[LLVMdev] JIT on armhf

[Renato Golin]

On 8 February 2013 21:42, David Given <dg at cowlark.com> wrote:

> The box itself is an Allwinner A10; armv7l. /proc/cpuinfo says it's got
> swp half thumb fastmult vfp edsp neon vfpv3.
>

Yes, it's a Cortex-A8.


I've been unable to find any values for CPU which are accepted (it just
> says 'unknown target CPU'. I've tried arm, armv7, armv7a, armv7l, arm7,
> armv4t... Any suggestions? Is there a way to get clang and llc to emit a
> list of what triples they support?
>

armv7 and armv7a should default to Cortex-A8, "arm" will default to 7TDMI.


Target: arm-unknown-linux-gnueabihf
>

Auto-detected wrongly (as expected),



> "/usr/bin/clang" -cc1 -triple armv4t-unknown-linux-gnueabihf -S
>

Defaulted to armv4t == ARM7TDMI.


I'm particularly curious about the way that the triple passed into the
> compiler backend starts 'armv4t' when it's rejected as a CPU type if I
> specify it manually.
>

What does: 'clang -v -mcpu=cortex-a8 -S -O3 test.c' prints as a target?

Just need to do that on the latest Clang, and only paste the target line.


But this *should* all be auto-detected, right?


Should, but it isn't. Unfortunately, auto-detection in the ARM world is not
as simple as in the Intel world, and it's just not implemented.

If your compiler's name is "armv7a-unknown-linux-gnueabihf-clang", you
might get it right, since that part is implemented and should guess
cortex-a8 (not because it'll detect your CPU, but because it's hard-coded
armv7 -> A8).


If I'm using the JIT, I
> shouldn't need platform-specific knowledge to set up the code generator?
>

You should. The IR (which the execution engine runs) is not platform
independent. The front-end has to make some assumptions when generating IR,
depending on the platform, so you need to generate the correct code to
begin with.

Also, as Amara said, you can set hard-float manually, in the execution
engine, but if your CPU is still v4, I don't think it'll work. You should
make sure you got a v7, than force hard-float, and NEON, and then you'll
get the execution correct. However, if your command line contains
"-mcpu=cortex-a8", you should get all that for free when you build your
Target with the triple above.

cheers,
--renato
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