[PATCH] D126341: Order implicitly instantiated global variable's initializer by the reverse instantiation order

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


================
Comment at: clang/lib/CodeGen/CGDeclCXX.cpp:563
+  } else if (IsInstantiation ||
              getContext().GetGVALinkageForVariable(D) == GVA_DiscardableODR ||
              D->hasAttr<SelectAnyAttr>()) {
----------------
rnk wrote:
> @rsmith , if inline global variable initialization has ordering requirements, we have a bug, because I believe this GVA_DiscardableODR codepath handles them, and we come through here to give them separate initializers in llvm.global_ctors. See the example with two separate global_ctors entries on godbolt:
> https://gcc.godbolt.org/z/5d577snqb
> 
> As long as LLVM doesn't provide ordering guarantees about same priority initializers in global_ctors, inline globals have the same problems as template instantiations. IMO whatever solution we use to order inline globals should be used for template instantiations.
> 
> Intuitively, that means LLVM should promise to run global_ctors in left-to-right order, and if all TUs instantiate initializers in the same order, everything should behave intuitively.
> 
> The question then becomes, why doesn't this work already?
> Intuitively, that means LLVM should promise to run global_ctors in left-to-right order

I don't think that's sufficient, due to the way we use COMDATs to discard duplicate global initializers. Consider:

TU 1 defines inline variable A.
TU 2 defines inline variable A and then inline variable B.
The standard guarantees that A is initialized before B in this scenario. But if (somehow) the linker picks the definition of A from TU 2, but orders the initializers from TU 1 first, then the resulting global_ctors order will be B then A, which is not allowed.

Either we need a guarantee that linkers will use the same ordering between objects when picking COMDATs as when concatenating `.init_array`, or we need to stop using the COMDAT trick for them (and either make LLVM respect the `@llvm.global_ctors` order or coalesce all inline variable initializers into the same function we run non-inline initializers from or something). Getting that guarantee seems like the best path to me, since the other option will presumably mean we check the guard variable once on startup for each TU that defines the variable, and it's something I expect linkers already happen to guarantee in practice.

> IMO whatever solution we use to order inline globals should be used for template instantiations.

That sounds like it would regress what globalopt is able to optimize, for no gain in conformance nor perhaps any real gain in initialization order guarantees. The inline variable case is different, both because we can guarantee an initialization order and because the standard requires us to do so. Should we add complexity to the compiler whose only purpose is to mask bugs, if that complexity doesn't actually define away the possibility of bad behavior?

If we can actually describe a rule that we provide for initialization order of instantiated variables, and we can easily implement that rule and be confident we won't want to substantially weaken it later, and we can thereby assure our users that we will satisfy that rule, then I think that could be interesting, but anything less than that doesn't seem worthwhile to me.

> The question then becomes, why doesn't this work already?

It looks like it mostly does.

One factor here is instantiation order. When we instantiate a variable, we add the variables that it references to our "to be instantiated" list, which is processed later:
```
template<int N> int Fib = Fib<N - 2> + Fib<N - 1>;
template<> int Fib<0> = 0;
template<> int Fib<1> = 1;
```
* instantiating `Fib<5>` will append `Fib<3>` and `Fib<4>` to the list
* then we visit `Fib<3>` and append `Fib<1>` and `Fib<2>` to the list
* then we visit `Fib<4>` and add no new entries

We pass declarations to the consumer when we're done with the instantiation step. *Sometimes* this includes instantiating variables referenced by that variable, and *sometimes* it doesn't. The difference is whether we're performing "recursive" instantiation or not.

When we're performing immediate instantiation of a variable (either because it was explicitly instantiated, or because we might need its value immediately because it might be usable in constant expressions), our instantiation step is non-recursive. We just add declarations to `Sema`'s "to be instantiated at end of TU" list. This is at least a little important semantically: we allow a matching specialization to be declared after the first use wherever possible. In that case, we'll pass a declaration to the consumer before we've instantiated the things it references.

When we're performing the end-of-TU instantiation of all referenced template specializations, we do that recursively, and that means that we will instantiate any referenced variables *before* we pass the referencing variable to the AST consumer.

