[llvm] [clang-tools-extra] [clang] [CodeGen] Revamp counted_by calculations (PR #70606)

[Yeoul Na via cfe-commits]

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@@ -859,53 +860,93 @@ CodeGenFunction::emitBuiltinObjectSize(const Expr *E, unsigned Type,
   }
 
   if (IsDynamic) {
-    LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
-        getLangOpts().getStrictFlexArraysLevel();
+    // The code generated here calculates the size of a struct with a flexible
+    // array member that uses the counted_by attribute. There are two instances
+    // we handle:
+    //
+    //       struct s {
+    //         unsigned long flags;
+    //         int count;
+    //         int array[] __attribute__((counted_by(count)));
+    //       }
+    //
+    //   1) bdos of the flexible array itself:
+    //
+    //     __builtin_dynamic_object_size(p->array, 1) ==
+    //         p->count * sizeof(*p->array)
+    //
+    //   2) bdos of a pointer into the flexible array:
+    //
+    //     __builtin_dynamic_object_size(&p->array[42], 1) ==
+    //         (p->count - 42) * sizeof(*p->array)
+    //
+    //   2) bdos of the whole struct, including the flexible array:
+    //
+    //     __builtin_dynamic_object_size(p, 1) ==
+    //        max(sizeof(struct s),
+    //            offsetof(struct s, array) + p->count * sizeof(*p->array))
+    //
     const Expr *Base = E->IgnoreParenImpCasts();
-
-    if (FieldDecl *FD = FindCountedByField(Base, StrictFlexArraysLevel)) {
-      const auto *ME = dyn_cast<MemberExpr>(Base);
-      llvm::Value *ObjectSize = nullptr;
-
-      if (!ME) {
-        const auto *DRE = dyn_cast<DeclRefExpr>(Base);
-        ValueDecl *VD = nullptr;
-
-        ObjectSize = ConstantInt::get(
-            ResType,
-            getContext().getTypeSize(DRE->getType()->getPointeeType()) / 8,
-            true);
-
-        if (auto *RD = DRE->getType()->getPointeeType()->getAsRecordDecl())
-          VD = RD->getLastField();
-
-        Expr *ICE = ImplicitCastExpr::Create(
-            getContext(), DRE->getType(), CK_LValueToRValue,
-            const_cast<Expr *>(cast<Expr>(DRE)), nullptr, VK_PRValue,
-            FPOptionsOverride());
-        ME = MemberExpr::CreateImplicit(getContext(), ICE, true, VD,
-                                        VD->getType(), VK_LValue, OK_Ordinary);
+    const Expr *Idx = nullptr;
+    if (const auto *UO = dyn_cast<UnaryOperator>(Base);
+        UO && UO->getOpcode() == UO_AddrOf) {
+      if (const auto *ASE = dyn_cast<ArraySubscriptExpr>(UO->getSubExpr())) {
----------------
rapidsna wrote:

Maybe we should do `dyn_cast<ArraySubscriptExpr>(UO->getSubExpr()->IgnoreParens())`? For case like this `&(f->fam[0])`?

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