On Wed, 20 Mar 2019, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
> gcc-4.8 and older dislike the use of __builtin_constant_p() within a
> constant expression context, and so we must use the magical
> __is_constexpr() instead.
>
> For example, with gcc-4.8.5:
> ../drivers/gpu/drm/i915/i915_reg.h:167:27: error: first argument to ‘__builtin_choose_expr’ not a constant
> ../include/linux/build_bug.h:16:45: error: bit-field ‘<anonymous>’ width not an integer constant
>
> Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Fixes: baa09e7d2f42 ("drm/i915: use REG_FIELD_PREP() to define register bitfield values")
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
> Cc: Imre Deak <imre.deak@xxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Thanks, Uma reported this too and confirmed __is_constexpr works.
Reported-by: Uma Shankar <uma.shankar@xxxxxxxxx>
Reviewed-by: Jani Nikula <jani.nikula@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_reg.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 85e8d1a1f70b..50d0b2ae89eb 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -161,10 +161,10 @@
> */
> #define REG_FIELD_PREP(__mask, __val) \
> ((u32)((((typeof(__mask))(__val) << __bf_shf(__mask)) & (__mask)) + \
> - BUILD_BUG_ON_ZERO(!__builtin_constant_p(__mask)) + \
> + BUILD_BUG_ON_ZERO(!__is_constexpr(__mask)) + \
> BUILD_BUG_ON_ZERO((__mask) == 0 || (__mask) > U32_MAX) + \
> BUILD_BUG_ON_ZERO(!IS_POWER_OF_2((__mask) + (1ULL << __bf_shf(__mask)))) + \
> - BUILD_BUG_ON_ZERO(__builtin_choose_expr(__builtin_constant_p(__val), (~((__mask) >> __bf_shf(__mask)) & (__val)), 0))))
> + BUILD_BUG_ON_ZERO(__builtin_choose_expr(__is_constexpr(__val), (~((__mask) >> __bf_shf(__mask)) & (__val)), 0))))
>
> /**
> * REG_FIELD_GET() - Extract a u32 bitfield value
--
Jani Nikula, Intel Open Source Graphics Center
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