[LLVMdev] [RFC] Internal command line options should not be statically initialized.

Chandler Carruth chandlerc at google.com
Thu Sep 19 10:34:47 PDT 2013

On Wed, Sep 18, 2013 at 9:06 PM, Andrew Trick <atrick at apple.com> wrote:

> On Sep 18, 2013, at 8:58 AM, Chris Lattner <clattner at apple.com> wrote:
> On Sep 17, 2013, at 10:10 AM, Andrew Trick <atrick at apple.com> wrote:
> LLVM's internal command line library needs to evolve. We have an immediate
> need to build LLVM as a library free of static initializers, but before
> brute-force fixing this problem, I'd like outline the incremental steps
> that will lead to a desirable long term solution. We want infrastructure in
> place to provide an evolutionary path.
> Thank you for tackling this, we should have fixed this years ago.
> Please do a pass over the cl::opts we have, and remove ones that are long
> dead or unused.   Do we still need -join-liveintervals? :-)
> On Sep 17, 2013, at 12:03 PM, Daniel Dunbar <daniel at zuster.org> wrote:
> On Tue, Sep 17, 2013 at 11:29 AM, Reid Kleckner <rnk at google.com> wrote:
>> Wait, I have a terrible idea.  Why don't we roll our own .init_array
>> style appending section?  I think we can make this work for all toolchains
>> we support.
> Andy and I talked about this, but I don't think its worth it. My opinion
> is:
> 1. For tool options (the top-level llc, opt, llvm-as etc. opts) it doesn't
> matter.
> 2. For experimental options (options that we would be happy if they were
> compiled out of a production compiler/JIT client/whatever), it doesn't
> matter.
> 3. For backend options that need to always be available, lots of them
> probably already need to get promoted to real API.
> 4. For the remaining options (ones that don't need to become API, but also
> aren't purely experimental), many of them can probably easily be
> initialized by some existing initialization hook (pass initialization,
> target initialization).
> 5. There aren't enough options left not in those categories to motivate
> some kind of clever solution.
> I think that this is a great summary of the problem.  Having cl::opt's
> compiled *out* of non-assert build by default makes a lot of sense to me,
> and having tool options use toolopt<> (or something) also makes perfect
> sense.
> If you're going to go and tackle pass-specific options, I think that we
> should consider changing the syntax and overall design of the command line
> options.   We already have some manual name mangling/namespacification of
> options (e.g. -tail-dup-limit=). Perhaps we should formalize this somehow?
> Obviously, based on the 18 responses I've gotten, the tone of my first
> email was misleading.
> I don’t want to stifle discussion, but to be clear, the only thing I
> propose to tackle immediately is the removal of static initializers from
> libraries. There are several isolated issues that Filip has found good
> workarounds for. cl::opt is the one pervasive problem that can't be weeded
> out one case at a time.
> The purpose of posting an RFC and opening up discussion was to find out
> from people who have already thought about this, how the ideal cl::opt
> framework should work. I won't be making that happen, rather I'll make sure
> that the changes we make don't get in the way of future progress.
> I would certainly love to see LLVM internal options be reorganized and
> help however I can, but I'll be very sad if that holds up removing static
> initializers.

=/ I think we should actually implement the right long-term design rather
than something short term.

Anyways, I feel like there are (at least) three possible problems you want
to solve here, and I'd like to understand which (maybe all) you're actually
trying to solve, and which ones seem most important:

1) threads still alive during program termination reading from flags that
are being destroyed along with all globals

2) initialization ordering issues between flags in different translation

3) the existence of (non-zero-initializing) static initializers at all

For me, #1 and #2 are things I care a lot about and would be happy to see
solved. But #3 doesn't seem necessary or even desirable. We have a lot of
registration patterns in LLVM that make working with it very simple and
easy. It's not clear why we would want to preclude this, or re-invent the
mechanisms that already exist to automatically trigger static
initialization with the arbitrary fan-out of 'initializeFoo' global
functions. So if #3 is really an important goal, I'm curious about the why.
=] This is especially relevant as it impacts all of the work I'm starting
to do on the pass management and registration system.

As a somewhat separable point, I completely agree that every flag which any
frontend actually needs to control for correct functionality should be
moved from flags to an actual, proper interface as global flags just don't
work for a library. Essentially, they should be "debugging" tools or
"developer" tools, not actual interfaces. This isn't true today, and the
most egregious cases are the emission of debug information. All of that is
controlled through global flags, which causes lots of problems today for
our library users of LLVM.

However, I don't think the flags should only be present in !NDEBUG builds.
I think its reasonable for developers to debug problems with released
binaries by causing these flags to be toggled using '-mllvm' or related
tools in the frontends to manually parse flags, or by 'opt' automatically
handing the flag parsing down to this layer.
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