[llvm-dev] Environment variables
David Greene via llvm-dev
llvm-dev at lists.llvm.org
Fri Sep 7 11:54:40 PDT 2018
David Blaikie <dblaikie at gmail.com> writes:
> Perhaps it'd make sense to just have one such environment variable
> entry point - perhaps in Clang's driver (& it'd use the environment
> variable as part of constructing the -cc1 command line - thus crash
> reports, etc, would still encode everything needed for reproducing the
> failure). Ensuring that all the options/configuration points are
> exposed at least via -mllvm style cl options (these are cheap to add -
> don't have to be plumbed through all the different layers, etc -
> intended for compiler-engineer tweaking/experiments/investigation).
>
> & that means not having lots of environment variable reading/testing
> all over the codebase, so probably not much need for generic/helper
> utilities
That sounds like a very reasonable approach. We've done similar things
in the past with other projects. The code I'm referencing is 30+ years
old, so the same design decisions almost certainly don't apply to much
newer code.
-David
> On Fri, Sep 7, 2018 at 7:59 AM David Greene via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>
> Sure, but we're not using this with ccache and other such things.
> We're
> very specifically using them for debugging purposes. It's very
> special-case and so we don't expect them to interact well with
> general
> tools. They're not meant for day-to-day use.
>
> -David
>
> Bruce Hoult <brucehoult at sifive.com> writes:
>
> > 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
> > >>>> _______________________________________________
> > >>>> LLVM Developers mailing list
> > >>>> llvm-dev at lists.llvm.org
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