why to (not) support "older" compiler versions

[Iain Buclaw via Digitalmars-d]

On 3 November 2015 at 12:57, drug via Digitalmars-d <
digitalmars-d at puremagic.com> wrote:

> On 03.11.2015 14:11, Iain Buclaw via Digitalmars-d wrote:
>
>> On 3 November 2015 at 11:35, Daniel Murphy via Digitalmars-d
>> <digitalmars-d at puremagic.com <mailto:digitalmars-d at puremagic.com>> wrote:
>>
>>     On 3/11/2015 7:52 PM, drug wrote:
>>
>>         On 03.11.2015 11:22, Johannes Pfau wrote:
>>
>>
>>             I guess it's to be compatible with the latest DMD, LDC and
>>             GDC. GDC
>>             currently only provides the 2.066.1 frontend.
>>
>>         A bit offtopic - will the situation change with ddmd accepted? I
>>         mean
>>         the situation with different frontend version in different
>>         compilers.
>>
>>
>>     While DDMD does not have any direct effect on our ability to keep
>>     the three compilers synced, some of the cleanup work that has been
>>     done does help.
>>
>>
>> Whilst other clean-up work has destroyed years of stable compatibility
>> between different 'ends'.  ;-)
>>
> Hmm, I asked because I've heard that using ddmd would help with keeping
> the compilers synced and we would have the same version of frontend
> everywhere...
>

Well, how would that work?  :-)

What you've probably misheard is half of a phrase.  Moving to towards ddmd
is not to be confused with moving towards a shared 'frontend' codebase, and
is the first half of the correct sentence.  The second half is that even
then, that has no guarantee of keeping things in sync without also
integrating other 'ends' into the CI process.

This requires that we set-up an infrastructure where:

- New PRs are tested against all compilers before merging.  This not to be
confused with our current set-up where all compilers build DMD.
Specifically new changes upstream must:
  1. Be able to apply the change cleanly in their local repositories
  2. Build themselves without error.
- We then need another process in place to keep each end in sync after
changes upstream are applied.

It was hoped that moving towards ddmd would force a lot of the ABI-specific
code to be moved into Target or Port (host) interfaces that are agnostic to
the backend.  There are still many target-specific areas where this is not
the case, and on top of that there are regressions in the host-specific
interfaces.

In short, there will always be a heavy maintenance burden regardless of
what language we're written in. :-)

Iain
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