[C11] PR24451: Allow restrict _Atomic pointers

[• d.zobnin.bugzilla]

Summary

Treat _Atomic pointers as pointer types (check value type of AtomicType) in case of handling the "restrict" qualifier. Accept such code (so does GCC):

int * restrict _Atomic p;

[rsmith]

I'm not convinced this change is correct. There are two possibilities for what int *_Atomic restrict could mean:

1. _Atomic(int *restrict): this seems to be ill-formed by 6.7.2.4/3, because the type name in an atomic type specifier cannot be a qualified type
2. _Atomic(int *) restrict: this seems to be ill-formed by 6.7.3/2, because (by 6.2.5) an atomic type is not a pointer type

If you're trying to simulate a GCC extension, this should at least have a corresponding ExtWarn, but you'll also need to make the case that the extension is useful.

[• d.zobnin.bugzilla]

Richard, thank you for the review and explanation!
I don't have any examples of useful code using this extension, I was trying to fix the PR... So, do you think we should add this GCC extension to Clang (with a proper warning, of course) for compatibility reasons, or it will be better to close PR24451 and abandon this review?

Thank you,
Denis Zobnin

[• d.zobnin.bugzilla]

Richard, please let me know what in your opinion we should do with this revision.

Thank you,
Denis Zobnin

[rsmith]

I mailed WG14 asking for clarification on how these rules are supposed to work (10 days ago) and am yet to hear anything in response. Is this apparent GCC extension/bug seeing much use in the wild? If not, I think our path forward is to file this as a bug against GCC and see if they want to fix it or accept it as an extension.

lib/Sema/SemaType.cpp

static std::string getPrintableNameForEntity(DeclarationName Entity) {
   return "type name";
 }
 
 QualType Sema::BuildQualifiedType(QualType T, SourceLocation Loc,
                                   Qualifiers Qs, const DeclSpec *DS) {
   if (T.isNull())
     return QualType();
 
+  QualType OriginalType = T;
+
   // Enforce C99 6.7.3p2: "Types other than pointer types derived from
   // object or incomplete types shall not be restrict-qualified."
   if (Qs.hasRestrict()) {
     unsigned DiagID = 0;
     QualType ProblemTy;
 
+    // Handle code like "int * _Atomic restrict p".
+    if (const AtomicType *AtomicT = T->getAs<AtomicType>())
+      T = AtomicT->getValueType();
+
     if (T->isAnyPointerType() || T->isReferenceType() ||
         T->isMemberPointerType()) {
       QualType EltTy;
       if (T->isObjCObjectPointerType())
         EltTy = T;
       else if (const MemberPointerType *PTy = T->getAs<MemberPointerType>())
         EltTy = PTy->getPointeeType();
       else
if (Qs.hasRestrict()) {
     }
 
     if (DiagID) {
       Diag(DS ? DS->getRestrictSpecLoc() : Loc, DiagID) << ProblemTy;
       Qs.removeRestrict();
     }
   }
 
-  return Context.getQualifiedType(T, Qs);
+  return Context.getQualifiedType(OriginalType, Qs);
 }
 
 QualType Sema::BuildQualifiedType(QualType T, SourceLocation Loc,
                                   unsigned CVRA, const DeclSpec *DS) {
   if (T.isNull())
     return QualType();
 
   // Convert from DeclSpec::TQ to Qualifiers::TQ by just dropping TQ_atomic.

test/Misc/ast-dump-restrict-atomic.c

+// RUN: %clang_cc1 -std=c11 -ast-dump %s | FileCheck %s
+// RUN: %clang_cc1 -std=c11 -verify %s
+// expected-no-diagnostics
+
+int * _Atomic restrict a;
+int * restrict _Atomic b;
+int _Atomic * restrict c;
+volatile _Atomic int * restrict _Atomic const * restrict d;
+
+// CHECK: a '__restrict _Atomic(int *)'
+// CHECK: b '__restrict _Atomic(int *)'
+// CHECK: c '_Atomic(int) *__restrict'
+// CHECK: d 'const __restrict _Atomic(volatile _Atomic(int) *) *__restrict'