Re: Minor issue: dialyzer on ARM: Compiling some key modules to native code => Illegal instruction (core dumped)

Wouldn't a generally better solution be to make the no_native the default
and instead have to explicitly turn it on with a --native flag? I have
found I unless I am checking very many files it usually goes faster to turn
it off, at least when looking at the total time.

Robert


On 1 December 2014 at 14:58, Kostis Sagonas <[email protected]> wrote:

> On 12/01/2014 10:42 AM, Mikael Pettersson wrote:
>
>> Background: ARM processors typically support two related but different
>> instruction sets and execution modes: ARM (the ordinary one) and Thumb
>> (an alternative one which offers higher code density at the expense of
>> lower performance).  With ARMv7, the Thumb mode has been improved and
>> some environments make it the default.
>> ...
>> It's possible to work around this problem by forcing the VM to be
>> compiled in ARM mode (by adding "-marm" to CFLAGS).  I prepared a
>> patch to do that, and it fixed the problem on Mattias' system.
>>
>> Since changing the compilation mode may or may not be what the
>> user or system builder intended, I'm reluctant to do this silently.
>> Therefore I'm considering removing the auto-enable of HiPE on ARM.
>> Users will have to explicitly enable HiPE, and accept that BEAM will
>> be in ARM mode not Thumb.
>>
>
> I do not have a strong opinion but, from what you are describing and from
> some googling on ARM vs thumb on the internet, it seems to me that adding
> -marm to the CFLAGS will result in BEAM itself executing faster (or at
> least not slower than with thumb), and the main advantage of the Thumb mode
> is in better memory (i-cache?) utilization.  I am not sure why some
> (recent?) C compilers make Thumb the default, but I guess it's because the
> ARM processors are often used in embedded applications and they primarily
> want to optimize for code size rather than speed.  Not sure this is the
> primary goal of Erlang developers who download and install Erlang/OTP and
> e.g. want to run dialyzer (as Mattias) on that machine.
>
> So my current vote would go to add the -marm option anyway when building
> BEAM, and leave HiPE enabled by default on that platform too.  Erlang
> developers who are primarily interested in the memory benefits of the Thumb
> mode can explicitly enable it with an appropriate flag which would also
> disable HiPE then, at least until the following is fixed.
>
> It would be nice to investigate where the incorrect mode switch between
> HiPE and BEAM is and eliminate this if it's not too much work.
>
> Kostis
>
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