Re: [PATCH] ipa-fnsummary: Remove inconsistent bp_pack_value

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Sep 21, 2021 at 4:09 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
>
> Hi Richi,
>
> Thanks for the review!
>
> on 2021/9/17 下午6:04, Richard Biener wrote:
> > On Fri, Sep 17, 2021 at 12:03 PM Richard Biener
> > <richard.guenther@gmail.com> wrote:
> >>
> >> On Fri, Sep 17, 2021 at 11:43 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
> >>>
> >>> Hi,
> >>>
> >>> When changing target_info with bitfield, I happened to find this
> >>> inconsistent streaming in and out.  We have the streaming in:
> >>>
> >>>           bp_pack_value (&bp, info->inlinable, 1);
> >>>           bp_pack_value (&bp, false, 1);
> >>>           bp_pack_value (&bp, info->fp_expressions, 1);
> >>>
> >>> while the streaming out:
> >>>
> >>>           info->inlinable = bp_unpack_value (&bp, 1);
> >>>           info->fp_expressions = bp_unpack_value (&bp, 1)
> >>>
> >>> The cleanup of Cilk Plus support seemed to miss to remove the bit
> >>> streaming out but change with streaming false.
> >>>
> >>> By hacking fp_expression_p to return true always, I can see it
> >>> reads the wrong fp_expressions value (false) out in wpa dumping.
> >>>
> >>> Bootstrapped and regress-tested on powerpc64le-linux-gnu Power9.
> >>>
> >>> Is it ok for trunk?
> >>
> >> OK for trunk and all affected branches (note we need to bump the
> >> LTO minor version there).  The issue comes from the removal
> >> of cilk+ in r8-4956 which removed the bp_unpack but replaced
> >> the bp_pack ...
> >>
> >> It's a correctness issue as we'll read fp_expressions as always 'false'
> >
> > Btw, on branches we could also simply unpack a dummy bit to avoid
> > changing the format.
> >
>
> Committed in r12-3721.  Thanks!
>
> As suggested, the patch for branches is listed below.
>
> Is ok for branches 9, 10 and 11 after some trunk burn in time?

It's OK for branches without waiting, maybe you can do a LTO bootstrap
on the branches for extra safety (just in case we're triggering some hidden
issues due to the fix).

Thanks,
Richard.

> BR,
> Kewen
> -----
> gcc/ChangeLog:
>
>         * ipa-fnsummary.c (inline_read_section): Unpack a dummy bit
>         to keep consistent with the side of streaming out.
>
> ---
> diff --git a/gcc/ipa-fnsummary.c b/gcc/ipa-fnsummary.c
> index 18bbae145b9..bf635c1f78a 100644
> --- a/gcc/ipa-fnsummary.c
> +++ b/gcc/ipa-fnsummary.c
> @@ -4403,13 +4403,20 @@ inline_read_section (struct lto_file_decl_data *file_data, const char *data,
>        bp = streamer_read_bitpack (&ib);
>        if (info)
>         {
> -          info->inlinable = bp_unpack_value (&bp, 1);
> -          info->fp_expressions = bp_unpack_value (&bp, 1);
> +         info->inlinable = bp_unpack_value (&bp, 1);
> +         /* On the side of streaming out, there is still one bit
> +            streamed out between inlinable and fp_expressions bits,
> +            which was used for cilk+ before but now always false.
> +            To remove the bit packing need to bump LTO minor version,
> +            so unpack a dummy bit here to keep consistent instead.  */
> +         bp_unpack_value (&bp, 1);
> +         info->fp_expressions = bp_unpack_value (&bp, 1);
>         }
>        else
>         {
> -          bp_unpack_value (&bp, 1);
> -          bp_unpack_value (&bp, 1);
> +         bp_unpack_value (&bp, 1);
> +         bp_unpack_value (&bp, 1);
> +         bp_unpack_value (&bp, 1);
>         }
>
>        count2 = streamer_read_uhwi (&ib);
>