linux-kernel - Re: [PATCH] [v2] drm/i915: use static const array for PICK macro

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Date:   Tue, 16 Jan 2018 17:42:27 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Imre Deak <imre.deak@...el.com>,
        Lionel Landwerlin <lionel.g.landwerlin@...el.com>,
        Intel Graphics <intel-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [v2] drm/i915: use static const array for PICK macro

On Mon, Dec 11, 2017 at 7:40 PM, Chris Wilson <chris@...is-wilson.co.uk> wrote:
> Quoting Chris Wilson (2017-12-11 12:51:42)
>> Quoting Arnd Bergmann (2017-12-11 12:46:22)
>> > v2: rebased after a1986f4174a4 ("drm/i915: Remove unnecessary PORT3 definition.")
>> > ---
>> >  drivers/gpu/drm/i915/i915_reg.h | 18 +++++++++---------
>> >  1 file changed, 9 insertions(+), 9 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
>> > index 09bf043c1c2e..36f4408503e1 100644
>> > --- a/drivers/gpu/drm/i915/i915_reg.h
>> > +++ b/drivers/gpu/drm/i915/i915_reg.h
>> > @@ -139,7 +139,7 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
>> >         return !i915_mmio_reg_equal(reg, INVALID_MMIO_REG);
>> >  }
>> >
>> > -#define _PICK(__index, ...) (((const u32 []){ __VA_ARGS__ })[__index])
>> > +#define _PICK(__index, ...) ({static const u32 __arr[] = { __VA_ARGS__ }; __arr[__index];})
>>
>> Is gcc smart enough for
>>         if (__builtin_context_p(__index)) {
>>                 ((const u32 []){ __VA_ARGS__ })[__index];
>>         } else {
>>                 static const u32 __arr[] = { __VA_ARGS__ };
>>                 __arr[__index];
>>         }
>> ?
>
> Not really, we don't have enough constants for it to make a substantial
> difference:
>
> add/remove: 1/0 grow/shrink: 3/5 up/down: 617/-604 (13)
> Function                                     old     new   delta
> cnl_ddi_vswing_program.isra                    -     574    +574
> bxt_ddi_phy_is_enabled                       220     241     +21
> bxt_ddi_phy_set_signal_level                 537     556     +19
> i9xx_get_pipe_config                        1474    1477      +3
> bxt_ddi_phy_verify_state                     411     408      -3
> _bxt_ddi_phy_init                            956     950      -6
> vlv_display_power_well_init                  470     461      -9
> bxt_ddi_pll_get_hw_state                     774     762     -12
> cnl_ddi_vswing_sequence                     1166     592    -574
> Total: Before=13461532, After=13461545, chg +0.00%
>
> Of particular note the size of __arr[] is not reduced, so gcc is already
> eliminating the static[] for constant index, or not eliminating the
> redundant branch here.

I noticed we never concluded here. Did you see anything wrong with my
workaround in the end or could we just apply it to avoid the stack
size regression?

       Arnd