[PATCH] D12312: Emiting invariant.group.barrier and adding -fstrict-vptrs

[Richard Smith via cfe-commits]

On Thu, Aug 27, 2015 at 3:49 PM, John McCall <rjmccall at gmail.com> wrote:

> rjmccall added inline comments.
>
> ================
> Comment at: include/clang/Driver/Options.td:990
> @@ -988,2 +989,3 @@
>             "value range">;
> +def fstrict_vptrs: Flag<["-"], "fstrict-vptrs">, Group<f_Group>,
> Flags<[CC1Option]>;
>  def fstrict_overflow : Flag<["-"], "fstrict-overflow">, Group<f_Group>;
> ----------------
> This needs documentation for the --help output, something like "Enable
> optimizations based on the strict rules for overwriting polymorphic C++
> objects".
>
> This option should eventually be promoted to be a driver option, so we
> might as well figure out the name now.  I'd rather not introduce "vptr" to
> the user lexicon.  I suggest -fstrict-vtable-pointers.
>
> ================
> Comment at: lib/CodeGen/CGClass.cpp:1279
> @@ +1278,3 @@
> +  if (CGM.getCodeGenOpts().StrictVPtrs && BaseVPtrsInitialized)
> +    CXXThisValue = Builder.CreateInvariantGroupBarrier(LoadCXXThis());
> +
> ----------------
> Prazek wrote:
> > rjmccall wrote:
> > > Should this just be in InitializeVTablePointers?
> > I want to add invariant.group.barrier only if it's needed. F.e. I don't
> want to put before I initialize vptrs for base, or when my class doesn't
> inherit frome anything. I want emit barrier after I will initialize some
> other vptrs.
> >
> > InitializeVptrs is called in EmitBaseInitializer, and also I woudnt want
> to put some extra flag if it must produce barrier or not (because it is
> hard to distinguish it from inside)
> Fair enough.
>
> Do we need to emit these barriers in unoptimized builds?
>

Yes, I think so, since we want to be able to LTO together optimized and
unoptimized code.

================
> Comment at: lib/CodeGen/CGClass.cpp:1487
> @@ +1486,3 @@
> +    if (!CanSkipVTablePointerInitialization(getContext(), Dtor)) {
> +      // Inserting llvm.invariant.group.barrier intrinsic before base
> vptrs
> +      // initialization to stop propagating previous vptr value.
> ----------------
> Grammar: "Insert the llvm.invariant.group.barrier intrinsic before
> initializing the vptrs to cancel any previous assumptions we might have
> made."
>
>
> http://reviews.llvm.org/D12312
>
>
>
>
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