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

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2021-05-13 at 19:31 -0400, Antoni Boucher wrote:
> Thanks for your answer.
> 
> See my answers below:
> 
> Le jeudi 13 mai 2021 à 18:13 -0400, David Malcolm a écrit :
> > 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?
> 
> I am not sure, but that seems like it's only created in c-family
> indeed.
> However, we do use it in libgccjit here:
> 
> https://github.com/gcc-mirror/gcc/blob/master/gcc/jit/jit-playback.c#L1180
> 
> > 
> > > > 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.
> 
> Ok, but do you think we'll want to support casts between integers and
> pointers?

Yes, though we probably want to reject truncating a pointer into a
smaller integer type.

> I opened an issue about this
> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95438) and would be
> willing to do a patch for it eventually.
> 
> > > 
> > > > 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!
> > > 
> > 
> > 
> 
>