Re: [PATCH][next] perf/x86: Avoid multiple -Wflex-array-member-not-at-end warnings

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 29, 2026 at 01:29:18PM -0600, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
> 
> Use the TRAILING_OVERLAP() helper to fix the following warnings:
> 
> 14 arch/x86/events/intel/../perf_event.h:326:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 6 arch/x86/events/amd/../perf_event.h:326:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 3 arch/x86/events/perf_event.h:326:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 1 arch/x86/xen/../events/perf_event.h:326:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 1 arch/x86/events/zhaoxin/../perf_event.h:326:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 1 ./arch/x86/include/generated/../../events/perf_event.h:326:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> This helper creates a union between a flexible-array member (FAM)
> and a set of members that would otherwise follow it. This overlays
> the trailing members onto the FAM while preserving the original
> memory layout.
> 
> Lastly, the static_assert() ensures the alignment between the FAM
> and struct perf_branch_entry lbr_entries[MAX_LBR_ENTRIES]; is not
> inadvertently changed, and it's intentionally placed inmediately
> after the related structure (that is, no blank line in between).
> 
> It's also worth mentioning that the entire Intel LBR bits block
> is moved just to avoid splitting it.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>  arch/x86/events/perf_event.h | 41 ++++++++++++++++++++----------------
>  1 file changed, 23 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
> index fad87d3c8b2c..9641b888cbee 100644
> --- a/arch/x86/events/perf_event.h
> +++ b/arch/x86/events/perf_event.h
> @@ -318,24 +318,6 @@ struct cpu_hw_events {
>  	/* Cached CFG_C values */
>  	u64			cfg_c_val[X86_PMC_IDX_MAX];
>  
> -	/*
> -	 * Intel LBR bits
> -	 */
> -	int				lbr_users;
> -	int				lbr_pebs_users;
> -	struct perf_branch_stack	lbr_stack;
> -	struct perf_branch_entry	lbr_entries[MAX_LBR_ENTRIES];
> -	u64				lbr_counters[MAX_LBR_ENTRIES]; /* branch stack extra */
> -	union {
> -		struct er_account		*lbr_sel;
> -		struct er_account		*lbr_ctl;
> -	};
> -	u64				br_sel;
> -	void				*last_task_ctx;
> -	int				last_log_id;
> -	int				lbr_select;
> -	void				*lbr_xsave;
> -
>  	/*
>  	 * Intel host/guest exclude bits
>  	 */
> @@ -384,7 +366,30 @@ struct cpu_hw_events {
>  	void				*kfree_on_online[X86_PERF_KFREE_MAX];
>  
>  	struct pmu			*pmu;
> +
> +	/*
> +	 * Intel LBR bits
> +	 */
> +	int				lbr_users;
> +	int				lbr_pebs_users;
> +	union {
> +		struct er_account		*lbr_sel;
> +		struct er_account		*lbr_ctl;
> +	};
> +	u64				br_sel;
> +	void				*last_task_ctx;
> +	int				last_log_id;
> +	int				lbr_select;
> +	void				*lbr_xsave;
> +
> +	/* Must be last as it ends in a flexible-array member. */
> +	TRAILING_OVERLAP(struct perf_branch_stack, lbr_stack, entries,
> +		struct perf_branch_entry	lbr_entries[MAX_LBR_ENTRIES];
> +		u64				lbr_counters[MAX_LBR_ENTRIES]; /* branch stack extra */
> +	);
>  };
> +static_assert(offsetof(struct cpu_hw_events, lbr_stack.entries) ==
> +	      offsetof(struct cpu_hw_events, lbr_entries));

This is horrible. That code was perfectly fine, and now you've made it
an unholy trainwreck just because compiler is stupid :-(

Also, we seem to be going around in circles, what was wrong with:

  https://lore.kernel.org/all/20240806113254.GG12673@noisy.programming.kicks-ass.net/