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

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

On 19/02/2026 10:36 am, James Clark wrote:
> 
> 
> 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/
> 
> 

Oh I see V1 was only the second patch, that's why I couldn't find it.

>>
>> 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))
>