Re: [PATCH] drm/xe/rtp: Fix build error with clang < 21 and non-const initializers

[Gustavo Sousa <gustavo.sousa@intel.com>]

Thomas Hellström <thomas.hellstrom@linux.intel.com> writes:

> Clang < 21 treats const-qualified compound literals at function scope as
> having static storage duration, which requires all initializer elements
> to be compile-time constants.  When xe_hw_engine.c initializes a local
> struct xe_rtp_table_sr using XE_RTP_TABLE_SR(), the compound literals in
> XE_RTP_TABLE_SR end up containing runtime values (e.g. blit_cctl_val
> derived from gt->mocs.uc_index), triggering:
>
>   xe_hw_engine.c:361: error: initializer element is not a compile-time constant
>   xe_hw_engine.c:416: error: initializer element is not a compile-time constant
>
> ARRAY_SIZE() cannot be used as a replacement because it expands through
> __must_be_array() -> __BUILD_BUG_ON_ZERO_MSG() -> _Static_assert inside
> sizeof(struct{}), which clang < 21 also rejects in the same context.
>
> Replace ARRAY_SIZE() with an open-coded sizeof(arr)/sizeof(elem) in
> XE_RTP_TABLE_SR and XE_RTP_TABLE to avoid both issues.
>
> Fixes: 5ff004fdc737 ("drm/xe/rtp: Add struct types for RTP tables")
> Cc: Matt Roper <matthew.d.roper@intel.com>
> Cc: Gustavo Sousa <gustavo.sousa@intel.com>
> Cc: Violet Monti <violet.monti@intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
> Cc: intel-xe@lists.freedesktop.org
> Reported-by: Mark Brown <broonie@kernel.org>
> Closes: https://lore.kernel.org/intel-xe/bfb0dee8-b243-47ba-a89d-71472b0d51c5@sirena.org.uk/
> Assisted-by: GitHub_Copilot:claude-sonnet-4.6
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>  drivers/gpu/drm/xe/xe_rtp.h | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/xe/xe_rtp.h b/drivers/gpu/drm/xe/xe_rtp.h
> index 4e3cfd69f922..2cc65053cd07 100644
> --- a/drivers/gpu/drm/xe/xe_rtp.h
> +++ b/drivers/gpu/drm/xe/xe_rtp.h
> @@ -461,14 +461,22 @@ struct xe_reg_sr;
>  		XE_RTP_PASTE_FOREACH(ACTION_, COMMA, (__VA_ARGS__))	\
>  	}
>  
> +/*
> + * Note: ARRAY_SIZE() cannot be used here because it expands through
> + * __must_be_array() -> __BUILD_BUG_ON_ZERO_MSG() -> _Static_assert inside
> + * sizeof(struct{}), which clang < 21 rejects when the compound literal
> + * contains non-compile-time-constant initializers.
> + */
>  #define XE_RTP_TABLE_SR(...) { \
>  	.entries = (const struct xe_rtp_entry_sr[]){__VA_ARGS__}, \
> -	.n_entries = ARRAY_SIZE(((const struct xe_rtp_entry_sr[]){__VA_ARGS__})), \
> +	.n_entries = sizeof((const struct xe_rtp_entry_sr[]){__VA_ARGS__}) / \
> +		sizeof(struct xe_rtp_entry_sr), \

What would happen if we used

  ARRAY_SIZE(((struct xe_rtp_entry_sr[]){__VA_ARGS__}))

instead?

Anyways, the patch looks correct to me as-is.  Hopefully we'll remember
to go back to use ARRAY_SIZE() when we bump the minimal clang version
required.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>

--
Gustavo Sousa

>  }
>  
>  #define XE_RTP_TABLE(...) { \
>  	.entries = (const struct xe_rtp_entry[]){__VA_ARGS__}, \
> -	.n_entries = ARRAY_SIZE(((const struct xe_rtp_entry[]){__VA_ARGS__})), \
> +	.n_entries = sizeof((const struct xe_rtp_entry[]){__VA_ARGS__}) / \
> +		sizeof(struct xe_rtp_entry), \
>  }
>  
>  #define XE_RTP_PROCESS_CTX_INITIALIZER(arg__) _Generic((arg__),							\
> -- 
> 2.54.0