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

[Nick Desaulniers via cfe-commits]

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@@ -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;
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nickdesaulniers wrote:

do we need 2 `Expr*`s?  Seems like we could use 1 variable?

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