Re: [PATCH] c++/modules: depending local enums [PR104919, PR106009]

[Patrick Palka <ppalka@redhat.com>]

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On Fri, 1 Mar 2024, Jason Merrill wrote:

> On 3/1/24 10:32, Jason Merrill wrote:
> > On 3/1/24 10:00, Patrick Palka wrote:
> > > On Fri, 1 Mar 2024, Jason Merrill wrote:
> > > 
> > > > On 2/29/24 15:56, Patrick Palka wrote:
> > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> > > > > OK for trunk?
> > > > > 
> > > > > -- >8 --
> > > > > 
> > > > > For local enums defined in a non-template function or a function
> > > > > template
> > > > > instantiation it seems we neglect to make the function depend on the
> > > > > enum
> > > > > definition, which ultimately causes streaming to fail due to the enum
> > > > > definition not being streamed before uses of its enumerators are
> > > > > streamed,
> > > > > as far as I can tell.
> > > > 
> > > > I would think that the function doesn't need to depend on the local enum
> > > > in
> > > > order for the local enum to be streamed before the use of the
> > > > enumerator,
> > > > which comes after the definition of the enum in the function body?
> > > > 
> > > > Why isn't streaming the body of the function outputting the enum
> > > > definition
> > > > before the use of the enumerator?
> > > 
> > > IIUC (based on observing the behavior for local classes) streaming the
> > > definition of a local class/enum as part of the function definition is
> > > what we want to avoid; we want to treat a local type definition as a
> > > logically separate definition and stream it separately (similar
> > > to class defns vs member defns I guess).  And by not registering a
> > > dependency
> > > between the function and the local enum, we end up never streaming out
> > > the local enum definition separately and instead stream it out as part
> > > of the function definition (accidentally) which we then can't stream in
> > > properly.
> > > 
> > > Perhaps the motivation for treating local type definitions as logically
> > > separate from the function definition is because they can leak out of a
> > > function with a deduced return type:
> > > 
> > >    auto f() {
> > >      struct A { };
> > >      return A();
> > >    }
> > > 
> > >    using type = decltype(f()); // refers directly to f()::A
> > 
> > Yes, I believe that's what modules.cc refers to as a "voldemort".
> > 
> > But for non-voldemort local types, the declaration of the function doesn't
> > depend on them, only the definition.  Why doesn't streaming them in the
> > definition work properly?
> 
> And does your 99426 patch address that problem?

I don't think so, that patch should only affect declaration merging (of
a streamed-in local type with the corresponding in-TU local type after
their containing function is merged).


> > This was nearly enough to make things work, except we now ran into
> > issues with the local TYPE/CONST_DECL copies when streaming the
> > constexpr version of a function body.  It occurred to me that we don't
> > need to make copies of local types when copying a constexpr function
> > body; only VAR_DECLs etc need to be copied for sake of recursive
> > constexpr calls.  So this patch adjusts copy_fn accordingly.
>
> Maybe adjust can_be_nonlocal instead?  It seems unnecessary in general
> to remap types and enumerators for inlining.

Unfortunately this approached caused a boostrap failure with Ada:

raised STORAGE_ERROR : stack overflow or erroneous memory access

The patch was

--- a/gcc/tree-inline.cc
+++ b/gcc/tree-inline.cc
@@ -725,6 +725,9 @@ can_be_nonlocal (tree decl, copy_body_data *id)
   if (TREE_CODE (decl) == FUNCTION_DECL)
     return true;
 
+  if (TREE_CODE (decl) == TYPE_DECL || TREE_CODE (decl) == CONST_DECL)
+    return true;
+
   /* Local static vars must be non-local or we get multiple declaration
      problems.  */
   if (VAR_P (decl) && !auto_var_in_fn_p (decl, id->src_fn))


> 
> Jason
> 
>