r350776 - [Sema] Mark target of __attribute__((alias("target"))) used for C

[Nico Weber via cfe-commits]

Aren't C names sometimes slightly mangled too? For example, on macOS
they're prefixed by a _, doesn't that have to be undone there?

On Wed, Jan 9, 2019 at 6:58 PM Nick Desaulniers via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> Author: nickdesaulniers
> Date: Wed Jan  9 15:54:55 2019
> New Revision: 350776
>
> URL: http://llvm.org/viewvc/llvm-project?rev=350776&view=rev
> Log:
> [Sema] Mark target of __attribute__((alias("target"))) used for C
>
> Summary:
> Prevents -Wunneeded-internal-delcaration warnings when the target has no
> other references. This occurs frequently in device drivers in the Linux
> kernel.
>
> Sema would need to invoke the demangler on the target, since in C++ the
> target name is mangled:
>
> int f() { return 42; }
> int g() __attribute__((alias("_Z1fv")));
>
> Sema does not have the ability to demangle names at this time.
>
> https://bugs.llvm.org/show_bug.cgi?id=39088
> https://github.com/ClangBuiltLinux/linux/issues/232
>
> Reviewers: rsmith, rjmccall
>
> Reviewed By: rsmith
>
> Subscribers: erik.pilkington, cfe-commits, pirama, srhines
>
> Differential Revision: https://reviews.llvm.org/D54188
>
> Added:
>     cfe/trunk/test/Sema/alias-unused.c
> Modified:
>     cfe/trunk/lib/Sema/SemaDeclAttr.cpp
>
> Modified: cfe/trunk/lib/Sema/SemaDeclAttr.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?rev=350776&r1=350775&r2=350776&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/Sema/SemaDeclAttr.cpp (original)
> +++ cfe/trunk/lib/Sema/SemaDeclAttr.cpp Wed Jan  9 15:54:55 2019
> @@ -1898,7 +1898,16 @@ static void handleAliasAttr(Sema &S, Dec
>      }
>    }
>
> -  // FIXME: check if target symbol exists in current file
> +  // Mark target used to prevent unneeded-internal-declaration warnings.
> +  if (!S.LangOpts.CPlusPlus) {
> +    // FIXME: demangle Str for C++, as the attribute refers to the mangled
> +    // linkage name, not the pre-mangled identifier.
> +    const DeclarationNameInfo target(&S.Context.Idents.get(Str),
> AL.getLoc());
> +    LookupResult LR(S, target, Sema::LookupOrdinaryName);
> +    if (S.LookupQualifiedName(LR, S.getCurLexicalContext()))
> +      for (NamedDecl *ND : LR)
> +        ND->markUsed(S.Context);
> +  }
>
>    D->addAttr(::new (S.Context) AliasAttr(AL.getRange(), S.Context, Str,
>
> AL.getAttributeSpellingListIndex()));
>
> Added: cfe/trunk/test/Sema/alias-unused.c
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Sema/alias-unused.c?rev=350776&view=auto
>
> ==============================================================================
> --- cfe/trunk/test/Sema/alias-unused.c (added)
> +++ cfe/trunk/test/Sema/alias-unused.c Wed Jan  9 15:54:55 2019
> @@ -0,0 +1,7 @@
> +// RUN: %clang_cc1 -triple x86_64-linux-gnu
> -Wunneeded-internal-declaration -x c -verify %s
> +// expected-no-diagnostics
> +static int f() { return 42; }
> +int g() __attribute__((alias("f")));
> +
> +static int foo [] = { 42, 0xDEAD };
> +extern typeof(foo) bar __attribute__((unused, alias("foo")));
>
>
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