[clang] [Clang] Implement the 'counted_by' attribute (PR #76348)

[Bill Wendling via cfe-commits]

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@@ -818,6 +819,189 @@ CodeGenFunction::evaluateOrEmitBuiltinObjectSize(const Expr *E, unsigned Type,
   return ConstantInt::get(ResType, ObjectSize, /*isSigned=*/true);
 }
 
+const FieldDecl *CodeGenFunction::FindFlexibleArrayMemberField(
+    ASTContext &Ctx, const RecordDecl *RD, uint64_t &Offset) {
+  const LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
+      getLangOpts().getStrictFlexArraysLevel();
+  unsigned FieldNo = 0;
+
+  for (const Decl *D : RD->decls()) {
+    if (const auto *Field = dyn_cast<FieldDecl>(D);
+        Field && Decl::isFlexibleArrayMemberLike(
+                     Ctx, Field, Field->getType(), StrictFlexArraysLevel,
+                     /*IgnoreTemplateOrMacroSubstitution=*/true)) {
+      const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(RD);
+      Offset += Layout.getFieldOffset(FieldNo);
+      return Field;
+    }
+
+    if (const auto *Record = dyn_cast<RecordDecl>(D))
+      if (const FieldDecl *Field =
+              FindFlexibleArrayMemberField(Ctx, Record, Offset)) {
+        const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(RD);
+        Offset += Layout.getFieldOffset(FieldNo);
+        return Field;
+      }
+
+    if (isa<FieldDecl>(D))
+      ++FieldNo;
+  }
+
+  return nullptr;
+}
----------------
bwendling wrote:

I thought about that, and they're just differently enough to not be too compatible. The best(?) we could do would be to have a generic "deep" iterator through all of the fields in a struct. (As it is, the iterators are only for the surface level.)

In fact, I think there should be methods added to `ASTRecordLayout` / `CGRecordLayout` to iterate through more than just the first-level decls. I'd like to do that as a follow-up commit though.

https://github.com/llvm/llvm-project/pull/76348