[llvm-dev] Environment variables

[Bruce Hoult via llvm-dev]

Env vars that change compiler output make it impossible to write tools such
as ccache or distcc. Including the entire env in the hash value that
determines whether ccache has a cache hit (as well as the compiler command
line and the preprocessed source file) would be ridiculous and would result
in very few cache hits.

On Thu, Sep 6, 2018 at 11:34 AM, Matthias Braun via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I can definitely relate to third party Makefiles being a huge pain to
> manipulate. And env vars can be an okay tool to help debugging (see also
> CCC_OVERRIDE_OPTIONS in clang for example). I also don't want to dispute
> that they may be the right solution in some cases.
> That said in my opinion we should not make it look like using environment
> variables is a good or encouraged thing because there are downsides:
>
> - The bug reproducetion instruction and file bundles  that clang produces
> when it crashes does not show environment variables, meaning you possibly
> cannot reproduce a bug...
> - Similarily when producing .ll files, going through jenkins output, etc.
> You typically have no good way to see the environment variables in effect.
> Even if you do, there are typically hundreds of them and it may be hard to
> know which ones affect the compiler.
> - env vars only work more reliably if they are sparsly used as soon as
> they have interesting/desirable effects you will see people using them in
> their makefiles as well, making the whole thing brittle again because now
> you have to wonder if someone overrides, resets, manipulates the env vars
> in their makefiles bringing you back to square one where you have to
> understand and change complex third party makefiles...
>
> - Matthias
>
> > On Sep 6, 2018, at 10:44 AM, David Greene <dag at cray.com> wrote:
> >
> > Yes, but in your example getenv is called every time enableFooBar needs
> > to be initialized.  What if your code is itself wrapped inside another
> > loop you can't see (for example, the PassManager invoking passes)?
> >
> > Maybe I'm being overly pedantic.
> >
> > We use a lot of environment variables in our compiler because it's
> > really super annoying and takes a lot of developer time to have to
> > update customer Makefiles to include the debugging options we want to
> > use to debug customer problems.  These are huge customer codes with
> > often many Makefiles which may be generated by custom tools we don't
> > understand at all.  :)  It's much easier to use the compiler flags that
> > are in the Makefiles and set some environment variables to override
> > things during "make."
> >
> > It seems odd that cl::ParseEnvironmentOptions exists but there is no
> > "official" way to get at environment variables.
> >
> > If this isn't something the community wants or needs, that's fine.  I
> > was just asking if a contribution would be welcomed if we end up
> > developing something.
> >
> >                       -David
> >
> > Matthias Braun <mbraun at apple.com> writes:
> >
> >>> On Sep 6, 2018, at 7:09 AM, David Greene via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>>
> >>> Ok, thanks!  I'm not dealing with UTF-8 so I don't think
> Process::GetEnv
> >>> will work.  I was looking for something that caches calls to getenv so
> >>> checks could be put into tight(-ish) loops without too much performance
> >>> impact.
> >>
> >> Sorry for the snarky answer but we already have that:
> >>
> >> // outside of loop
> >> bool enableFooBar = getenv("ENABLE_FOO_BAR");
> >> while (...) {
> >>    // it's not getting re-checked every loop iteration:
> >>    enableFooBar;
> >> }
> >>
> >> Generally we don't really look at env vars today (I think for clang
> >> you can mostly affect some search paths with them) and IMO it is a
> >> good thing to force being explicit on the command line instead...
> >>
> >> - Matthias
> >>
> >>>
> >>>                          -David
> >>>
> >>> Reid Kleckner via llvm-dev <llvm-dev at lists.llvm.org> writes:
> >>>
> >>>> llvm::Process::GetEnv looks like it does the right thing.
> >>>>
> >>>> I think we added it to deal with Unicode on Windows, though. We have
> >>>> plenty of calls to getenv that are mostly looking for '1', '0', or
> >>>> variable presence, and they pretty much work.
> >>>>
> >>>> On Wed, Sep 5, 2018 at 2:12 PM David Greene via llvm-dev
> >>>> <llvm-dev at lists.llvm.org> wrote:
> >>>>
> >>>>   Is there an LLVM-ish way to handle environment variables?
> >>>>   Specifically,
> >>>>   I want to check existence and/or value of an environment variable
> >>>>   and
> >>>>   take appropriate action.
> >>>>
> >>>>   I was kind of surprised that LLVM doesn't seem to have any
> >>>>   special/optimized way to deal with environment variables. The one
> >>>>   Stackoverflow I found on it suggested using getenv().
> >>>>
> >>>>   Thanks!
> >>>>
> >>>>   -David
> >>>>   _______________________________________________
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