[llvm-dev] The state of ARMConstantIslandPass in 4.0.[01]

[Ariel Ben-Yehuda via llvm-dev]

On 5/30/17, Renato Golin <renato.golin at linaro.org> wrote:
> On 29 May 2017 at 22:41, Ariel Ben-Yehuda via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Are these fixes important? Should we backport them? Do we need to keep
>> anything in mind?
>
> Hi Ariel,
>
> Those fixes are important, but as Tom said, only for 4.0.2 if we do one.
>
> However, they'll need to be looked at with care, as they fix some odd
> bugs in the compiler and I'm not sure how stable they really are.
>
> They may behave well in trunk because of other patches that were
> noticed and fixed, but back-porting just those patches may introduce
> instabilities that weren't fixed in 4.0.
>
> Given that we release a new version every 6 months, and that we
> (Still) don't have (community) support for older versions, you may be
> better off using 5.0 instead.
>
> Thanks for creating the merge requests, though. Let's look at them
> when/if 4.0.2 comes along.
>
> cheers,
> --renato
>

> They may behave well in trunk because of other patches that were
noticed and fixed, but back-porting just those patches may introduce
instabilities that weren't fixed in 4.0.

That's what's worrying me. Because we don't want to ship a version of
Rust on which developing on ARM is a minefield, we have to apply the
first 2 patches. However, it looks that we might also need to apply
the other patches, and I'm worried about whether and how can they be
safely backported. Could the patch authors pitch in?

> Given that we release a new version every 6 months, and that we (Still) don't have (community) support for older versions, you may be better off using 5.0 instead.

In our experience, every new version of LLVM both requires some
porting work and introduces new issues in non-P1 targets that need to
be fixed. We'll upgrade to LLVM 5.0 as soon as it gets released and we
port and apply the fixes. In the meanwhile we'll like a version in
which ARM is less of a minefield.

 - Ariel