[PATCH][GCC][ARM] Fix can_change_mode_class for big-endian

[Tamar Christina]

Hi Kyrill,

Many thanks for the review!

The 06/20/2018 09:43, Kyrill Tkachov wrote:
> Hi Tamar,
> 
> On 19/06/18 15:14, Tamar Christina wrote:
> > Hi All,
> >
> > This patch requires https://gcc.gnu.org/ml/gcc-patches/2018-06/msg01145.html to work,
> > it has been accepted once already but caused a regression on certain configuratoins.
> > I am re-submitting it with the required mid-end change and requesting a back-port.
> >
> > --- original patch.
> >
> > Taking the subreg of a vector mode on big-endian may result in an infinite
> > recursion and eventually a segfault once we run out of stack space.
> >
> > As an example, taking a subreg of V4HF to SImode we end up in the following
> > loop on big-endian:
> >
> > #861 0x00000000008462e9 in operand_subword_force src/gcc/gcc/emit-rtl.c:1787
> > #862 0x0000000000882a90 in emit_move_multi_word src/gcc/gcc/expr.c:3621
> > #863 0x000000000087eea1 in emit_move_insn_1 src/gcc/gcc/expr.c:3698
> > #864 0x000000000087f350 in emit_move_insn src/gcc/gcc/expr.c:3757
> > #865 0x000000000085e326 in copy_to_reg src/gcc/gcc/explow.c:603
> > #866 0x00000000008462e9 in operand_subword_force src/gcc/gcc/emit-rtl.c:1787
> >
> > The reason is that operand_subword_force will always fail. When the value is in
> > a register that can't be accessed as a multi word the code tries to create a new
> > psuedo register and emit the value to it. Eventually you end up in simplify_gen_subreg
> > which calls validate_subreg.
> >
> > validate_subreg will however always fail because of the REG_CAN_CHANGE_MODE_P check.
> >
> > On little endian this check always returns true. On big-endian this check is supposed
> > to prevent values that have a size larger than word size, due to those being stored in
> > VFP registers.
> >
> > However we are only interested in a subreg of the vector mode, so we should be checking
> > the unit size, not the size of the entire mode. Doing this fixes the problem.
> >
> > Regtested on armeb-none-eabi and no regressions.
> > Bootstrapped on arm-none-linux-gnueabihf and no issues.
> >
> > Ok for trunk? and for backport to GCC 8?
> >
> > Thanks,
> > Tamar
> >
> > gcc/
> > 2018-06-19  Tamar Christina  <tamar.christina@arm.com>
> >
> >         PR target/84711
> >         * config/arm/arm.c (arm_can_change_mode_class): Use GET_MODE_UNIT_SIZE
> >         instead of GET_MODE_SIZE when comparing Units.
> >
> > gcc/testsuite/
> > 2018-06-19  Tamar Christina  <tamar.christina@arm.com>
> >
> >         PR target/84711
> >         * gcc.target/arm/big-endian-subreg.c: New.
> >
> > -- 
> 
> 
> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> index 90d62e699bce9594879be2e3016c9b36c7e064c8..703632240822e762a906570964b949c783df56f3 100644
> --- a/gcc/config/arm/arm.c
> +++ b/gcc/config/arm/arm.c
> @@ -31508,8 +31508,8 @@ arm_can_change_mode_class (machine_mode from, machine_mode to,
>   {
>     if (TARGET_BIG_END
>         && !(GET_MODE_SIZE (from) == 16 && GET_MODE_SIZE (to) == 8)
> -      && (GET_MODE_SIZE (from) > UNITS_PER_WORD
> -	  || GET_MODE_SIZE (to) > UNITS_PER_WORD)
> +      && (GET_MODE_UNIT_SIZE (from) > UNITS_PER_WORD
> +	  || GET_MODE_UNIT_SIZE (to) > UNITS_PER_WORD)
> 
> Does GET_MODE_UNIT_SIZE do what you want? Its documentation in rtl.texi says:
> "Returns the size in bytes of the subunits of a datum of mode @var{m}.
>   This is the same as @code{GET_MODE_SIZE} except in the case of complex
>   modes.  For them, the unit size is the size of the real or imaginary
>   part."
> 
> Does it also do the right thing for vector modes (GET_MODE_SIZE (GET_MODE_INNER (mode))) ?
> If so, this patch is ok, but we'll need to update the documentation to make it more explicit.

I don't know what the documentation is trying to say here, but the key is the first part:

"returns the size in bytes of the subunits of a datum of mode m"

MODE: V4SI, GET_MODE_UNIT_SIZE: 4, GET_MODE_SIZE: 16

So GET_MODE_UNIT_SIZE (m) * GET_MODE_NUNITS(m) == GET_MODE_SIZE (m)

or GET_MODE_UNIT_SIZE (m) == GET_MODE_SIZE (GET_MODE_INNER (mode)).

>From this the only time GET_MODE_UNIT_SIZE is equal to GET_MODE_SIZE is on
non-vector modes of V1 vector modes.

Kind Regards,
Tamar

> 
> Thanks for the patch,
> Kyrill
> 
> 
> 

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