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

[Chris Wilson]

Quoting Chris Wilson (2017-12-11 12:51:42)
> Quoting Arnd Bergmann (2017-12-11 12:46:22)
> > The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
> > to shrink the i915 kernel module by around 1000 bytes. However, the
> > downside is a size regression with CONFIG_KASAN, as I found from stack size
> > warnings with gcc-7.0.1:
> > 
> > before:
> > drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_get_hw_state':
> > drivers/gpu/drm/i915/intel_dpll_mgr.c:1644:1: error: the frame size of 176 bytes is larger than 100 bytes [-Werror=frame-larger-than=]
> > drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_enable':
> > drivers/gpu/drm/i915/intel_dpll_mgr.c:1548:1: error: the frame size of 224 bytes is larger than 100 bytes [-Werror=frame-larger-than=]
> > 
> > after:
> > drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_get_hw_state':
> > drivers/gpu/drm/i915/intel_dpll_mgr.c:1644:1: error: the frame size of 1016 bytes is larger than 1000 bytes [-Werror=frame-larger-than=]
> > drivers/gpu/drm/i915/intel_dpll_mgr.c: In function 'bxt_ddi_pll_enable':
> > drivers/gpu/drm/i915/intel_dpll_mgr.c:1548:1: error: the frame size of 1960 bytes is larger than 1000 bytes [-Werror=frame-larger-than=]
> > 
> > I also checked the module sizes and got with gcc-7.0.1
> > 
> > original:
> >    text    data     bss     dec     hex filename
> > 2380830 1155436    4448 3540714  3606ea drivers/gpu/drm/i915/i915-kasan.o
> > 1298054  543692    2884 1844630  1c2596 drivers/gpu/drm/i915/i915-nokasan.o
> > 
> > after ce64645d86ac:
> >    text    data     bss     dec     hex filename
> > 2389515 1154476    4448 3548439  362517 drivers/gpu/drm/i915/i915-kasan.o
> > 1299639  543692    2884 1846215  1c2bc7 drivers/gpu/drm/i915/i915-nokasan.o
> > 
> > with this patch:
> >    text    data     bss     dec     hex filename
> > 2381275 1163884    4448 3549607  3629a7 drivers/gpu/drm/i915/i915-kasan.o
> > 1296038  543692    2884 1842614  1c1db6 drivers/gpu/drm/i915/i915-nokasan.o
> > 
> > Actually showing a code size growth in .text both with and without kasan,
> > and my version gets most of it back at the expense of larger .data when
> > kasan is enabled.
> > 
> > Fixes: ce64645d86ac ("drm/i915: use variadic macros and arrays to choose port/pipe based registers")
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80114
> > Link: https://lkml.org/lkml/2017/3/20/1022
> > Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> > 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.
-Chris