Re: [PATCH][simplify-rtx] Zero-initialise local array in simplify_immed_subreg

[Andrew Pinski <pinskia at gmail dot com>]

On Fri, Oct 7, 2016 at 7:08 AM, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:
> Hi all,
>
> I've encountered another wrong-code bug with the store merging pass. This
> time it's in RTL.
> The test gcc.target/aarch64/aapcs64/test_27.c on aarch64 merges a few __fp16
> values at GIMPLE level but
> during RTL dse1 one of the constants generated gets wrongly misinterpreted
> from HImode to HFmode
> by simplify_immed_subreg. The HFmode value it ends up producing is
> completely bogus.
>
> By stepping through the code with GDB the problem is in the hunk touched by
> this patch when it
> fills in an array of longs with the value bytes before passing it down to
> real_from_target.
> It fills in the array by orring in each byte.
>
> However, the array it declared to use for this doesn not get properly
> zero-initialised for modes
> less that 32 bits wide (HFmode in this case). The fix in this patch is to
> just use an array initialiser
> to zero it out. This makes the failure go away.
>
> Bootstrapped and tested on aarch64, x86_64.
>
> Ok for trunk?

Even though this has been approved I think it is best to do write this
code as this:
long tmp[MAX_BITSIZE_MODE_ANY_MODE / 32];
     /* real_from_target wants its input in words affected by
        FLOAT_WORDS_BIG_ENDIAN.  However, we ignore this,
        and use WORDS_BIG_ENDIAN instead; see the documentation
        of SUBREG in rtl.texi.  */
memset (tmp, 0, sizeof(tmp));

Thanks,
Andrew Pinski


> Thanks,
> Kyrill
>
> 2016-10-06  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     * simplify-rtx.c (simplify_immed_subreg): Zero-initialize tmp array
>     before merging in bytes to pass down to real_from_target.