RFR: Add support for ARMv6 to ARM JIT

[Edward Nevill]

On Thu, Jan 22, 2015 at 10:00 AM, Andrew Haley <aph at redhat.com> wrote:

> On 22/01/15 09:53, Edward Nevill wrote:
> > On Wed, Jan 21, 2015 at 5:16 PM, Andrew Haley <aph at redhat.com> wrote:
> >
> >> On 01/21/2015 05:06 PM, Edward Nevill wrote:
> >>
> >>> Because of the size of the changes I have introduced a new file
> >>> arm32JIT.cpp rather than modifying the existing thumb2.cpp. This is
> >>> switched on a symbol ARM32JIT. By default it will continue to build
> >>> the Thumb2 JIT. To change it to build the ARM32 JIT by default add
> >>> -DARM32JIT to CFLAGS in zeroshark.make as in the following patch.
> >>
> >> I don't get this.  Surely we should have both, switchable at runtime.
> >
> > arm32jit.cpp supports generation for ARMV6, ARMV6K and ARMV6T2+.
> >
> > For ARMV6 and ARMV6K it will generate 32 bit ARM instructions.
> >
> > For ARMV6T2 and later it will generate Thumb2 instructions by defaulty,
> but
> > this can be changed by setting the T2_COMPILE_ARM environment variable.
>
> So what is thumb2.cpp for?  I still don't get it.
>

The idea was to minimise the change to reduce the risk of breaking anything
for distros. The migration from thumb2 JIT to arm JIT would be done in 3
stages.

Stage 1:

Introduced arm32JIT.cpp but still have the default build from the old
thumb2.cpp which is unchanged apart from the one #ifndef ARM32JIT.

Distros can then elect to choose the arm32 JIT by applying the 1 small
patch which adds -DARM32JIT to zeroshark.make. This patch can be carried
within the distro packaging. Alternatively distros can ignore the arm32 JIT
and continue with the unchanged thumb2 JIT as before.

Stage 2:


When everyone is happy with the arm32 JIT make ARM32JIT the default.
Distros can still elect to build the thumb2 JIT by backing out the
-DARM32JIT in zeroshark.make

Stage 3:

Sometime in the future delete thumb2.cpp

Of course if people are happy to go straight to stage 3 that is fine by me.
The only question then is whether the changes should just be applied to
thumb2.cpp which would now generate thumb2 or arm32 code or whether to
rename it to arm32JIT.cpp.

All the best,
Ed.
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