Re: [PATCH] c++: canonicity of fn types w/ complex eh specs [PR115159]

[Jason Merrill <jason@redhat.com>]

On 5/21/24 21:55, Patrick Palka wrote:
> On Tue, 21 May 2024, Jason Merrill wrote:
> 
>> On 5/21/24 17:27, Patrick Palka wrote:
>>> On Tue, 21 May 2024, Jason Merrill wrote:
>>>
>>>> On 5/21/24 15:36, Patrick Palka wrote:
>>>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
>>>>> OK for trunk?
>>>>>
>>>>> Alternatively, I considered fixing this by incrementing
>>>>> comparing_specializations around the call to comp_except_specs in
>>>>> cp_check_qualified_type, but generally for types whose identity
>>>>> depends on whether comparing_specializations is set we need to
>>>>> use structural equality anyway IIUC.
>>>>
>>>> Why not both?
>>>
>>> I figured the latter change isn't necessary/observable since
>>> comparing_specializations would only make a difference for complex
>>> exception specifications, and with this patch we won't even call
>>> cp_check_qualified_type on a complex eh spec.
>>
>> My concern is that if we're building a function type multiple times with the
>> same noexcept-spec, this patch would mean creating multiple equivalent
>> function types instead of reusing one already created for the same function.
>>
>>>>> +  bool complex_p = (cr && cr != noexcept_true_spec
>>>>> +		    && !UNPARSED_NOEXCEPT_SPEC_P (cr));
>>>>
>>>> Why treat unparsed specs differently from parsed ones?
>>>
>>> Unparsed specs are unique according to cp_tree_equal, so in turn
>>> function types with unparsed specs are unique, so it should be safe to
>>> treat such types as canonical.  I'm not sure if this optimization
>>> matters though; I'm happy to remove this case.
>>
>> The idea that this optimization could make a difference raised the concern
>> above.
> 
> Aha, makes sense.  To that end it seems we could strengthen the ce_exact
> in comp_except_specs to require == instead of cp_tree_equal equality
> when comparing two noexcept-specs; the only ce_exact callers are
> cp_check_qualified_type and cxx_type_hash_eq, which should be fine with
> that strengthening.  This way, we at least do try to reuse a variant if
> the (complex or unparsed) noexcept-spec is exactly the same.

Sounds good.

Given that, we probably still want to move the canonical_eh_spec up in 
build_cp_fntype_variant, and pass that to cp_check_qualified_type?

