Re: [PATCH] Fix decimal floating-point LTO streaming for offloading compilation

[Thomas Schwinge <thomas@codesourcery.com>]

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Hi Julian!

On 2019-11-28T14:24:02+0000, Julian Brown <julian@codesourcery.com> wrote:
> As mentioned in PR91985, offloading compilation is broken at present
> because of an issue with LTO streaming. With thanks to Joseph for
> hints, here's a solution.
>
> Unlike e.g. the _FloatN types, when decimal floating-point types are
> enabled, common tree nodes are created for each float type size (e.g.
> dfloat32_type_node) and also a pointer to each type is created
> (e.g. dfloat32_ptr_type_node). tree-streamer.c:record_common_node emits
> these like:
>
>   <float:32>     (dfloat32_type_node)
>   <float:64>     (dfloat64_type_node)
>   <float:128>    (dfloat128_type_node)
>   <float:32> *   (dfloat32_ptr_type_node)
>   <float:32>
>   <float:64> *   (dfloat64_ptr_type_node)
>   <float:64>
>   <float:128> *  (dfloat128_ptr_type_node)
>   <float:128>
>
> I.e., with explicit emission of a copy of the pointed-to type following
> the pointer itself.

I also do see that, but I fail to understand why that duplication: the
first '<float:32>' and the second one (after the '<float:32> *') are the
same node, or aren't they?

> When DFP is disabled, we instead get:
>
>   <<< error >>>
>   <<< error >>>
>   <<< error >>>
>   <<< error >>>
>   <<< error >>>
>   <<< error >>>

(With that expectedly being any 'NULL_TREE's converted to
'error_mark_node' in 'gcc/tree-streamer.c:record_common_node'.)

> So, the number of nodes emitted during LTO write-out in the host/read-in
> in the offload compiler do not match.

ACK.

> This patch restores the number of nodes emitted by creating
> dfloatN_ptr_type_node as generic pointers rather than treating them as
> flat error_type_nodes. I don't think there's an easy way of creating an
> "error_type_node *", nor do I know if that would really be preferable.
>
> Tested with offloading to NVPTX & bootstrapped. OK to apply?

> commit 17119773a8a45af098364b4faafe68f2e868479a
> Author: Julian Brown <julian@codesourcery.com>
> Date:   Wed Nov 27 18:41:56 2019 -0800
>
>     Fix decimal floating-point LTO streaming for offloading compilation
>     
>             gcc/
>             * tree.c (build_common_tree_nodes): Use pointer type for
>             dfloat32_ptr_type_node, dfloat64_ptr_type_node and
>             dfloat128_ptr_type_node when decimal floating point support is disabled.
>
> diff --git a/gcc/tree.c b/gcc/tree.c
> index 5ae250ee595..db3f225ea7f 100644
> --- a/gcc/tree.c
> +++ b/gcc/tree.c
> @@ -10354,6 +10354,15 @@ build_common_tree_nodes (bool signed_char)
>        layout_type (dfloat128_type_node);
>        dfloat128_ptr_type_node = build_pointer_type (dfloat128_type_node);
>      }
> +  else
> +    {
> +      /* These must be pointers else tree-streamer.c:record_common_node will emit
> +	 a different number of nodes depending on DFP availability, which breaks
> +	 offloading compilation.  */
> +      dfloat32_ptr_type_node = ptr_type_node;
> +      dfloat64_ptr_type_node = ptr_type_node;
> +      dfloat128_ptr_type_node = ptr_type_node;
> +    }
>  
>    complex_integer_type_node = build_complex_type (integer_type_node, true);
>    complex_float_type_node = build_complex_type (float_type_node, true);

(Maybe that's indeed better than my "hamfisted" patch.)  ;-)

But it still reads a bit like a workaround (explicitly setting only
'dfloat*_ptr_type_node' here, leaving the actual 'dfloat*_type_node'
untouched (and then later on implicitly converted to 'error_mark_node' as
mentioned).

I guess we need somebody with more experience to review this.


Grüße
 Thomas

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