Building an ARM/RPI-B release (hacked) on CURRENT/AMD64.

[Ian Lepore]

On Thu, 2014-04-17 at 19:01 +0100, Mark R V Murray wrote:
> On 17 Apr 2014, at 13:49, Ian Lepore <ian at FreeBSD.org> wrote:
> 
> > U-boot requires that a global register be set aside by the compiler and
> > it's used to access all global vars.  As I vaguely understand it, u-boot
> > used to want r8 for this, and clang didn't used to support the concept
> > at all.  Now clang supports it, but only for r9, and apparently more
> > recent u-boot expects r9 rather than r8.  So the fix is probably to use
> > more recent u-boot sources (I've been using 2014.01 for imx6 stuff), and
> > probably to add the new -ffixed-r9 flag for a clang build.
> 
> Correct.
> 
> The pig in trying to build u-boot 2004.04 with Clang/XDEV is the use of
> 
> #define DECLARE_GLOBAL_DATA_PTR                register volatile gd_t *gd asm ("r9”)
> 
> which means “gd is an alias for the r9 register and is a pointer to type …”
> 
> … I think. :-)
> 
> Clang doesn’t like this one bit. First objection is to “global register variables”, so if I experimentally knock out the “register”, I simply get the second objection - to "multiple instances of the r9 global variable”.
> 
> Googling a bit suggests that Clang just plain can’t do this. :-(
> 
> M

Hmmm.  After a bit of poking around in the llvm code, it looks like the
full extent of the support for -ffixed-r9 is that it doesn't consider
that register available for use by the code generator; that's only part
of what u-boot needs.  

Some online notes I found for clang 3.5 claim that global register
variables aren't supported, and aren't likely to be any time soon.

-- Ian