You can see this happening here: https://godbolt.org/z/4sj1Y7W4G (look at the order in which we get the warnings: for `Fib<5>` then `x` then `Fib<3>`, then `Fib<2>`, then `Fib<4>`, and note that we initialize `Fib<5>` first, because it's the first thing added to the consumer. Then `Fib<2>`, `Fib<3>`, and `Fib<4>` get passed to the consumer *in that order*, because that is the order in which the instantiations of their definitions *finish*. But `Fib<5>`'s instantiation finishes first, because that's a non-recursive instantiation.

Without the explicit instantiation, we pass the variables to the AST consumer in the order `Fib<2>`, `Fib<3>`, `Fib<4>`, `Fib<5>`: https://godbolt.org/z/sax1dvh7z leading to an "intuitive" result. They end up dependency-ordered because that's the order in which their instantiations happen to finish. (Example with a static data member: https://godbolt.org/z/9fsbPvWTP)

Another factor (and the reason why my examples are working and @ychen's very similar examples are not) is `CodeGenModule`'s deferred emission of variables. If an instantiated variable has an initializer with no side effects, `CodeGenModule` won't emit it unless it emits a reference to it (there's no point emitting something that will just be discarded by the linker). And `CodeGenModule`'s process for emitting deferred declarations does all kinds of reordering. The way this works is:

`CodeGenModule` is handed the globals, sees they don't need to be emitted, and ignores them. Then:

* it emits a reference to `Fib<5>` and decides that `Fib<5>` needs to be emitted and adds it to a queue of deferred declarations to emit
* it emits the deferred declaration `Fib<5>`, which adds `Fib<3>` and `Fib<4>` to a new queue for things used by `Fib<5>`
* it emits the things used by `Fib<5>`: `Fib<3>` and `Fib<4>`
  * emitting `Fib<3>` adds `Fib<2>` to a new queue for things used by `Fib<3>`, so `Fib<2>` is emitted next
  * emitting `Fib<4>` adds nothing new to the queue

So when the initializers don't have side-effects, the variables are initialized in the order `Fib<5>`, `Fib<3>`, `Fib<2>`, `Fib<4>`.

So the "reversing" has at least two sources (beyond anything that globalopt or the linker might do):
* non-recursive instantiations in `Sema` will cause an instantiated variable to be passed to the consumer before the things it references; this can mostly be avoided by not using explicit instantiations
* instantiated variables with side-effect-free initializers have their initializers emitted on use rather than in instantiation order; this can be avoided by building with `-femit-all-decls` or by adding a dummy side-effect to the initializer

If we want to fix just the `CodeGen` side of this, I think the thing to do would be to follow the model that `CodeGen` uses for ordered initialization (it tracks a `DelayedCXXInitPosition` map giving the order in which the variables should be initialized, if their initializers are actually emitted). We could do the same thing for instantiated variables, allocating each one handed to CodeGen a slot which either gets filled in with that initializer, or doesn't get emitted if the variable is not emitted. But, as noted above, I'm not convinced it's worth it unless this leads to some actual user-facing behavior guarantee.


================
Comment at: clang/lib/CodeGen/CGDeclCXX.cpp:582
 
-    AddGlobalCtor(Fn, 65535, COMDATKey);
+    AddGlobalCtor(Fn, 65535, COMDATKey, IsInstantiation);
     if (COMDATKey && (getTriple().isOSBinFormatELF() ||
----------------
rsmith wrote:
> This is effectively initializing instantiated variables in reverse instantiation order. That seems like it'll make things worse as much as it makes things better. For example, given:
> 
> ```
> #include <iostream>
> template<typename T> int a = (std::cout << "hello, ", 0);
> template<typename T> int b = (std::cout << "world", 0);
> int main() {
>   (void)a<int>;
>   (void)b<int>;
> }
> ```
> 
> ... we currently print `"hello, world"`, but with this change we'll print `"worldhello, "`.
> 
> If we want a sensible initialization order, I think we need a different strategy, that will probably require `Sema` to be a lot more careful about what order it instantiates variables in and what order it passes them to the AST consumer: if an instantiation A triggers another instantiation B, we should defer passing A to the consumer until B has been instantiated and passed to the consumer. That's probably not too hard to implement, by adding an entry to the pending instantiation list to say "now pass this to the consumer" in the case where one instantiation triggers another. But I do wonder whether that level of complexity is worthwhile, given that code relying on this behavior is broken.
Please see my other (long) comment; `Sema` actually already does what I describe here, except in the cases where it does an eager, non-recursive instantiation.


Repository:
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  https://reviews.llvm.org/D126341/new/

https://reviews.llvm.org/D126341