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

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: #64509
Link: https://lore.kernel.org/[email protected]/
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110947

Reviewed By: aaron.ballman, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D157435

Author: nathanchance

clang/docs/ReleaseNotes.rst

@@ -123,6 +123,8 @@ Improvements to Clang's diagnostics
   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
 -------------------------

clang/lib/Sema/SemaDecl.cpp

@@ -14144,6 +14144,7 @@ void Sema::CheckCompleteVariableDeclaration(VarDecl *var) {
       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,

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

@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -Wmissing-variable-declarations -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+register unsigned long current_stack_pointer asm("rsp");