[patch][pr22217] Use the most recent decl for mangling

[Richard Smith]

On Thu, Jan 29, 2015 at 4:04 PM, John McCall <rjmccall at apple.com> wrote:

> On Jan 29, 2015, at 3:59 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Thu, Jan 29, 2015 at 3:57 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> On Thu, Jan 29, 2015 at 3:47 PM, John McCall <rjmccall at apple.com> wrote:
>>
>>>
>>> On Jan 29, 2015, at 2:47 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>>
>>> On Fri, Jan 23, 2015 at 12:25 AM, John McCall <rjmccall at apple.com>
>>> wrote:
>>>
>>>> > On Jan 22, 2015, at 4:52 PM, Rafael Espíndola <
>>>> rafael.espindola at gmail.com> wrote:
>>>> >
>>>> > Sent the email a bit early.
>>>> >
>>>> >
>>>> >>> That is not what I am seeing with gcc. Given
>>>> >>>
>>>> >>> int pr22217_foo;
>>>> >>> int *b = &pr22217_foo;
>>>> >>> extern int pr22217_foo __attribute__((section("zed")));
>>>>
>>>> This should be an error in both C and C++.  I see absolutely no reason
>>>> to allow a declaration following a definition (even a tentative definition)
>>>> to add a section attribute.  We should not be afraid to reject
>>>> stupidly-written code when it abuses language extensions, even when they’re
>>>> not “our” extensions.
>>>>
>>>
>>> I completely agree with the principle here. It is not reasonable to
>>> write attributes that affect a definition after the definition. It is not
>>> reasonable to write attributes that affect how a symbol is referenced (such
>>> as an asm label) after the first use (and perhaps we should simply require
>>> them on the first declaration).
>>>
>>> (Segue away from attributes and towards tentative definitons follows...)
>>>
>>> I don't agree with what you said about tentative definitions. The C
>>> standard is very clear on the model for tentative definitions: they act
>>> exactly like non-defining declarations until you get to the end of the
>>> translation unit; if you've not seen a non-tentative definition by that
>>> point "then the behavior is exactly as if the translation unit contains a
>>> file scope declaration of that identifier, with the composite type as of
>>> the end of the translation unit, with an initializer equal to 0.”
>>>
>>>
>>> So, this is interesting.  Unix C compilers have traditionally defaulted
>>> to -fcommon, i.e. to treating uninitialized variables as common definitions
>>> that are overridable not just within a translation unit, but within the
>>> entire program.  (I’m not sure whether ELF platforms implement this as
>>> “program” or “linkage unit”.  Darwin uses “linkage unit”.)  Whether that’s
>>> actually compliant is arguable, but regardless, it’s the semantics we use,
>>> and so we really do have to maintain the tri-state, because tentative
>>> definitions are semantically quite different from non-tentative definitions.
>>>
>>> But in the sense that non-tentative definitions fully replace tentative
>>> definitions, I agree that the correct behavior is probably to allow a
>>> non-tentative definition with a section attribute to “override” a tentative
>>> definition which lacks the attribute.
>>>
>>> That's reasonable as long as section attributes never affect the
>>> code-generation of accesses to an object.  I think we can agree that
>>> section attributes that do affect code-generation of references (in an
>>> incompatible way) would clearly need to be on all declarations.  But that’s
>>> more like an address-space attribute than a section attribute.
>>>
>>> Based on that simple semantic model, it is not reasonable for us to
>>> reject this:
>>>
>>> int pr22217_foo;
>>> int *b = &pr22217_foo;
>>> extern int pr22217_foo __attribute__((section("zed")));
>>> int pr22217_foo = 123;
>>>
>>> See also PR20688, which is a rejects-valid for standard C11 code due to
>>> our being confused about how tentative definitions work.
>>>
>>> And here's another case we get wrong:
>>>
>>>   int a[];
>>>   extern int a[5];
>>>
>>> We're required to emit a definition of 'a' with type 'int[5]', but we
>>> emit it with type 'int[1]'. We get the corresponding case with an
>>> incomplete struct correct:
>>>
>>>   struct foo x; // ok, tentative definition
>>>   struct foo { int n, m; };
>>>   // definition emitted now and has complete type; initializer is {0}.
>>>
>>> There are lots of ways we can fix this; perhaps the easiest one would be
>>> to literally follow what the C standard says: synthesize a definition for
>>> each tentatively-defined variable at the end of the translation unit. Then
>>> we can change isThisDeclarationADefinition to simply return 'bool' instead
>>> of an enum, and have it return 'false' for tentative definitions. Sema
>>> would track the tentative definitions it's seen, and consider converting
>>> each one to a definition at end-of-TU.
>>>
>>>
>>> Like I mentioned above, this isn’t actually allowed under -fcommon.
>>>
>>
>> I don't see why not. We just need to make sure that the definition we
>> create at the end of TU is emitted as a common definition.
>>
>
> Ah.  I see what you’re saying.  This does become a channel of out-of-bound
> information that other clients have to be aware of, but yes, it’s possible.
>

Yeah, we were intending on tracking this on VarDecl so that consumers who
care can find out that a particular implicit definition was synthesized to
represent the definition of a tentatively-defined variable. I contend that
very few consumers of the AST will need to be aware of this; fewer than
need to be aware of the isThisDeclarationADefinition special case today.

> (There are also a small number of checks we need to turn off for the
> synthesized definition; in particular, for weird cases like:
>
> int a[];
> void f() { extern int a[5]; }
>
> ... we should not reject even though the synthesized definition has type
> 'int a[1];' which is not compatible with the declaration in 'f'. I suppose
> we could even detect this case and not emit a definition of 'a' at all
> here, because we know that another TU must be providing one with the right
> size.)
>
>
> Odds that that wouldn’t cause a failure somewhere? :)
>

If it does, the code is doing scary things today -- we'll emit 'a' as an
array of 1 element. But just because we could, doesn't mean we should... =)
*jurassic park flashback*
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