Re: [PATCH] libgccjit: Handle truncation and extension for casts [PR 95498]

[David Malcolm <dmalcolm@redhat.com>]

On Sat, 2021-02-20 at 17:17 -0500, Antoni Boucher via Gcc-patches
wrote:
> Hi.
> Thanks for your feedback!
> 

Sorry about the delay in responding.

In the past I was hesitant about adding more cast support to libgccjit
since I felt that the user could always just create a union to do the
cast.  Then I tried actually using the libgccjit API to do this, and
realized how much work it adds, so I now think we do want to support
casting more types.


> See answers below:
> 
> On Sat, Feb 20, 2021 at 11:20:35AM -0700, Tom Tromey wrote:
> > > > > > > "Antoni" == Antoni Boucher via Gcc-patches <   
> > > > > > > gcc-patches@gcc.gnu.org> writes:
> > 
> > Antoni> gcc/jit/
> > Antoni>         PR target/95498
> > Antoni>         * jit-playback.c: Add support to handle truncation
> > and extension
> > Antoni>         in the convert function.
> > 
> > Antoni> +  switch (dst_code)
> > Antoni> +    {
> > Antoni> +    case INTEGER_TYPE:
> > Antoni> +    case ENUMERAL_TYPE:
> > Antoni> +      t_ret = convert_to_integer (dst_type, expr);
> > Antoni> +      goto maybe_fold;
> > Antoni> +
> > Antoni> +    default:
> > Antoni> +      gcc_assert (gcc::jit::active_playback_ctxt);
> > Antoni> +      gcc::jit::active_playback_ctxt->add_error (NULL,
> > "unhandled conversion");
> > Antoni> +      fprintf (stderr, "input expression:\n");
> > Antoni> +      debug_tree (expr);
> > Antoni> +      fprintf (stderr, "requested type:\n");
> > Antoni> +      debug_tree (dst_type);
> > Antoni> +      return error_mark_node;
> > Antoni> +
> > Antoni> +    maybe_fold:
> > Antoni> +      if (TREE_CODE (t_ret) != C_MAYBE_CONST_EXPR)

Do we even get C_MAYBE_CONST_EXPR in libgccjit?  That tree code is
defined in c-family/c-common.def; how can nodes of that kind be created
outside of the c-family?

> > Antoni> +       t_ret = fold (t_ret);
> > Antoni> +      return t_ret;
> > 
> > It seems weird to have a single 'goto' to maybe_fold, especially
> > inside
> > a switch like this.
> > 
> > If you think the maybe_fold code won't be reused, then it should just
> > be
> > hoisted up and the 'goto' removed.
> 
> This actually depends on how the support for cast between integers and 
> pointers will be implemented (see below).
> If we will support truncating pointers (does that even make sense? and
> I 
> guess we cannot extend a pointer unless we add the support for 
> uint128_t), that label will be reused for that case.
> Otherwise, it might not be reused.
> 
> So, please tell me which option to choose and I'll update my patch.

FWIW I don't think we'll want to support truncating or extending
pointers.

> 
> > On the other hand, if the maybe_fold code might be reused for some
> > other
> > case, then I suppose I would have the case end with 'break' and then
> > have this code outside the switch.
> > 
> > 
> > In another message, you wrote:
> > 
> > Antoni> For your question, the current code already works with
> > boolean and
> > Antoni> reals and casts between integers and pointers is currently
> > not
> > Antoni> supported.
> > 
> > I am curious why this wasn't supported.  It seems like something that
> > one might want to do.
> 
> I have no idea as this is my first contribution to gcc.
> But this would be indeed very useful and I opened an issue about this: 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95438
> 
> > thanks,
> > Tom
> 
> Thanks!
>