Re: [PATCH] locking/local_lock: Reduce local_[un]lock_nested_bh() overhead

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-03-09 15:52:34 [+0100], Eric Dumazet wrote:
> > +void ericeric(void);
> > +void ericeric(void)
> > +{
> > +       raw_cpu_read_long(this_cpu_off);
> > +       raw_cpu_read_long(this_cpu_off);
> > +}
> >
> > I am guessing __raw_cpu_read() is forcing the asm ?
> 
> Might be a clang issue. Oh well.

So the difference is that with gcc we have USE_X86_SEG_SUPPORT and with
llvm we don't. This leads to two asm statements with LLVM of which only
one is eliminated. This optimisation origins in commit ca4256348660c
("x86/percpu: Use C for percpu read/write accessors").

__seg_fs and __seg_gs is supported by LLVM but enabling it leads to tons
warnings and aborts later.

Is there something missing in LLVM? The generated code for
raw_cpu_read_long(this_cpu_off) looks fine.

Sebastian