On Thu, Jan 29, 2015 at 3:47 PM, John McCall <[email protected]> wrote:
> > On Jan 29, 2015, at 2:47 PM, Richard Smith <[email protected]> wrote: > > On Fri, Jan 23, 2015 at 12:25 AM, John McCall <[email protected]> wrote: > >> > On Jan 22, 2015, at 4:52 PM, Rafael Espíndola < >> [email protected]> 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. > Or we can try to keep our current model with a tristate for whether a > declaration is a definition, but that requires both Sema and IRGen to get a > lot smarter with regard to handling of tentative definitions. > > > I think this is reasonable. IRGen should be able to just completely > replace an existing tentative definition. As I mentioned up-thread, IRGen > needs to hold persistent references to global variables with handles anyway > just because types can change. > > John. >
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