Re: [PATCH] c++/modules: Stream unmergeable temporaries by value again [PR114856]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 2 May 2024, Nathaniel Shead wrote:

> Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk/14.2?
> 
> Another alternative would be to stream such !DECL_NAME temporaries with
> a merge key of MK_unique rather than attempting to find the matching
> (nonexistant) field of the class context.

Both approaches sound good to me, hard to say which one is preferable..

The handling of function-scope vs class-scope temporaries seems to start
diverging in:

@@ -8861,28 +8861,6 @@ trees_out::decl_node (tree decl, walk_kind ref)
       return false;
     }
 
!  tree ctx = CP_DECL_CONTEXT (decl);
!  depset *dep = NULL;
!  if (streaming_p ())
!    dep = dep_hash->find_dependency (decl);
!  else if (TREE_CODE (ctx) != FUNCTION_DECL
!	   || TREE_CODE (decl) == TEMPLATE_DECL
!	   || DECL_IMPLICIT_TYPEDEF_P (decl)
!	   || (DECL_LANG_SPECIFIC (decl)
!	       && DECL_MODULE_IMPORT_P (decl)))
!    {
!      auto kind = (TREE_CODE (decl) == NAMESPACE_DECL
!		   && !DECL_NAMESPACE_ALIAS (decl)
!		   ? depset::EK_NAMESPACE : depset::EK_DECL);
!      dep = dep_hash->add_dependency (decl, kind);
!    }
!
!  if (!dep)
!    {
!      /* Some internal entity of context.  Do by value.  */
!      decl_value (decl, NULL);
!      return false;
!    }
 
   if (dep->get_entity_kind () == depset::EK_REDIRECT)
     {

where for a class-scope temporary we add a dependency for it, stream
it by reference, and then stream it by value separately, which seems
unnecessary.

So if we decide to keep the create_temporary_var change, we probably
would want to unify this code path's handling of temporaries (i.e.
don't add_dependency a temporary regardless of its context).

If we decide your partially revert the create_temporary_var change,
your patch LGTM.

> 
> -- >8 --
> 
> In r14-9266-g2823b4d96d9ec4 I gave all temporary vars a DECL_CONTEXT,
> including those at namespace or global scope, so that they could be
> properly merged across importers.  However, not all of these temporary
> vars are actually supposed to be mergeable.
> 
> For instance, in the attached testcase we have an unnamed temporary var
> used in the NSDMI of a class member, which cannot properly merged -- but
> it also doesn't need to be, as it'll be thrown away when the class type
> itself is merged anyway.
> 
> This patch reverts the change made above and instead makes a weaker
> adjustment that only causes temporary vars with linkage have a
> DECL_CONTEXT to merge from.  This way these unnamed, "unmergeable"
> temporaries are properly streamed by value again.
> 
> 	PR c++/114856
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (make_temporary_var_for_ref_to_temp): Set context for
> 	temporaries with linkage.
> 	* init.cc (create_temporary_var): Revert to only set context
> 	when in a function decl.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/modules/pr114856.h: New test.
> 	* g++.dg/modules/pr114856_a.H: New test.
> 	* g++.dg/modules/pr114856_b.C: New test.
> 
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  gcc/cp/call.cc                            |  1 +
>  gcc/cp/init.cc                            |  2 +-
>  gcc/testsuite/g++.dg/modules/pr114856.h   | 12 ++++++++++++
>  gcc/testsuite/g++.dg/modules/pr114856_a.H |  5 +++++
>  gcc/testsuite/g++.dg/modules/pr114856_b.C |  5 +++++
>  5 files changed, 24 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/g++.dg/modules/pr114856.h
>  create mode 100644 gcc/testsuite/g++.dg/modules/pr114856_a.H
>  create mode 100644 gcc/testsuite/g++.dg/modules/pr114856_b.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index dbdd7c29fe8..3b8889ac301 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13799,6 +13799,7 @@ make_temporary_var_for_ref_to_temp (tree decl, tree type)
>  
>        tree name = mangle_ref_init_variable (decl);
>        DECL_NAME (var) = name;
> +      DECL_CONTEXT (var) = current_scope ();
>        SET_DECL_ASSEMBLER_NAME (var, name);
>      }
>    else
> diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> index a93ce00800c..e758a8c8568 100644
> --- a/gcc/cp/init.cc
> +++ b/gcc/cp/init.cc
> @@ -4287,7 +4287,7 @@ create_temporary_var (tree type)
>    TREE_USED (decl) = 1;
>    DECL_ARTIFICIAL (decl) = 1;
>    DECL_IGNORED_P (decl) = 1;
> -  DECL_CONTEXT (decl) = current_scope ();
> +  DECL_CONTEXT (decl) = current_function_decl;
>  
>    return decl;
>  }
> diff --git a/gcc/testsuite/g++.dg/modules/pr114856.h b/gcc/testsuite/g++.dg/modules/pr114856.h
> new file mode 100644
> index 00000000000..b1a3c2cd834
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/pr114856.h
> @@ -0,0 +1,12 @@
> +// PR c++/114856
> +
> +#include <initializer_list>
> +struct A {
> +  ~A();
> +};
> +struct V {
> +  V(std::initializer_list<A>);
> +};
> +struct data {
> +  V v{{}};
> +};
> diff --git a/gcc/testsuite/g++.dg/modules/pr114856_a.H b/gcc/testsuite/g++.dg/modules/pr114856_a.H
> new file mode 100644
> index 00000000000..6195277dbde
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/pr114856_a.H
> @@ -0,0 +1,5 @@
> +// PR c++/114856
> +// { dg-additional-options "-fmodule-header" }
> +// { dg-module-cmi {} }
> +
> +#include "pr114856.h"
> diff --git a/gcc/testsuite/g++.dg/modules/pr114856_b.C b/gcc/testsuite/g++.dg/modules/pr114856_b.C
> new file mode 100644
> index 00000000000..f81dc8b81d5
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/pr114856_b.C
> @@ -0,0 +1,5 @@
> +// PR c++/114856
> +// { dg-additional-options "-fmodules-ts" }
> +
> +#include "pr114856.h"
> +import "pr114856_a.H";
> -- 
> 2.43.2
> 
>