[llvm-commits] [PATCH] Fix Alias Bug

[David Blaikie]

General feedback: please send all patches for review as attachments,
not inline, so that mail clients don't mangle your patches.

On Tue, Jan 8, 2013 at 1:15 PM, David Greene <dag at cray.com> wrote:
> Use a union to do type punning instead of a cast.  A cast
> breaks strict aliasing rules.
> ---
>  llvm/unittests/Support/YAMLIOTest.cpp |   10 ++++++++--
>  1 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/llvm/unittests/Support/YAMLIOTest.cpp b/llvm/unittests/Support/YAMLIOTest.cpp
> index afa71cc..f148c8d 100644
> --- a/llvm/unittests/Support/YAMLIOTest.cpp
> +++ b/llvm/unittests/Support/YAMLIOTest.cpp
> @@ -783,10 +783,16 @@ namespace yaml {
>      static void mapping(IO &io, KindAndFlags& kf) {
>        io.mapRequired("kind",  kf.kind);
>        // type of flags field varies depending on kind field
> +      union {
> +        uint32_t i;
> +        AFlags aflags;
> +        BFlags bflags;
> +      } flags = { kf.flags };
> +

It's still UB to access the non-active member of a union. I believe
the only valid way to do this sort of thing is with memcpy (you're
allowed to read/write the bytes of such simple types through char*s) &
let the compiler optimize that into the same trivial copy.

>        if ( kf.kind == kindA )
> -        io.mapRequired("flags", *((AFlags*)&kf.flags));
> +        io.mapRequired("flags", flags.aflags);
>        else
> -        io.mapRequired("flags", *((BFlags*)&kf.flags));
> +        io.mapRequired("flags", flags.bflags);
>      }
>    };
>  }
> --
> 1.7.8.4
>
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