com.ua> <500A93FE.1080700@FreeBSD.org> <20120721114316.GY2676@deviant.kiev.zoral.com.ua>
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[-- Attachment #1 --] On Sat, Jul 21, 2012 at 02:43:16PM +0300, Konstantin Belousov wrote: > On Sat, Jul 21, 2012 at 01:35:26PM +0200, Dimitry Andric wrote: > > On 2012-07-21 01:16, Konstantin Belousov wrote: > > > On Fri, Jul 20, 2012 at 10:07:05PM +0100, David Chisnall wrote: > > >> On 20 Jul 2012, at 17:33, Konstantin Belousov wrote: > > >> > > >>> It is not related to dtrace at all, and indeed OFFSETOF_CURTHREAD is 0. > > >>> This is a bug in clang, we compile our kernel in freestanding environment. > > >> > > >> The copies of the C spec that I have do not differentiate between > > >> freestanding and hosted environments for the validity of dereferencing > > >> a pointer value of 0. Doing so is undefined in all cases and any > > >> standards-compliant compiler is quite at liberty to eat your dog in > > >> such situations - it is explicitly not guaranteed to read the memory at > > >> linear address 0 (this is undefined for at least two reasons that I can > > >> think of from the C spec, and probably more). > > > > > > Ok, I stand corrected. But the standard does not say what you claim > > > either. It only specifies that NULL pointer is unequal to any pointer > > > to object or function (implicitely saying that you can create a C object > > > or function pointer to which is equal to NULL). > > > > > > So, lets reformulate it other way: freestanding implementation in clang > > > has no use, at least for general purpose kernel. Especially ridiculous > > > is the fact that clang throws it hands for asm inline wanting to get > > > null address, on the machine with linearly addressable memory. > > > > Oh come on, that's just hyperbole. Everybody understands that directly > > dereferencing a NULL pointer is very unusual, in any environment. It's > > perfectly sane to warn about it. Is it such a big problem to simply > > insert a cast to tell the compiler you really want to do this, even if > > it is highly unusual? Oh, just for record. I forgot to note this first time, and almost missed it now in response. The code in question _does not_ dereference NULL pointer. It is artificial quirk in the GNU inline asm syntax that rvalue is needed when I am passing memory _address_ to the asm. > > The point of existence of the inline __pure2 __curthread() is to allow > a compiler to cache the result of the call. Basically, the curthread > dereference uses %gs basing, which typically adds a measurable penalty > on the frontend and sometimes on the execution as well. > > Putting a volatile somewhere prevents the caching, right ? I am probably > fine with something along the lines of > #ifdef CLANG /* XXX what to put there */ > #define VOLATILE volatile > #else > #define VOLATILE > #endif > and then use VOLATILE in the cast. > > Could you recomment the best #if test ? > > How to test the change ? Is CC=clang make buildkernel enough ? > [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlAKllwACgkQC3+MBN1Mb4izJwCg9fKUNEB3UhDuXB6An0ROjbfK wMkAn2gJ8tZ78pt+1DdYlm/khRFkhpUO =Al1c -----END PGP SIGNATURE-----help
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