Type-punning warnings [was Re: PATCH: silence warnings in unwind-dw2-fde.c]

[Dave Korn <dave dot korn dot cygwin at googlemail dot com>]

Ben Elliston wrote:
> This patch silences the following warnings when building libgcc:
> 
> unwind-dw2-fde.c:321: warning: dereferencing type-punned pointer will break
> strict-aliasing rules

> -  const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) x->pc_begin;
> -  const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) y->pc_begin;
> +  _Unwind_Ptr x_ptr, y_ptr;
> +  memcpy (&x_ptr, x->pc_begin, sizeof (_Unwind_Ptr));
> +  memcpy (&y_ptr, y->pc_begin, sizeof (_Unwind_Ptr));

  Say, I've been in the habit in the past (GCC 3.x) of avoiding these warnings
by doing like:

> -  const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) x->pc_begin;
> -  const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) y->pc_begin;
> +  const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) (void *) x->pc_begin;
> +  const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) (void *) y->pc_begin;

... which I see no longer works.  Was that ever correct, or was I just losing
information during the cast-to-void that meant GCC just couldn't diagnose a
potential problem, but it was still there?  I believed that casting through
void would make the result could-alias-anything, which I thought would prevent
any untoward consequences.

  Also, maybe rather than warning, we could translate the assignment to a
built-in memcpy internally?  Or perhaps best of all do both?

    cheers,
      DaveK