> Like so?
> 
> -- >8 --
> 
> Subject: [PATCH] c++: canonicity of fn types w/ complex eh specs [PR115159]
> 
> Here the member functions QList::g and QList::h are given the same
> function type since their exception specifications are equivalent
> according to cp_tree_equal.  In doing so however this means that the
> type of QList::h refers to a function parameter from QList::g, which
> ends up confusing modules streaming.
> 
> I'm not sure if modules can be fixed to handle this situation, but
> regardless it seems weird in principle that a function parameter can
> escape in such a way.  The analogous situation with a trailing return
> type and decltype
> 
>    auto g(QList &other) -> decltype(f(other));
>    auto h(QList &other) -> decltype(f(other));
> 
> behaves better because we don't canonicalize decltype, and so the
> function types of g and h are non-canonical and therefore not shared.
> 
> In light of this, it seems natural to treat function types with complex
> eh specs as non-canonical as well so that each such function declaration
> is given a unique function/method type node.  The main benefit of type
> canonicalization is to speed up repeated type comparisons, but it should
> rare for us to repeatedly compare two otherwise compatible function
> types with complex exception specifications, so foregoing canonicalization
> should not cause any problems.
> 
> To that end, this patch strengthens the ce_exact case of comp_except_specs
> to require identity instead of equivalence of the exception specification
> so that build_cp_fntype_variant doesn't reuse a variant when it shouldn't.
> And in build_cp_fntype_variant we need to use structural equality for types
> with a complex eh spec.  In turn we could simplify the code responsible
> for adjusting unparsed eh spec variants.
> 
> 	PR c++/115159
> 
> gcc/cp/ChangeLog:
> 
> 	* tree.cc (build_cp_fntype_variant): Always use structural
> 	equality for types with a complex exception specification.
> 	(fixup_deferred_exception_variants): Always use structural
> 	equality for adjusted variants.
> 	* typeck.cc (comp_except_specs): Require == instead of
> 	cp_tree_equal for noexcept-spec comparison in the ce_exact case.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/modules/noexcept-2_a.H: New test.
> 	* g++.dg/modules/noexcept-2_b.C: New test.
> ---
>   gcc/cp/tree.cc                              | 47 +++++----------------
>   gcc/cp/typeck.cc                            |  4 +-
>   gcc/testsuite/g++.dg/modules/noexcept-2_a.H | 24 +++++++++++
>   gcc/testsuite/g++.dg/modules/noexcept-2_b.C |  4 ++
>   4 files changed, 41 insertions(+), 38 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/modules/noexcept-2_a.H
>   create mode 100644 gcc/testsuite/g++.dg/modules/noexcept-2_b.C
> 
> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> index 9d37d255d8d..93a64322418 100644
> --- a/gcc/cp/tree.cc
> +++ b/gcc/cp/tree.cc
> @@ -2794,7 +2794,12 @@ build_cp_fntype_variant (tree type, cp_ref_qualifier rqual,
>     /* Canonicalize the exception specification.  */
>     tree cr = flag_noexcept_type ? canonical_eh_spec (raises) : NULL_TREE;
>   
> -  if (TYPE_STRUCTURAL_EQUALITY_P (type))
> +  /* Always use structural equality for function types with a complex
> +     exception specification since their identity may depend on e.g.
> +     whether comparing_specializations is set.  */
> +  bool complex_eh_spec_p = (cr && cr != noexcept_true_spec
> +			    && !UNPARSED_NOEXCEPT_SPEC_P (cr));
> +  if (TYPE_STRUCTURAL_EQUALITY_P (type) || complex_eh_spec_p)
>       /* Propagate structural equality. */
>       SET_TYPE_STRUCTURAL_EQUALITY (v);
>     else if (TYPE_CANONICAL (type) != type || cr != raises || late)
> @@ -2812,55 +2817,23 @@ build_cp_fntype_variant (tree type, cp_ref_qualifier rqual,
>   /* TYPE is a function or method type with a deferred exception
>      specification that has been parsed to RAISES.  Fixup all the type
>      variants that are affected in place.  Via decltype &| noexcept
> -   tricks, the unparsed spec could have escaped into the type system.
> -   The general case is hard to fixup canonical types for.  */
> +   tricks, the unparsed spec could have escaped into the type system.  */
>   
>   void
>   fixup_deferred_exception_variants (tree type, tree raises)
>   {
>     tree original = TYPE_RAISES_EXCEPTIONS (type);
> -  tree cr = flag_noexcept_type ? canonical_eh_spec (raises) : NULL_TREE;
>   
>     gcc_checking_assert (UNPARSED_NOEXCEPT_SPEC_P (original));
>   
> -  /* Though sucky, this walk will process the canonical variants
> -     first.  */
> -  tree prev = NULL_TREE;
>     for (tree variant = TYPE_MAIN_VARIANT (type);
> -       variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant))
> +       variant; variant = TYPE_NEXT_VARIANT (variant))
>       if (TYPE_RAISES_EXCEPTIONS (variant) == original)
>         {
>   	gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type));
>   
> -	if (!TYPE_STRUCTURAL_EQUALITY_P (variant))
> -	  {
> -	    cp_cv_quals var_quals = TYPE_QUALS (variant);
> -	    cp_ref_qualifier rqual = type_memfn_rqual (variant);
> -
> -	    /* If VARIANT would become a dup (cp_check_qualified_type-wise)
> -	       of an existing variant in the variant list of TYPE after its
> -	       exception specification has been parsed, elide it.  Otherwise,
> -	       build_cp_fntype_variant could use it, leading to "canonical
> -	       types differ for identical types."  */
> -	    tree v = TYPE_MAIN_VARIANT (type);
> -	    for (; v; v = TYPE_NEXT_VARIANT (v))
> -	      if (cp_check_qualified_type (v, variant, var_quals,
> -					   rqual, cr, false))
> -		{
> -		  /* The main variant will not match V, so PREV will never
> -		     be null.  */
> -		  TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant);
> -		  break;
> -		}
> -	    TYPE_RAISES_EXCEPTIONS (variant) = raises;
> -
> -	    if (!v)
> -	      v = build_cp_fntype_variant (TYPE_CANONICAL (variant),
> -					   rqual, cr, false);
> -	    TYPE_CANONICAL (variant) = TYPE_CANONICAL (v);
> -	  }
> -	else
> -	  TYPE_RAISES_EXCEPTIONS (variant) = raises;
> +	SET_TYPE_STRUCTURAL_EQUALITY (variant);
> +	TYPE_RAISES_EXCEPTIONS (variant) = raises;
>   
>   	if (!TYPE_DEPENDENT_P (variant))
>   	  /* We no longer know that it's not type-dependent.  */
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 75b696e32e0..d535746fd43 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -1227,7 +1227,9 @@ comp_except_specs (const_tree t1, const_tree t2, int exact)
>     if ((t1 && TREE_PURPOSE (t1))
>         || (t2 && TREE_PURPOSE (t2)))
>       return (t1 && t2
> -	    && cp_tree_equal (TREE_PURPOSE (t1), TREE_PURPOSE (t2)));
> +	    && (exact == ce_exact
> +		? TREE_PURPOSE (t1) == TREE_PURPOSE (t2)
> +		: cp_tree_equal (TREE_PURPOSE (t1), TREE_PURPOSE (t2))));
>   
>     if (t1 == NULL_TREE)			   /* T1 is ...  */
>       return t2 == NULL_TREE || exact == ce_derived;
> diff --git a/gcc/testsuite/g++.dg/modules/noexcept-2_a.H b/gcc/testsuite/g++.dg/modules/noexcept-2_a.H
> new file mode 100644
> index 00000000000..b7144f42d7e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/noexcept-2_a.H
> @@ -0,0 +1,24 @@
> +// PR c++/115159
> +// { dg-additional-options -fmodule-header }
> +// { dg-module-cmi {} }
> +
> +struct QDebug;
> +
> +template<class T> void f(T);
> +
> +template<class T> struct QList {
> +  QDebug g(QList &other) noexcept(noexcept(f(other)));
> +  QDebug h(QList &other) noexcept(noexcept(f(other)));
> +};
> +
> +struct QObjectData {
> +  QList<int> children;
> +};
> +
> +struct QIODevice {
> +  QObjectData d_ptr;
> +};
> +
> +struct QDebug {
> +  QDebug(QIODevice);
> +};
> diff --git a/gcc/testsuite/g++.dg/modules/noexcept-2_b.C b/gcc/testsuite/g++.dg/modules/noexcept-2_b.C
> new file mode 100644
> index 00000000000..d34c63add10
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/modules/noexcept-2_b.C
> @@ -0,0 +1,4 @@
> +// PR c++/115159
> +// { dg-additional-options "-fmodules-ts -fno-module-lazy" }
> +
> +import "noexcept-2_a.H";