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

Peter Zijlstra
2026-05-04T08:32:44Z

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: