Re: thumb2 support

[Rich Felker <dalias@libc.org>]

On Sun, May 04, 2014 at 02:56:49PM +0100, Justin Cormack wrote:
> I was wrong, sorry, __thumb__ and __thumb2__ are defined. Maybe
> __ARM_ARCH_ISA_THUMB does seem to be not defined always.

OK, so the patch as committed should at least be ok, even if not
optimal.

> Some armv6 machines have thumb2, but not many (ARMv6T2 apparently, I
> don't have one I don't think).

In any case it's not broken on them, just using the kuser helpers
rather than inlines.

> All v7 machines do have (only) thumb2.
> So you could test ARM_ARCH_6T2 or __thumb2__.

We could change the !__thumb__ test to (!__thumb__||__thumb2__).

> Alternatively it seems you can compile with -Wa,-mimplicit-it=always
> and you don't need the patch at all. That could make it easier in
> future.

Yes, but it doesn't fix the fact that we have to omit the asm entirely
on thumb1. Also adding mandatory arch-specific CFLAGS is probably more
ugly than tucking away this logic in the arch-specific atomic.h file.

> So far all the thumb2-only hardware has no MMU, so less likely to be
> using musl...

And if it is using musl it will probably need a special
bare-metal-type port with its own asm dirs rather than using the
existing Linux/ARM asm. (Of course rdp has an approach that shares the
Linux asm and syscall framework despite being "bare-metal".)

Rich