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

[James Clark]

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@xxxxxxx/
>
> and
>
> https://lore.kernel.org/linux-perf-users/20250320105235.3498106-1- 
> leo.yan@xxxxxxx/

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@xxxxxxxxxx>
> > ---
> >   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))