[cxx-abi-dev] manglings for exception specifications in function types

[John McCall]

> On Oct 11, 2016, at 2:11 PM, Richard Smith <richardsmith at google.com> wrote:
> Under
>   http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0012r1.html <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0012r1.html>
> 
> the noexceptness of a function type is now part of the type. As a result, we need manglings for exception-specifications on function pointer/reference types:
> 
> void f(void()) {}
> void f(void() noexcept) {} // ok, overload not redefinition
> 
> (It's not clear to me whether or not this was also necessary prior to C++17 to handle dependent exception specifications that appear lexically within the parameter list of a function template, and actual implementation practice varies as to whether such exception specifications are SFINAEable.)
> 
> 
> In order to handle overloading/SFINAE on exception specifications in dependent cases, we need to be able to mangle not only "noexcept", but also "noexcept(expression)" and "throw(<types>)". Suggestion for manglings:
> 
> <exception-spec> ::=
>   nx  -- non-throwing exception specification
>   nX <expression> E  -- computed (value-dependent) noexcept
>   tw <type>* E  -- throw (types)
> 
> <function-type> ::= [<CV-qualifiers>] [<exception-spec>] [Dx] F [Y] <bare-function-type> [<ref-qualifier>] E
> 
> In the case of throw(a, b, c), we could omit types that are neither instantiation-dependent nor pack expansions (if that omits all types, we can use the 'nx' mangling instead), since C++17 says you can't overload on the actual types in the dynamic exception specification, and we otherwise only need them to be present if they might result in a substitution failure.
> 
> Thoughts?

I think this is an amazingly late change to the language with pretty thin justification; does that count?

This really is a major change which can reasonably be expected to cause substantial source and binary breakage.  The proposal mentions transaction_safe as a feature that added similar complexity, but that analogy is weak because (1) TM is expected to be an optional TS, whereas noexcept is a mandatory core language feature, and (2) existing code does not use the transaction_safe attribute, whereas noexcept and throw() have seen widespread adoption, in the latter case for years.

If it is a goal of this proposal to eliminate the underspecified fake type system around exception specifications, it is worth noting that it completely fails to do so, since the checking rules for direct function pointer assignments are still quite a bit stronger than those provided by the new type system.

Furthermore, while the proposal does mention a fairly unlikely problem arising with template argument deduction, it fails to note the much larger one which is likely to break (or cause silently possible-miscompiles in) many metaprogramming systems where suddenly function types have acquire an entire new axis of differentiation.  For example, this code only type-checks because of special rules allowing a conversion:

template <class R, class... A> void take_fn(R (*fn)(A...));
...
extern void my_fn() noexcept;
take_fn(my_fn);

But, of course, a metaprogram inspecting a function type will completely fail to recognize a noexcept function type:

  template <class R, class... A> struct function_result<R(A...)> { using type = R; };

And in fact, this adds yet another dimension to the combinatorial explosion of specializations required in order to match all function types:

  template <class R, class... A> struct function_result<R(A...) > { using type = R; };
  template <class R, class... A> struct function_result<R(A...) const> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) volatile> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) const volatile> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) &> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) const &> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) volatile &> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) const volatile &> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) &&> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) const &&> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) volatile &&> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) const volatile &&> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept > { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept const> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept volatile> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept const volatile> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept &> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept const &> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept volatile &> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept const volatile &> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept &&> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept const &&> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept volatile &&> { using type = R; };
  template <class R, class... A> struct function_result<R(A...) noexcept const volatile &&> { using type = R; };

You will note that I have omitted the necessary specializations for "transaction_safe", as well as the incredibly common extension of specialized calling conventions.

This also breaks source compatibility for template matching, and basically every function template in the standard library is going to change manglings (and become *much* larger) due to noexcept expressions now being mangled.

And the entire proposal seems to have forgotten about reference-to-function types.

But if we're just talking about manglings, then yes, I think your ABI proposal is basically fine. :)  It's a little unfortunate to include this kind of discrimination so early in the mangling, because some object/image file symbol tables optimize for symbols with common prefixes, but our mangling scheme is generally poor at achieving that anyway.

John.
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