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

[Bill Wendling via cfe-commits]

================
@@ -859,53 +859,60 @@ CodeGenFunction::emitBuiltinObjectSize(const Expr *E, unsigned Type,
   }
 
   if (IsDynamic) {
-    LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
-        getLangOpts().getStrictFlexArraysLevel();
-    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);
-      }
-
-      // At this point, we know that \p ME is a flexible array member.
-      const auto *ArrayTy = getContext().getAsArrayType(ME->getType());
+    // 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 the whole struct, including the flexible array:
+    //
+    //     __builtin_dynamic_object_size(p, 1) ==
+    //        sizeof(*p) + p->count * sizeof(*p->array)
----------------
bwendling wrote:

@apple-fcloutier Ah, I didn't see the square brackets. So this only works if the `counted_by` attribute is specified on the fam and set to the correct number. In the documentation, it's explicitly stated that it's up to the programmer to ensure that the count is correct if the size somehow changes.

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