Re: [PATCH v2 1/2] perf bench: Fix initialization of union

[James Clark <james.clark@linaro.org>]

On 19/02/2026 12:44 am, Namhyung Kim wrote:
> Recent compilers don't initialize all members (or the largest member) in
> an union.  Instead it seems to set the first member only.  So some perf
> bench output shows invalid numbers like below on my system with GCC 15.

I couldn't find V1 of this so I wasn't sure if it was discussed already, 
but wasn't the plan to fix this with a compiler option:

https://lore.kernel.org/linux-perf-users/20250319110454.3230687-1-leo.yan@arm.com/

and

https://lore.kernel.org/linux-perf-users/20250320105235.3498106-1-leo.yan@arm.com/


> 
> Before:
>    $ perf bench mem mmap
>    # Running 'mem/mmap' benchmark:
>    # function 'demand' (Demand loaded mmap())
>    # Copying 1MB bytes ...
> 
>           0.011127 bytes/sec
>    # function 'populate' (Eagerly populated mmap())
>    # Copying 1MB bytes ...
> 
>           0.011127 bytes/sec
> 
> After:
>    $ perf bench mem mmap
>    # Running 'mem/mmap' benchmark:
>    # function 'demand' (Demand loaded mmap())
>    # Copying 1MB bytes ...
> 
>           2.209417 GB/sec
>    # function 'populate' (Eagerly populated mmap())
>    # Copying 1MB bytes ...
> 
>           7.875504 GB/sec
> 
> Because the first member of bench_clock is u64 ('cycles'), the default
> struct timeval wasn't fully initialized.  Let's use memset to reset the
> whole memory.
> 
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>   tools/perf/bench/mem-functions.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/bench/mem-functions.c b/tools/perf/bench/mem-functions.c
> index 2908a3a796c932d0..676c8d18f4c2e259 100644
> --- a/tools/perf/bench/mem-functions.c
> +++ b/tools/perf/bench/mem-functions.c
> @@ -193,9 +193,10 @@ static void __bench_mem_function(struct bench_mem_info *info, struct bench_param
>   {
>   	const struct function *r = &info->functions[r_idx];
>   	double result_bps = 0.0;
> -	union bench_clock rt = { 0 };
> +	union bench_clock rt;
>   	void *src = NULL, *dst = NULL;
>   
> +	memset(&rt, 0, sizeof(rt));
>   	printf("# function '%s' (%s)\n", r->name, r->desc);
>   
>   	if (r->fn.init && r->fn.init(info, p, &src, &dst))