Re: [PATCH] c++/modules: Prevent overwriting arguments for duplicates [PR112588]

[Patrick Palka <ppalka@redhat.com>]

On Mon, 8 Jan 2024, Nathaniel Shead wrote:

> On Sat, Jan 06, 2024 at 05:32:37PM -0500, Nathan Sidwell wrote:
> > I;m not sure about this, there was clearly a reason I did it the way it is,
> > but perhaps that reasoning became obsolete -- something about an existing
> > declaration and reading in a definition maybe?
> > 
> > nathan
> 
> So I took a bit of a closer look and this is actually a regression,
> seeming to start with r13-3134-g09df0d8b14dda6. I haven't looked more
> closely at the actual change though to see whether this implies a
> different fix yet though.

Interesting..  FWIW I applied your patch to the gcc 12 release branch,
which doesn't have r13-3134, and there were no modules testsuite
regressions there either, which at least suggests that this maybe_dup
logic isn't directly related to the optimization that r13-3134 removed.

Your patch also seems to fix PR99244 (which AFAICT is not a regression)

> 
> Nathaniel
> 
> > On 11/22/23 06:33, Nathaniel Shead wrote:
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu. I don't have write
> > > access.
> > > 
> > > -- >8 --
> > > 
> > > When merging duplicate instantiations of function templates, currently
> > > read_function_def overwrites the arguments with that of the existing
> > > duplicate. This is problematic, however, since this means that the
> > > PARM_DECLs in the body of the function definition no longer match with
> > > the PARM_DECLs in the argument list, which causes issues when it comes
> > > to generating RTL.
> > > 
> > > There doesn't seem to be any reason to do this replacement, so this
> > > patch removes that logic.
> > > 
> > > 	PR c++/112588
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > > 	* module.cc (trees_in::read_function_def): Don't overwrite
> > > 	arguments.
> > > 
> > > gcc/testsuite/ChangeLog:
> > > 
> > > 	* g++.dg/modules/merge-16.h: New test.
> > > 	* g++.dg/modules/merge-16_a.C: New test.
> > > 	* g++.dg/modules/merge-16_b.C: New test.
> > > 
> > > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > > ---
> > >   gcc/cp/module.cc                          |  2 --
> > >   gcc/testsuite/g++.dg/modules/merge-16.h   | 10 ++++++++++
> > >   gcc/testsuite/g++.dg/modules/merge-16_a.C |  7 +++++++
> > >   gcc/testsuite/g++.dg/modules/merge-16_b.C |  5 +++++
> > >   4 files changed, 22 insertions(+), 2 deletions(-)
> > >   create mode 100644 gcc/testsuite/g++.dg/modules/merge-16.h
> > >   create mode 100644 gcc/testsuite/g++.dg/modules/merge-16_a.C
> > >   create mode 100644 gcc/testsuite/g++.dg/modules/merge-16_b.C
> > > 
> > > diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> > > index 4f5b6e2747a..2520ab659cc 100644
> > > --- a/gcc/cp/module.cc
> > > +++ b/gcc/cp/module.cc
> > > @@ -11665,8 +11665,6 @@ trees_in::read_function_def (tree decl, tree maybe_template)
> > >         DECL_RESULT (decl) = result;
> > >         DECL_INITIAL (decl) = initial;
> > >         DECL_SAVED_TREE (decl) = saved;
> > > -      if (maybe_dup)
> > > -	DECL_ARGUMENTS (decl) = DECL_ARGUMENTS (maybe_dup);
> > >         if (context)
> > >   	SET_DECL_FRIEND_CONTEXT (decl, context);
> > > diff --git a/gcc/testsuite/g++.dg/modules/merge-16.h b/gcc/testsuite/g++.dg/modules/merge-16.h
> > > new file mode 100644
> > > index 00000000000..fdb38551103
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/modules/merge-16.h
> > > @@ -0,0 +1,10 @@
> > > +// PR c++/112588
> > > +
> > > +void f(int*);
> > > +
> > > +template <typename T>
> > > +struct S {
> > > +  void g(int n) { f(&n); }
> > > +};
> > > +
> > > +template struct S<void>;

If we use a partial specialization here instead (which would have disabled
the removed optimization, demonstrating how fragile/inconsistent it was)

  void f(int*);

  template <typename T>
  struct S { };

  template<typename T>
  struct S<T*> {
    void g(int n) { f(&n); }
  };

  template struct S<void*>;

then the ICE appears earlier, since GCC 12 instead of 13.

> > > diff --git a/gcc/testsuite/g++.dg/modules/merge-16_a.C b/gcc/testsuite/g++.dg/modules/merge-16_a.C
> > > new file mode 100644
> > > index 00000000000..c243224c875
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/modules/merge-16_a.C
> > > @@ -0,0 +1,7 @@
> > > +// PR c++/112588
> > > +// { dg-additional-options "-fmodules-ts" }
> > > +// { dg-module-cmi merge16 }
> > > +
> > > +module;
> > > +#include "merge-16.h"
> > > +export module merge16;
> > > diff --git a/gcc/testsuite/g++.dg/modules/merge-16_b.C b/gcc/testsuite/g++.dg/modules/merge-16_b.C
> > > new file mode 100644
> > > index 00000000000..8c7b1f0511f
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/modules/merge-16_b.C
> > > @@ -0,0 +1,5 @@
> > > +// PR c++/112588
> > > +// { dg-additional-options "-fmodules-ts" }
> > > +
> > > +#include "merge-16.h"
> > > +import merge16;
> > 
> > -- 
> > Nathan Sidwell
> > 
> 
>