Date: Sat, 21 Jul 2012 13:35:26 +0200 From: Dimitry Andric <dim@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Kim Culhan <w8hdkim@gmail.com>, freebsd-current@freebsd.org, David Chisnall <theraven@freebsd.org> Subject: Re: -current build failure Message-ID: <500A93FE.1080700@FreeBSD.org> In-Reply-To: <20120720231604.GT2676@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>
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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?
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