r224329 - Renamed RefersToEnclosingLocal bitfield to RefersToCapturedVariable.

[Richard Smith]

On Sun, Jan 4, 2015 at 7:57 PM, John McCall <rjmccall at apple.com> wrote:

> On Jan 4, 2015, at 6:28 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> On 5 Jan 2015 02:08, "John McCall" <rjmccall at apple.com> wrote:
> >>
> >> On Jan 4, 2015, at 1:11 AM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> >> On Sat, Jan 3, 2015 at 1:46 PM, John McCall <rjmccall at apple.com> wrote:
> >>>>
> >>>> On Jan 2, 2015, at 7:10 AM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> >>>> On Tue, Dec 16, 2014 at 12:01 AM, Alexey Bataev <a.bataev at hotmail.com>
> wrote:
> >>>>>
> >>>>> Author: abataev
> >>>>> Date: Tue Dec 16 02:01:48 2014
> >>>>> New Revision: 224329
> >>>>>
> >>>>> URL: http://llvm.org/viewvc/llvm-project?rev=224329&view=rev
> >>>>> Log:
> >>>>> Renamed RefersToEnclosingLocal bitfield to RefersToCapturedVariable.
> >>>>> Bitfield RefersToEnclosingLocal of Stmt::DeclRefExprBitfields
> renamed to RefersToCapturedVariable to reflect latest changes introduced in
> commit 224323. Also renamed method Expr::refersToEnclosingLocal() to
> Expr::refersToCapturedVariable() and comments for constant arguments.
> >>>>> No functional changes.
> >>>>
> >>>>
> >>>> This seems like a bad idea for me. It's incorrect: for a lambda, the
> flag means that the DeclRefExpr refers to an enclosing local, and does
> *not* imply that the variable is necessarily captured. This confusion has
> already led to a bug (fixed in r225060).
> >>>
> >>>
> >>> If that’s actually a useful property to track, I have no complaint
> about tracking the two things separately.  I don’t think DRE is short of
> bits.
> >>
> >>
> >> The problem is that we can't reasonably track whether a DRE refers to a
> captured variable, because that is not a local property of the DRE; it
> depends on non-trivial properties of the context in which the DRE is used
> (and more generally it depends on how else that same variable is used
> elsewhere in the lambda, but we can largely ignore that).
> >>
> >> I'd be fine having separate flags for 'refers to enclosing local' and
> 'refers to captured global' (if the latter is the point of this change),
> but we should not claim a DRE refers to a capture when it does not.
> >
> >
> > Richard, I understand that not all DREs are potentially evaluated and
> thus may not actually induce capturing, and I concede your point that the
> flag's name shouldn’t use the word “capture” if it’s set on unevaluated
> DREs.  The question is this: what information do we actually need to track
> on DREs?
> >
> > It is very useful for IR-gen to be able to immediately know whether a
> DRE refers to a captured local or global variable, no matter why that
> capture was performed.  As far as I’m aware, IR-gen does not care about
> whether the flag is set on unevaluated references to enclosing variables,
> because IR-gen should never see an unevaluated use in normal expression
> emission.
>
> This is not about unevaluated references, it's about evaluated references
> that aren't odr uses, which depends on the context in which the DRE
> appears. IRGen does need to deal with these cases.
>
> Ugh, fine, I will go haul out the standard to figure out what you’re
> talking about, since you are apparently in a vague mood tonight.
>

Apologies for the terseness, was replying from a phone.


> Okay, I believe what you’re saying is:
> 1. A reference to an entity (a variable/this) within a lambda only causes
> a capture if it's an ODR use.
> 2. All ODR uses are PE references, but a PE reference isn't an ODR use if
> the variable can be used in a constant expression and it's a potential
> result of an expression that’s either ignored or loaded.
> 3. Whether an expression is ignored or loaded cannot be decided
> immediately from context; you have to semantically process the entire
> expression, and it may be different in different template instantiations.
>  (This dependence is also true of PPE-ness, I think.)
> 4. Since we sometimes share DRE nodes, we can’t retroactively alter a node
> after doing the necessary semantic analysis.
> 5. AST clients are likely to want to know whether a DRE is not an ODR-use
> and can only be constant-evaluated.  For example, it’s important that
> IR-gen not actually emit a reference to a static member variable of a class
> template if that member variable hasn’t been ODR-used.  Clients should
> similarly not be confused by an evaluated reference to an uncaptured
> enclosing local.
>
> I think point #4 tells us that the desired information in #5 can’t
> actually be directly represented in the AST, though.
>

Yes, that's an excellent summary. For concreteness, here's an example where
this happens:

void f(int, int);
void f(const int&, double);
auto f() {
  const int n = 0;
  return [=](auto t) { return [=]() { f(n, t); } };
}
void g() { f()(0)(); f()(0.)(); }

Here, the lambda returned by f()(0) does not capture 'n', but the lambda
returned by f()(0.) does, and they share a DeclRefExpr for n.

But as the first example shows, client interests in #5 are much broader
> than just references to enclosing locals.  IR-gen would benefit from
> knowing whether an arbitrary expression was constant-evaluable; currently
> it has hacky code to try to constant-evaluate specific nodes, and it lets
> all the other nodes fall out from LLVM’s own constant-folding and
> optimization, but it would clearly be better to, say, know that a
> particular call is a call to a constexpr function with constant arguments
> and so we can just directly try to fold it instead of emitting a whole
> bunch of unnecessary code that will get cleaned up later.  Using “this is
> an enclosing local” as an incomplete approximation of that isn’t actually
> useful.
>

The only cases where we need to emit different code based on whether an
expression is a constant expression (outside unevaluated contexts) are when
initializing a variable of static/thread_local storage duration, and when
an expression refers to an enclosing local variable that it doesn't
capture. In all other cases, it's correct and often reasonable to evaluate
the expression at runtime. In the case where a lambda expression refers to
an enclosing local but doesn't capture it, we should emit a copy of that
local's value as a constant global.

So it seems to me that we just need one bit, called
> refersToEnclosingVariableOrCapture(), with the meaning that it’s set if the
> DRE refers to an enclosing local (captured or not) or captured global
> variable, regardless of whether this reference is an ODR use.
>

That seems fine to me.
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