[Sema] Do not emit -Wmissing-variable-declarations for register variables

[nathanchance]

Summary

When building the Linux kernel with -Wmissing-variable-declarations,
there are several instances of warnings around variables declared with
register storage:

arch/x86/include/asm/asm.h:208:24: warning: no previous extern declaration for non-static variable 'current_stack_pointer' [-Wmissing-variable-declarations]
register unsigned long current_stack_pointer asm(_ASM_SP);
                       ^
arch/x86/include/asm/asm.h:208:10: note: declare 'static' if the variable is not intended to be used outside of this translation unit
register unsigned long current_stack_pointer asm(_ASM_SP);
         ^
1 warning generated.

The suggestion is invalid, as the variable cannot have both static and
register storage. Do not emit -Wmissing-variable-declarations for
register variables.

Closes: https://github.com/llvm/llvm-project/issues/64509
Link: https://lore.kernel.org/[e-mail]/

[nickdesaulniers]

Thanks for the patch; I think it's better not to warn rather than expect users to add an
unsigned long currest_stack_pointer; declaration before the definition for register storage variables.

Maybe worth add a link to the GCC bug report in the commit message, too?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110947

clang/test/Sema/warn-missing-variable-declarations-register.c

2

@aaron.ballman do we need this REQUIRES clause?

[aaron.ballman]

LGTM aside from some minor nits.

clang/docs/ReleaseNotes.rst

126–127

Sphinx fixes

clang/test/Sema/warn-missing-variable-declarations-register.c

2

Nope, no need for REQUIRES here.

[nathanchance]

• Update formatting in release notes
• Add GCC bug link to commit message
• Remove unnecessary REQUIRES: in test

clang/docs/ReleaseNotes.rst

 --------------------------
 
 Improvements to Clang's diagnostics
 -----------------------------------
 - Clang constexpr evaluator now prints template arguments when displaying
   template-specialization function calls.
 - Clang contexpr evaluator now displays notes as well as an error when a constructor
   of a base class is not called in the constructor of its derived class.
+- Clang no longer emits `-Wmissing-variable-declarations` for variables declared
+  with the `register` storage class.
 
 Bug Fixes in This Version
 -------------------------
 - Fixed an issue where a class template specialization whose declaration is
   instantiated in one module and whose definition is instantiated in another
   module may end up with members associated with the wrong declaration of the
   class, which can result in miscompiles in some cases.
 - Fix crash on use of a variadic overloaded operator.

clang/lib/Sema/SemaDecl.cpp

void Sema::CheckCompleteVariableDeclaration(VarDecl *var) {
   // prior declaration.  We only want to do this for global
   // declarations, but we also specifically need to avoid doing it for
   // class members because the linkage of an anonymous class can
   // change if it's later given a typedef name.
   if (var->isThisDeclarationADefinition() &&
       var->getDeclContext()->getRedeclContext()->isFileContext() &&
       var->isExternallyVisible() && var->hasLinkage() &&
       !var->isInline() && !var->getDescribedVarTemplate() &&
+      var->getStorageClass() != SC_Register &&
       !isa<VarTemplatePartialSpecializationDecl>(var) &&
       !isTemplateInstantiation(var->getTemplateSpecializationKind()) &&
       !getDiagnostics().isIgnored(diag::warn_missing_variable_declarations,
                                   var->getLocation())) {
     // Find a previous declaration that's not a definition.
     VarDecl *prev = var->getPreviousDecl();
     while (prev && prev->isThisDeclarationADefinition())
       prev = prev->getPreviousDecl();

clang/test/Sema/warn-missing-variable-declarations-register.c

+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -Wmissing-variable-declarations -fsyntax-only -verify %s
+// REQUIRES: x86-registered-target
+// expected-no-diagnostics
+
+register unsigned long current_stack_pointer asm("rsp");