[llvm-dev] unusual use of getenv

[Luke Drummond via llvm-dev]

Hi folks

I noticed something interesting when debugging a program that uses llvm
for JIT compilation.

Running `ltrace` surfaced a number of `getenv("bar")` calls coming from
llvm. It turns out these are not "real" `getenv` calls, but are an
optimization "do nothing" escape hatch which have been in
`llvm/include/llvm/LinkAllPasses.h` [for over 15years](1) - apparently
as a way to prevent the compiler eliminating symbol references to
optimization pass initialization functions. I took a look at the code
and couldn't really work out what issue is being solved as the commit
messages from 2005 have something to be desired ;)

I removed the whole function body from my local tree and `ninja check`
was happy in release mode (amd64-linux-gcc-10.2). Given its age, and the
fact that it's been through several iterations, I guess I've stumbled
upon a Chesterton Fence and would appreciate some input on whether this
is still needed. I see the original commit was Windows only, and was
then updated to use `getenv` as a way to support this behaviour
cross-platform.

It's more weird than pernicious given that nothing is done with the
result, but to me it feels dirty and confusing to query the process
environment in this way. As such, I wonder 3 things:

    1. Is this still needed? I don't know enough about the original
       toolchains affected to know if the problem still exists, but my
       limited testing shows that it doesn't seem to affect Linux
       builds.
    2. If 1: Is there a better way e.g. define our own function that
       can't be eliminated instead of `getenv` or use features of newer
       language standards and toolchains introduced since 2005 that might
       make the original problem go away on its own (I don't know what
       these might be).
    3. If 1 and not 2: could we make it more obvious that this comes
       from LLVM for those in my situation e.g.
       `getenv("LLVM_IGNORE_THIS_GETENV")` or similar instead of the
       unhelpful "bar" variable?

If it's no longer needed in any case, I can post a removal patch.

Any input is appreciated.

All the Best

Luke

[1] https://github.com/llvm/llvm-project/commit/00d5508496c1e#diff-7206f3725623127339dd17671577a6888ee3402d2e667ae9dd1457ea3600f4e7R3

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