[llvm-dev] Environment variables (original) (raw)

David Greene via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 7 07:56:55 PDT 2018

I agree with all of this. We have many environment variable hooks in the compiler but they're all for internal use and have very specific debugging purposes (turn off this-or-that, tune this parameter, etc.). We don't share them with customers at all.

They are a very useful debugging tool but need to be used judiciously.

                       -David

Matthias Braun <mbraun at apple.com> writes:

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 CCCOVERRIDEOPTIONS 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("ENABLEFOOBAR"); 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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