r226436 - Sema: Variable definitions cannot be __attribute__((alias))

[Richard Smith]

On Mon, Jan 19, 2015 at 1:00 AM, David Majnemer
<david.majnemer at gmail.com> wrote:
> Author: majnemer
> Date: Mon Jan 19 03:00:28 2015
> New Revision: 226436
>
> URL: http://llvm.org/viewvc/llvm-project?rev=226436&view=rev
> Log:
> Sema: Variable definitions cannot be __attribute__((alias))
>
> Things that are OK:
>   extern int var1 __attribute((alias("v1")));
>   static int var2 __attribute((alias("v2")));

This seems surprising; it looks like there are two definitions of var2
here (one from the alias and the other from the variable definition).
I don't see that this is different from var3.

> Things that are not OK:
>   int var3 __attribute((alias("v3")));
>   extern int var4 __attribute((alias("v4"))) = 4;

Is this OK:

const int var6 __attribute__((alias("v6")));

It implicitly has internal linkage, and adding 'extern' to it would
change semantics, but it looks more like var3 than var2.

> We choose to accpet:
>   struct S { static int var5 __attribute((alias("v5"))); };
>
>   This code causes assertion failues in GCC 4.8 and ICC 13.0.1, we have
>   no reason to reject it.
>
> This partially fixes PR22217.
>
> Modified:
>     cfe/trunk/include/clang/Basic/Attr.td
>     cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
>     cfe/trunk/lib/Sema/SemaDecl.cpp
>     cfe/trunk/lib/Sema/SemaDeclAttr.cpp
>     cfe/trunk/test/Misc/ast-dump-attr.cpp
>     cfe/trunk/test/Sema/alias-redefinition.c
>
> Modified: cfe/trunk/include/clang/Basic/Attr.td
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/Attr.td?rev=226436&r1=226435&r2=226436&view=diff
> ==============================================================================
> --- cfe/trunk/include/clang/Basic/Attr.td (original)
> +++ cfe/trunk/include/clang/Basic/Attr.td Mon Jan 19 03:00:28 2015
> @@ -345,6 +345,8 @@ def AddressSpace : TypeAttr {
>  def Alias : Attr {
>    let Spellings = [GCC<"alias">];
>    let Args = [StringArgument<"Aliasee">];
> +  let Subjects = SubjectList<[Function, GlobalVar], ErrorDiag,
> +                             "ExpectedFunctionGlobalVarMethodOrProperty">;
>    let Documentation = [Undocumented];
>  }
>
>
> Modified: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td?rev=226436&r1=226435&r2=226436&view=diff
> ==============================================================================
> --- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td (original)
> +++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td Mon Jan 19 03:00:28 2015
> @@ -3902,10 +3902,8 @@ def warn_missing_variable_declarations :
>  def err_static_data_member_reinitialization :
>    Error<"static data member %0 already has an initializer">;
>  def err_redefinition : Error<"redefinition of %0">;
> -def err_alias_after_tentative :
> -  Error<"alias definition of %0 after tentative definition">;
> -def err_tentative_after_alias :
> -  Error<"tentative definition of %0 after alias definition">;
> +def err_alias_is_definition :
> +  Error<"definition %0 cannot also be an alias">;
>  def err_definition_of_implicitly_declared_member : Error<
>    "definition of implicitly declared %select{default constructor|copy "
>    "constructor|move constructor|copy assignment operator|move assignment "
>
> Modified: cfe/trunk/lib/Sema/SemaDecl.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDecl.cpp?rev=226436&r1=226435&r2=226436&view=diff
> ==============================================================================
> --- cfe/trunk/lib/Sema/SemaDecl.cpp (original)
> +++ cfe/trunk/lib/Sema/SemaDecl.cpp Mon Jan 19 03:00:28 2015
> @@ -2219,18 +2219,8 @@ static void checkNewAttributesAfterDef(S
>      const Attr *NewAttribute = NewAttributes[I];
>
>      if (isa<AliasAttr>(NewAttribute)) {
> -      if (FunctionDecl *FD = dyn_cast<FunctionDecl>(New))
> -        S.CheckForFunctionRedefinition(FD, cast<FunctionDecl>(Def));
> -      else {
> -        VarDecl *VD = cast<VarDecl>(New);
> -        unsigned Diag = cast<VarDecl>(Def)->isThisDeclarationADefinition() ==
> -                                VarDecl::TentativeDefinition
> -                            ? diag::err_alias_after_tentative
> -                            : diag::err_redefinition;
> -        S.Diag(VD->getLocation(), Diag) << VD->getDeclName();
> -        S.Diag(Def->getLocation(), diag::note_previous_definition);
> -        VD->setInvalidDecl();
> -      }
> +      FunctionDecl *FD = cast<FunctionDecl>(New);
> +      S.CheckForFunctionRedefinition(FD, cast<FunctionDecl>(Def));
>        ++I;
>        continue;
>      }
> @@ -5103,6 +5093,18 @@ static void checkAttributesAfterMerging(
>      if (ND.isExternallyVisible()) {
>        S.Diag(Attr->getLocation(), diag::err_attribute_weakref_not_static);
>        ND.dropAttr<WeakRefAttr>();
