[BPF] Restrict preserve_access_index attribute to C only

[yonghong-song]

Summary

This patch is a follow-up for commit 4e2ce228ae79

[BPF] Add preserve_access_index attribute for record definition

to restrict attribute for C only. A new test case is added
to check for this restriction.

Additional code polishing is done based on
Aaron Ballman's suggestion in https://reviews.llvm.org/D69759/new/.

[aaron.ballman]

LGTM aside from a small nit. Thank you!

clang/lib/CodeGen/CGExpr.cpp

3431–3435

You can drop the getTypePtr() here and just use the magic -> overload.

[yonghong-song]

clang/lib/CodeGen/CGExpr.cpp

3431–3435

Thanks! This indeed better:

+    const auto *PointeeT = PtrT->getPointeeType()
+                             ->getUnqualifiedDesugaredType();

clang/include/clang/Basic/Attr.td

def AMDGPUNumVGPR : InheritableAttr {
   let Subjects = SubjectList<[Function], ErrorDiag, "kernel functions">;
 }
 
 def BPFPreserveAccessIndex : InheritableAttr,
                              TargetSpecificAttr<TargetBPF>  {
   let Spellings = [Clang<"preserve_access_index">];
   let Subjects = SubjectList<[Record], ErrorDiag>;
   let Documentation = [BPFPreserveAccessIndexDocs];
+  let LangOpts = [COnly];
 }
 
 def WebAssemblyImportModule : InheritableAttr,
                               TargetSpecificAttr<TargetWebAssembly> {
   let Spellings = [Clang<"import_module">];
   let Args = [StringArgument<"ImportModule">];
   let Documentation = [WebAssemblyImportModuleDocs];
   let Subjects = SubjectList<[Function], ErrorDiag>;

clang/lib/CodeGen/CGExpr.cpp

 }
 
 /// Given an array base, check whether its member access belongs to a record
 /// with preserve_access_index attribute or not.
 static bool IsPreserveAIArrayBase(CodeGenFunction &CGF, const Expr *ArrayBase) {
   if (!ArrayBase || !CGF.getDebugInfo())
     return false;
 
-  const auto *ImplicitCast = dyn_cast<ImplicitCastExpr>(ArrayBase);
-  if (!ImplicitCast)
-    return false;
-
   // Only support base as either a MemberExpr or DeclRefExpr.
   // DeclRefExpr to cover cases like:
   //    struct s { int a; int b[10]; };
   //    struct s *p;
   //    p[1].a
   // p[1] will generate a DeclRefExpr and p[1].a is a MemberExpr.
   // p->b[5] is a MemberExpr example.
-  const Expr *E = ImplicitCast->getSubExpr();
-  const auto *MemberCast = dyn_cast<MemberExpr>(E);
-  if (MemberCast)
-    return MemberCast->getMemberDecl()->hasAttr<BPFPreserveAccessIndexAttr>();
-
-  const auto *DeclRefCast = dyn_cast<DeclRefExpr>(E);
-  if (DeclRefCast) {
-    const VarDecl *VarDef = dyn_cast<VarDecl>(DeclRefCast->getDecl());
+  const Expr *E = ArrayBase->IgnoreImpCasts();
+  if (const auto *ME = dyn_cast<MemberExpr>(E))
+    return ME->getMemberDecl()->hasAttr<BPFPreserveAccessIndexAttr>();
+
+  if (const auto *DRE = dyn_cast<DeclRefExpr>(E)) {
+    const auto *VarDef = dyn_cast<VarDecl>(DRE->getDecl());
     if (!VarDef)
       return false;
 
-    const auto *PtrT = dyn_cast<PointerType>(VarDef->getType().getTypePtr());
+    const auto *PtrT = VarDef->getType()->getAs<PointerType>();
     if (!PtrT)
       return false;
-    const auto *PointeeT = PtrT->getPointeeType().getTypePtr();
-
-    // Peel off typedef's
-    const auto *TypedefT = dyn_cast<TypedefType>(PointeeT);
-    while (TypedefT) {
-      PointeeT = TypedefT->desugar().getTypePtr();
-      TypedefT = dyn_cast<TypedefType>(PointeeT);
-    }
-
-    // Not a typedef any more, it should be an elaborated type.
-    const auto ElaborateT = dyn_cast<ElaboratedType>(PointeeT);
-    if (!ElaborateT)
-      return false;
-
-    const auto *RecT = dyn_cast<RecordType>(ElaborateT->desugar().getTypePtr());
-    if (!RecT)
-      return false;
 
+    const auto *PointeeT = PtrT->getPointeeType()
+                             ->getUnqualifiedDesugaredType();
+    if (const auto *RecT = dyn_cast<RecordType>(PointeeT))
       return RecT->getDecl()->hasAttr<BPFPreserveAccessIndexAttr>();
+    return false;
   }
 
   return false;
 }
 
 static Address emitArraySubscriptGEP(CodeGenFunction &CGF, Address addr,
                                      ArrayRef<llvm::Value *> indices,
                                      QualType eltType, bool inbounds,

clang/test/Sema/bpf-attr-preserve-access-index.cpp

+// RUN: %clang_cc1 -x c++ -triple bpf-pc-linux-gnu -dwarf-version=4 -fsyntax-only -verify %s
+
+#define __reloc__ __attribute__((preserve_access_index))
+
+struct t1 {
+  int a;
+  int b[4];
+  int c:1;
+} __reloc__; // expected-warning {{'preserve_access_index' attribute ignored}}