Date: Sat, 21 Jul 2012 12:59:07 +0100 From: David Chisnall <theraven@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Dimitry Andric <dim@freebsd.org>, freebsd-current@freebsd.org, Kim Culhan <w8hdkim@gmail.com> Subject: Re: -current build failure Message-ID: <8A03FF40-9A33-402F-AA01-E7C25EFF7430@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 21 Jul 2012, at 00:16, Konstantin Belousov wrote:
> 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).
See 6.5.3.2, paragraph 4. In particular:
> If an invalid value has been assigned to the pointer, the behavior of =
the unary * operator is undefined.
and then (footnote 87 from the same page):
> Among the invalid values for dereferencing a pointer by the unary * =
operator are a null pointer
This clearly states that any dereference of the null pointer is =
explicitly invalid. You can look up the correct paragraph for the other =
reason yourself (that any valid pointer must be the result of pointer =
arithmetic on a valid pointer).
> So, lets reformulate it other way: freestanding implementation in =
clang
> has no use, at least for general purpose kernel.
The term 'freestanding' has a well-defined meaning in the C =
specification. It does not mean 'the C memory model doesn't apply', it =
means that there is no underlying operating system providing facilities =
such as I/O. Compiling for a freestanding target does not let you opt =
out of the C memory model.
> 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.
C is not a macro assembler, it is a well-defined (mostly) language with =
a complex memory model. The relevant bit of code is:
> (*(char *)OFFSETOF_CURTHREAD)
This is dereferencing the NULL pointer if OFFSETOF_CURTHREAD is 0. If =
you want to pass the 0 into the assembly then passing it as something =
other than a memory operand would make sense. Or, as the warning =
suggests, add a volatile qualification to indicate that the asm is not =
actually dereferencing this pointer.
However...
> 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.
Clang designates address space 256 for GS-relative addressing (and 257 =
for FS-relative addressing), so you don't need to use any assembly if =
all that you want to do is load something relative to one of these =
segments. =46rom the clang manual:
> Annotating a pointer with address space #256 causes it to be code =
generated relative to the X86 GS segment register, and address space =
#257 causes it to be relative to the X86 FS segment. Note that this is a =
very very low-level feature that should only be used if you know what =
you're doing (for example in an OS kernel).
>=20
> Here is an example:
>=20
> #define GS_RELATIVE __attribute__((address_space(256)))
> int foo(int GS_RELATIVE *P) {
> return *P;
> }
>=20
> Which compiles to (on X86-32):
>=20
> _foo:
> movl 4(%esp), %eax
> movl %gs:(%eax), %eax
> ret
Using this instead of the inline asm allows the front end to perform =
numerous optimisations because it no longer has a bit of magic embedded =
in the middle of the code that the optimisers don't understand.
David=
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