Date: Sat, 21 Jul 2012 14:43:16 +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: <20120721114316.GY2676@deviant.kiev.zoral.com.ua> In-Reply-To: <500A93FE.1080700@FreeBSD.org> 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>
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--aSb4XDBEo1qAP/IB Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 environ= ment. > >> > >> 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). > >=20 > > 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). > >=20 > > 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. >=20 > 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? 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=3Dclang make buildkernel enough ? --aSb4XDBEo1qAP/IB Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlAKldQACgkQC3+MBN1Mb4gy4ACgrfWAQU17xAURP6Ku9JMeZ1sX uQ4AoMan5lkR572n0BeHWRjy9s1CEzy/ =4ZPB -----END PGP SIGNATURE----- --aSb4XDBEo1qAP/IB--
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