[llvm-dev] Enabling statistics in release builds / static constructors

[Matthias Braun via llvm-dev]

> On Dec 13, 2016, at 12:56 PM, Reid Kleckner <rnk at google.com> wrote:
> 
> Given that LLVM has so many auto-registration systems (cl::opt, target registry, pass registry, statistics, I'm sure there's more), maybe we should spend the time to build an auto-registration system that doesn't involve static constructors?
I would volunteer to do the work, however this obviously needs some consensus first on how that would look.

> 
> It needs custom code for every supported object file format, and is hard to get right when DSOs are involved, but in the long run it's probably worth fixing this problem once and for all.
I assume you are thinking about creating custom linker sections with list of init functions; Similar to the existing constructors sections but running at a time controlled by llvm code. While the compiler/linker nerd in me would love doing that, I could see this being very tricky to pull off consistenly on all platforms.

We should not forget that there is a portable and proven solution: Just write the code!
So here comes the strawman:


static Statistic NumBlips("blips");
static cl::opt MyOpt("my-cool-option", cl::desc("bla"));
static cl::opt AnotherOpt("bla", cl::desc("foo bar"));
// Note that the constructors of Statistic and cl::opt would be reworked to be pure constexpr and do not run any code

static void init_globals() {
  NumBlips.init();
  MyOpt.init();
  AnotherOpt.init();
}
// Note that the init_globals() function is pretty mechanical so hopefully easy to understand and maintain.

We have to call init_gloabals somewhere early:
- Put an init_globals() call into code that runs early.
- We already have a lot of early running functions called initializeXXXPass() which we can use for this
- For the remaining files we probably have to export init_globals() and call it from some common place of the library.

If someone comes up a with working solution for putting initializers into a section we can later replace the
init_globals() function with that so the refactoring work to split into constexpr constructor and init() is not wasted either way!

- Matthias