[libcxx-commits] [clang] [libcxx] [clang] Add builtin to clear padding bytes (prework for P0528R3) (PR #75371)

[Eli Friedman via libcxx-commits]

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@@ -3124,7 +3124,71 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
     return BuiltinIsWithinLifetime(*this, TheCall);
   case Builtin::BI__builtin_trivially_relocate:
     return BuiltinTriviallyRelocate(*this, TheCall);
+  case Builtin::BI__builtin_clear_padding: {
+    if (checkArgCount(TheCall, 1))
+      return ExprError();
+
+    const Expr *PtrArg = TheCall->getArg(0);
+    const QualType PtrArgType = PtrArg->getType();
+    if (!PtrArgType->isPointerType()) {
+      Diag(PtrArg->getBeginLoc(), diag::err_typecheck_convert_incompatible)
+          << PtrArgType << "pointer" << 1 << 0 << 3 << 1 << PtrArgType
+          << "pointer";
+      return ExprError();
+    }
+    QualType PointeeType = PtrArgType->getPointeeType();
+    if (PointeeType.isConstQualified()) {
+      Diag(PtrArg->getBeginLoc(), diag::err_typecheck_assign_const)
+          << TheCall->getSourceRange() << 4 /*ConstUnknown*/;
+      return ExprError();
+    }
+    if (RequireCompleteType(PtrArg->getBeginLoc(), PointeeType,
+                            diag::err_typecheck_decl_incomplete_type))
+      return ExprError();
+
+    // For non trivially copyable types, we try to match gcc's behaviour.
+    // i.e. __builtin_clear_padding(&var) is OK as long as var is a complete
+    // object, either a local variable or a function parameter passed by value
+    auto IsAddrOfDeclExpr = [&]() {
+      const Expr *IgnoreCastsAndParens = PtrArg->IgnoreCasts();
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efriedma-quic wrote:

IgnoreCasts is probably too aggressive.  Casts in general can do a lot of weird things.

I think we have an IgnoreNoopCasts or something like that?

https://github.com/llvm/llvm-project/pull/75371