[clang] [CodeGen] Revamp counted_by calculations (PR #70606)

[Nick Desaulniers via cfe-commits]

================
@@ -966,9 +962,68 @@ static llvm::Value *getArrayIndexingBound(CodeGenFunction &CGF,
   return nullptr;
 }
 
-FieldDecl *CodeGenFunction::FindCountedByField(
-    const Expr *Base,
-    LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel) {
+Expr *CodeGenFunction::BuildCountedByFieldExpr(Expr *Base,
+                                               const ValueDecl *CountedByVD) {
+  // Find the outer struct expr (i.e. p in p->a.b.c.d).
+  Base = Base->IgnoreImpCasts();
+  Base = Base->IgnoreParenNoopCasts(getContext());
+
+  // Work our way up the expression until we reach the DeclRefExpr.
+  while (!isa<DeclRefExpr>(Base))
+    if (auto *ME = dyn_cast<MemberExpr>(Base->IgnoreImpCasts())) {
+      Base = ME->getBase()->IgnoreImpCasts();
+      Base = Base->IgnoreParenNoopCasts(getContext());
+    }
+
+  // Add back an implicit cast to create the required pr-value.
+  Base =
+      ImplicitCastExpr::Create(getContext(), Base->getType(), CK_LValueToRValue,
+                               Base, nullptr, VK_PRValue, FPOptionsOverride());
+
+  Expr *CountedByExpr = Base;
+
+  if (const auto *IFD = dyn_cast<IndirectFieldDecl>(CountedByVD)) {
+    // The counted_by field is inside an anonymous struct / union. The
+    // IndirectFieldDecl has the correct order of FieldDecls to build this
+    // easily. (Yay!)
+    for (NamedDecl *ND : IFD->chain()) {
+      ValueDecl *VD = cast<ValueDecl>(ND);
+      CountedByExpr =
+          MemberExpr::CreateImplicit(getContext(), CountedByExpr,
+                                     CountedByExpr->getType()->isPointerType(),
+                                     VD, VD->getType(), VK_LValue, OK_Ordinary);
+    }
+  } else {
+    CountedByExpr = MemberExpr::CreateImplicit(
+        getContext(), CountedByExpr, CountedByExpr->getType()->isPointerType(),
+        const_cast<ValueDecl *>(CountedByVD), CountedByVD->getType(), VK_LValue,
+        OK_Ordinary);
+  }
+
+  return CountedByExpr;
+}
+
+const ValueDecl *
+CodeGenFunction::FindFlexibleArrayMemberField(ASTContext &Ctx,
+                                              const RecordDecl *RD) {
+  LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
+      getLangOpts().getStrictFlexArraysLevel();
+
+  for (const Decl *D : RD->decls()) {
+    if (const ValueDecl *VD = dyn_cast<ValueDecl>(D);
----------------
nickdesaulniers wrote:

if (const auto *VD = ...

https://github.com/llvm/llvm-project/pull/70606