Re: [PATCH] kcsan: Add option to allow watcher interruptions

[peterz]

On Sat, Jul 25, 2020 at 10:10:13PM +0200, peterz@xxxxxxxxxxxxx wrote:
> On Sat, Jul 25, 2020 at 12:39:09PM -0700, Paul E. McKenney wrote:

> > This gets me the following for __rcu_read_lock():
> > 
> > 00000000000000e0 <__rcu_read_lock>:
> >       e0:	48 8b 14 25 00 00 00 	mov    0x0,%rdx
> >       e7:	00 
> >       e8:	8b 82 e0 02 00 00    	mov    0x2e0(%rdx),%eax
> >       ee:	83 c0 01             	add    $0x1,%eax
> >       f1:	89 82 e0 02 00 00    	mov    %eax,0x2e0(%rdx)
> >       f7:	c3                   	retq   
> >       f8:	0f 1f 84 00 00 00 00 	nopl   0x0(%rax,%rax,1)
> >       ff:	00 
> > 
> > One might hope for a dec instruction, but this isn't bad.  We do lose
> > a few instructions compared to the C-language case due to differences
> > in address calculation:
> > 
> > 00000000000000e0 <__rcu_read_lock>:
> >       e0:	48 8b 04 25 00 00 00 	mov    0x0,%rax
> >       e7:	00 
> >       e8:	83 80 e0 02 00 00 01 	addl   $0x1,0x2e0(%rax)
> >       ef:	c3                   	retq   
> 
> Shees, that's daft... I think this is one of the cases where GCC is
> perhaps overly cautious when presented with 'volatile'.
> 
> It has a history of generating excessively crap code around volatile,
> and while it has improved somewhat, this seems to show there's still
> room for improvement...
> 
> I suppose this is the point where we go bug a friendly compiler person.

Having had a play with godbolt.org, it seems clang isn't affected by
this particular flavour of crazy, but GCC does indeed refuse to fuse the
address calculation and the addition.