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

[David Malcolm <dmalcolm@redhat.com>]

On Tue, 2021-05-25 at 20:16 -0400, Antoni Boucher wrote:
> I updated the patch according to the comments by Tom Tromey.
> 
> There's one question left about your question regarding
> C_MAYBE_CONST_EXPR, David:
> 
> I am not sure if we can get a C_MAYBE_CONST_EXPR from libgccjit, and
> it
> indeed seems like it's only created in c-family.
> However, we do use it in libgccjit here:
> https://github.com/gcc-mirror/gcc/blob/master/gcc/jit/jit-playback.c#L1180
> 
> I tried removing the condition `if (TREE_CODE (t_ret) !=
> C_MAYBE_CONST_EXPR)` and all the tests of libgccjit still pass.
> 
> That code was copied from here:
> https://github.com/gcc-mirror/gcc/blob/master/gcc/c/c-convert.c#L175
> and might not be needed in libgccjit.
> 
> Should I just remove the condition, then?

I think so.

Thanks
Dave

> 
> Le jeudi 13 mai 2021 à 19:58 -0400, David Malcolm a écrit :
> > 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!
> > > > > 
> > > > 
> > > > 
> > > 
> > > 
> > 
> > 
>