[clang] [Clang] Generate the GEP instead of adding AST nodes (PR #73730)

[Eli Friedman via cfe-commits]

================
@@ -956,42 +958,112 @@ static llvm::Value *getArrayIndexingBound(CodeGenFunction &CGF,
   return nullptr;
 }
 
-const Expr *
-CodeGenFunction::BuildCountedByFieldExpr(const Expr *Base,
-                                         const ValueDecl *CountedByVD) {
+namespace {
+
+/// \p MemberExprBaseVisitor returns the base \p DeclRefExpr of a field access.
+class MemberExprBaseVisitor
+    : public StmtVisitor<MemberExprBaseVisitor, Expr *> {
+  const RecordDecl *ExpectedRD;
+
+  bool IsExpectedRecordDecl(const Expr *E) const {
+    QualType Ty = E->getType();
+    if (Ty->isPointerType())
+      Ty = Ty->getPointeeType();
+    return ExpectedRD == Ty->getAsRecordDecl();
+  }
+
+public:
+  MemberExprBaseVisitor(const RecordDecl *ExpectedRD)
+      : ExpectedRD(ExpectedRD) { }
+
+  //===--------------------------------------------------------------------===//
+  //                            Visitor Methods
+  //===--------------------------------------------------------------------===//
+
+  // Note: if we build C++ support for counted_by, then we'll have to handle
+  // horrors like this:
+  //
+  //     struct S {
+  //       int x, y;
+  //       int blah[] __attribute__((counted_by(x)));
+  //     } s;
+  //
+  //     int foo(int index, int val) {
+  //       int (S::*IHatePMDs)[] = &S::blah;
+  //       (s.*IHatePMDs)[index] = val;
+  //     }
+
+  Expr *Visit(Expr *E) {
+    return StmtVisitor<MemberExprBaseVisitor, Expr *>::Visit(E);
+  }
+
+  Expr *VisitCastExpr(CastExpr *E) {
+    return IsExpectedRecordDecl(E) ? E : Visit(E->getSubExpr());
+  }
+  Expr *VisitCompoundLiteralExpr(CompoundLiteralExpr *E) {
+    return IsExpectedRecordDecl(E) ? E : nullptr;
+  }
+  Expr *VisitDeclRefExpr(DeclRefExpr *E) {
+    return IsExpectedRecordDecl(E) ? E : nullptr;
+  }
+  Expr *VisitMemberExpr(MemberExpr *E) {
+    Expr *Res = Visit(E->getBase());
+    return !Res && IsExpectedRecordDecl(E) ? E : Res;
----------------
efriedma-quic wrote:

The recursion here doesn't look right.  Silly example:

```
struct S { struct S *p; int count; int[] values; }
int f(int idx, struct S *p) {
  return p->p->values[idx];
}
```

https://github.com/llvm/llvm-project/pull/73730