[LLVMdev] Bi-Endian Compiler

[C Bergström]

On Fri, Jul 31, 2015 at 12:15 AM, Herbie Robinson
<HerbieRobinson at verizon.net> wrote:
> I have been quietly working on a port of LLVM to Stratus VOS.  The OS is
> always big endian (because customers need to migrate data to new hardware
> quickly and can't afford to convert).  This entails presenting a big endian
> programming model no matter what the hardware does.  We did this about 15
> years ago with the Stratus compilers and GCC.  We offered the GCC changes
> back to the community, but the GCC folks declined to take them...
>
> In my copious spare time, I've already written (but not debugged) an LLVM
> pass that reverses the endianess of the underlying processor.  The LLVM IR
> is actually quite good for this.  Believe it or not, the hard part is
> actually dealing with initialization!
>
> I am leaving on vacation as soon as my GPS maps get updated; so, I don't
> have time to read your paper, now.  I will look at it when I get back.
> Perhaps we can collaborate.  If nothing else, I participated peripherally in
> the Stratus implementation.  I would like to say a lot more, but I need to
> hit the road...

We've done this as well.
http://www.pathscale.com/Bi-ENDIAN

The key piece will be a few things
1) Allowing a way to force endian based off an attribute. This can be
helpful for when you flip the bits, but need to try to triage exactly
where the problem is
2) As other stated - the llvm ir is relatively easy for this - most of
our changes are on the clang side
3) Be *very* careful about patents. There are a number of compiler
companies that patents around this area.

IANAL and I would not typically say this, but it's something which
llvm should approach cautiously and probably get advice on before
accepting patches.

/* I have full docs on what we've done, but it's not public. I can
share with any interested parties */

Thanks