[PATCH] D63161: Devirtualize destructor of final class.

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


================
Comment at: lib/CodeGen/CGExprCXX.cpp:1867-1868
 
-      if (Dtor->isVirtual()) {
+      if (Dtor->isVirtual() &&
+          !(Dtor->hasAttr<FinalAttr>() || RD->hasAttr<FinalAttr>())) {
         CGF.CGM.getCXXABI().emitVirtualObjectDelete(CGF, DE, Ptr, ElementType,
----------------
yamauchi wrote:
> rsmith wrote:
> > Can we use `getDevirtualizedMethod` here instead? That catches a few other cases that just checking for `final` will miss, and disables this for cases where it will currently do the wrong thing (eg, `-fapple-kext`). See `EmitCXXMemberOrOperatorMemberCallExpr` for where we do this for other virtual calls.
> > 
> > As a particularly terrible example:
> > 
> > ```
> > struct A { virtual ~A() final = 0; };
> > void evil(A *p) { delete p; }
> > ```
> > 
> > is (amazingly) valid, and must not be devirtualized because there is no requirement that the destructor of `A` have a definition in this program. (It would, however, be correct to optimize out the entire `delete` expression in this case, on the basis that `p` must always be `nullptr` whenever it's reached. But that doesn't really seem worthwhile.)
> Updated. Hopefully this does the right thing.
Please can you add that test to your testcases too (eg, with a `CHECK-NOT` to ensure we don't reference the destructor of `A`)? It's subtle enough that I could imagine us getting that wrong in the future.


================
Comment at: lib/CodeGen/CGExprCXX.cpp:1887
+            Dtor = DevirtualizedDtor;
+            Ptr = CGF.EmitPointerWithAlignment(Inner);
+          } else {
----------------
In this case we'll emit the inner expression (including its side-effects) twice.

The simplest thing to do would be to just drop this `else if` case for now and add a FIXME.


Repository:
  rC Clang

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D63161/new/

https://reviews.llvm.org/D63161