[aarch64-port-dev ] RFR (M) 8228400: Remove built-in AArch64 simulator

[Andrew Dinn]

On 28/07/2019 21:03, Aleksey Shipilev wrote:
> On 7/28/19 6:52 PM, Dmitry Samersoff wrote:
>> 1. Do I understand correctly that we no longer use number_of_arguments
>> parameter?
> 
> Yes, I think so.

Yes we don't need to use this parameter. Indeed we could probably change
the signature to match the fact that we never supply an argument for it.
However, ... The reason it is declared in the Aarch64 code is because it
mirrors the code for x86_64. The argument is not needed for x86_64
either but the code is dual purpose for x86_32 where the number of
parameters is needed. So, by dropping this parameter we would be
choosing to diverge from x86_64.

I'm not sure how much virtue there is in doing that. When we first
ported the x86_64 code to Aarch64 we tried to keep the code for the two
ports aligned as far as possible (i.e. only diverge when the
architecture and/or performance required it). n.b. that's much the same
tactic as is adopted when backporting and happens for much the same
reasons -- many innovations happen first in x86, hence need /cross/
porting to AArch64.

This policy has occasionally led to minor oddities like this one but it
has also made development and maintenance much easier. Diverging on this
specific point probably wouldn't matter too much one way or the other.
So long as whoever is maintaining the code knows that it is derived from
x86_64 they can easily make allowance such a minor differences. However,
it gets more and more difficult to port code as these sort of changes
accumulate. Personally, I would prefer to keep the two ports aligned as
far as possible because my experience is that it has made it a lot
easier to avoid errors and spot defects. i.e. I think it is a benefit
rather than a problem that maintainers really need to keep that
alignment in mind.

>> Should we remove it and version of call_VM_leaf on l. 1430
> 
> Maybe? I would leave it as follow-up. The change would be local and easy to test separately.
> Unfortunately, it would invalidate lots of testing already done for this patch. I can see how much
> hassle that would be, and maybe fold that improvement here...

See above. However, Aleksey is right that this should be done as a
follow-up patch if at all.

regards,


Andrew Dinn
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