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

[Jason Merrill <jason@redhat.com>]

On 1/8/24 12:04, Patrick Palka wrote:
> 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?
>>
>> 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)

It seems to me we always want the DECL_ARGUMENTS corresponding to the 
actual definition we're using, which since "installing" is true, is the 
new definition.  In duplicate_decls when we merge a new definition into 
an old declaration, we give the old declaration the new DECL_ARGUMENTS.

The patch is OK.

>>> 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
>>>
>>
>>
>