Re: [Xen-devel] [PATCH 02/34] xen: clang: Disable built-in assembler

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Sat, 2014-03-29 at 22:55 +0000, Julien Grall wrote:
> Hi Tim,
> 
> On 27/03/14 18:01, Tim Deegan wrote:
> >
> > The patch below works for me (at least as far as building
> > asm-offsets.h on x86) by wrapping everything in a string.  I did try
> > just prefixing with '#' but clang 3.5 also strips the comments out.
> > That seems unhelpful, since I know some people put comments in their
> > inline assembler too. :(
> 
> I'm able to build correctly x86 with your patch, and this patch (e.g #2) 
> reverted.
> 
> But for ARM ... it breaks in another place :(
> 
> vfp.c:8:25: error: invalid operand for instruction
>      v->arch.vfp.fpexc = READ_CP32(FPEXC);
> <inline asm>:1:6: note: instantiated into assembly here
>          mrc p10, 7, r1, c8, c0, 0;
>              ^
> 
> Coprocessor p10 (and p11) are used for Neon instruction are clang 
> doesn't allow to use it directly. 
> (http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20131104/194170.html).
> 
> Depends on which part of the ARM ARM you are reading, p10 and p11 should 
> not be used directly with mrc/mcr instruction ... but gas accept it.
> 
> I guess the best solution is to use directly the VFP instructions but it 
> would mean to re-enable VFP at compile time in Xen (see 
> http://www.gossamer-threads.com/lists/xen/devel/284653?do=post_view_threaded).

Do the VFP instructions have different encodings? I thought this was an
assembler mnemonic difference only.

Ian.


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