> +      ND.dropAttr<AliasAttr>();
> +    }
> +  }
> +
> +  if (auto *VD = dyn_cast<VarDecl>(&ND)) {
> +    if (VD->hasInit()) {
> +      if (const auto *Attr = VD->getAttr<AliasAttr>()) {
> +        assert(VD->isThisDeclarationADefinition() &&
> +               !VD->isExternallyVisible() && "Broken AliasAttr handled late!");
> +        S.Diag(Attr->getLocation(), diag::err_alias_is_definition) << VD;
> +        VD->dropAttr<AliasAttr>();
> +      }
>      }
>    }
>
> @@ -9617,18 +9619,6 @@ Sema::FinalizeDeclaration(Decl *ThisDecl
>      }
>    }
>
> -  if (!VD->isInvalidDecl() &&
> -      VD->isThisDeclarationADefinition() == VarDecl::TentativeDefinition) {
> -    if (const VarDecl *Def = VD->getDefinition()) {
> -      if (Def->hasAttr<AliasAttr>()) {
> -        Diag(VD->getLocation(), diag::err_tentative_after_alias)
> -            << VD->getDeclName();
> -        Diag(Def->getLocation(), diag::note_previous_definition);
> -        VD->setInvalidDecl();
> -      }
> -    }
> -  }
> -
>    const DeclContext *DC = VD->getDeclContext();
>    // If there's a #pragma GCC visibility in scope, and this isn't a class
>    // member, set the visibility of this variable.
>
> Modified: cfe/trunk/lib/Sema/SemaDeclAttr.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?rev=226436&r1=226435&r2=226436&view=diff
> ==============================================================================
> --- cfe/trunk/lib/Sema/SemaDeclAttr.cpp (original)
> +++ cfe/trunk/lib/Sema/SemaDeclAttr.cpp Mon Jan 19 03:00:28 2015
> @@ -1492,6 +1492,20 @@ static void handleAliasAttr(Sema &S, Dec
>      return;
>    }
>
> +  // Aliases should be on declarations, not definitions.
> +  if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
> +    if (FD->isThisDeclarationADefinition()) {
> +      S.Diag(Attr.getLoc(), diag::err_alias_is_definition) << FD;
> +      return;
> +    }
> +  } else {
> +    const auto *VD = cast<VarDecl>(D);
> +    if (VD->isThisDeclarationADefinition() && VD->isExternallyVisible()) {
> +      S.Diag(Attr.getLoc(), diag::err_alias_is_definition) << VD;
> +      return;
> +    }
> +  }
> +
>    // FIXME: check if target symbol exists in current file
>
>    D->addAttr(::new (S.Context) AliasAttr(Attr.getRange(), S.Context, Str,
>
> Modified: cfe/trunk/test/Misc/ast-dump-attr.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Misc/ast-dump-attr.cpp?rev=226436&r1=226435&r2=226436&view=diff
> ==============================================================================
> --- cfe/trunk/test/Misc/ast-dump-attr.cpp (original)
> +++ cfe/trunk/test/Misc/ast-dump-attr.cpp Mon Jan 19 03:00:28 2015
> @@ -79,7 +79,7 @@ void TestInt(void) __attribute__((constr
>  // CHECK:      FunctionDecl{{.*}}TestInt
>  // CHECK-NEXT:   ConstructorAttr{{.*}} 123
>
> -int TestString __attribute__((alias("alias1")));
> +static int TestString __attribute__((alias("alias1")));
>  // CHECK:      VarDecl{{.*}}TestString
>  // CHECK-NEXT:   AliasAttr{{.*}} "alias1"
>
>
> Modified: cfe/trunk/test/Sema/alias-redefinition.c
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Sema/alias-redefinition.c?rev=226436&r1=226435&r2=226436&view=diff
> ==============================================================================
> --- cfe/trunk/test/Sema/alias-redefinition.c (original)
> +++ cfe/trunk/test/Sema/alias-redefinition.c Mon Jan 19 03:00:28 2015
> @@ -23,22 +23,5 @@ void fun4(void);
>  void f5() {}
>  void __attribute((alias("f5"))) fun5(void) {} // expected-error {{redefinition of 'fun5'}} // expected-note {{previous definition}}
>
> -int v1;
> -int var1 __attribute((alias("v1"))); // expected-note {{previous definition}}
> -int var1 __attribute((alias("v1"))); // expected-error {{redefinition of 'var1'}}
> -
> -int v2;
> -int var2 = 2; // expected-note {{previous definition}}
> -int var2 __attribute((alias("v2"))); // expected-error {{redefinition of 'var2'}}
> -
> -int v3;
> -int var3 __attribute((alias("v3"))); // expected-note {{previous definition}}
> -int var3 = 2; // expected-error {{redefinition of 'var3'}}
> -
> -int v4;
> -int var4; // expected-note {{previous definition}}
> -int var4 __attribute((alias("v4"))); // expected-error {{alias definition of 'var4' after tentative definition}}
> -
> -int v5;
> -int var5 __attribute((alias("v5"))); // expected-note {{previous definition}}
> -int var5; // expected-error {{tentative definition of 'var5' after alias definition}}

Might it be useful to keep these testcases (and mark the definitions
with the alias attribute as as 'extern' to restore the former behavior
of the tests)?

> +int var1 __attribute((alias("v1"))); // expected-error {{definition 'var1' cannot also be an alias}}
> +static int var2 __attribute((alias("v2"))) = 2; // expected-error {{definition 'var2' cannot also be an alias}}
>
>
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