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Re: [Xen-devel] [PATCH 5/7] x86/alt: Support for automatic padding calculations

On Mon, Feb 12, 2018 at 03:04:21PM +0000, Andrew Cooper wrote:
> On 12/02/18 14:39, Wei Liu wrote:
> > On Mon, Feb 12, 2018 at 11:23:05AM +0000, Andrew Cooper wrote:
> >>  .macro ALTERNATIVE oldinstr, newinstr, feature
> >>  .L\@_orig_s:
> >>      \oldinstr
> >>  .L\@_orig_e:
> >> +     .skip (-((repl_len(1) - orig_len) > 0) * (repl_len(1) - orig_len)), 
> >> 0x90
> > Seeing the negation at the beginning, I suppose this should also be a
> > gas specific macro?
> The build failures are because clang's integrated assembler can't cope
> with non-absolute references with .skip, but we already know about this
> and have code identical to this in tree.  (I temporarily removed it in
> patch 4).

Newer clang (6) supports .skip with labels, but doesn't support the
(-(... And it's having some issues with the rest of the expression,
will have to check more closely tomorrow.

I wonder, what's Linux doing in this regard? It seems like clang/llvm
is quite committed to support building Linux, so it might be good to
follow suit in this case.


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