Re: [Xen-devel] [PATCH 02/12] xen/arm: fix get_cpu_info() when built with clang

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

Sorry, I am picking up this series again.

On 4/18/19 7:03 PM, Stefano Stabellini wrote:
> On Wed, 17 Apr 2019, Julien Grall wrote:
> > Hi,
> >
> > On 4/17/19 9:45 PM, Stefano Stabellini wrote:
> > > On Wed, 27 Mar 2019, Julien Grall wrote:
> > > > Clang understands the GCCism in use here, but still complains that sp is
> > > > unitialised. In such cases, resort to the older versions of this code,
> > > > which directly read sp into the temporary variable.
> > > >
> > > > Note that we still keep the GCCism in the default case, as it causes GCC
> > > > to create rather better assembly.
> > > >
> > > > This is based on the x86 counterpart.
> > > I understand this is based on an existing approach but what about other
> > > compilers? I have a suggestion below.
> > What if the compiler actually support named registers? Why would we make the
> > code less efficient?
> It is not my intention to make the code less efficient for other
> compilers. However, reading the commit message and the patch I have the
> impression that the clang version is more likely to be applicable to
> other compilers, compared to the gcc version. More "standard". The
> reason is that the clang version only requires asm inline, while the gcc
> version requires both asm inline and named registers. For the sake of
> getting Xen to compile out of the box with any C compiler, I think it is
> best if we default to the less demanding version of the implementation
> for unknown compilers.
While building Xen out of box is nice goal to have, this is likely be 
very hard to reach out because Xen is using a lot of GCCism. It mostly 
work with Clang because they have adopted some of them.
I would be happy to revert the condition, but then AFAICT there are no 
pretty way to now that we are using GCC. While the define __GNUC__ is 
meant to tell you this is compiled with GCC, clang is also defining it.
So the condition would have to be

#if !defined(__clang__) && defined(__GNUC__)

But then if clang is already defining __GNUC__, what actually prevents 
any other to do it?
I have yet to see anyone wanted to build Xen with another compiler other 
than clang and GCC. So I will leave this patch as is. Feel free to 
suggest a different approach if you are not happy with this.
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel