Linux Virtualization - Jul 2020 - [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y

On Tue, Jun 30, 2020 at 7:39 PM Will Deacon <will at kernel.org>
wrote:
> +#define __READ_ONCE(x)                                                 \
> +({                                                                     \
> +       int atomic = 1;                                                 \
> +       union { __unqual_scalar_typeof(x) __val; char __c[1]; } __u;    \
> +       typeof(&(x)) __x = &(x);                                   
\
> +       switch (sizeof(x)) {                                            \
...
> +       atomic ? (typeof(x))__u.__val : (*(volatile typeof(x) *)__x);   \
> +})
This expands (x) nine times (five in __unqual_scala_typeof()), which can
lead to significant code bloat after preprocessing if something passes a
compound expression into READ_ONCE().
The compiler works it out eventually, but we've seen an actual slowdown
in compile speed from this recently, especially on clang.

I think if you move the

        typeof(&(x)) __x = &(x);

line first, all other instances can use typeof(*__x) instead of typeof(x)
and avoid this problem. Once we make gcc-4.9 the minimum version,
this could be further improved to

       __auto_type __x = &(x);

       Arnd

On Tue, Jun 30, 2020 at 09:25:03PM +0200, Arnd Bergmann
wrote:
> On Tue, Jun 30, 2020 at 7:39 PM Will Deacon <will at kernel.org>
wrote:
> > +#define __READ_ONCE(x)                                               
\
> > +({                                                                   
\
> > +       int atomic = 1;                                               
\
> > +       union { __unqual_scalar_typeof(x) __val; char __c[1]; } __u;  
\
> > +       typeof(&(x)) __x = &(x);                              
\
> > +       switch (sizeof(x)) {                                          
\
> ...
> > +       atomic ? (typeof(x))__u.__val : (*(volatile typeof(x) *)__x); 
\
> > +})
> 
> This expands (x) nine times (five in __unqual_scala_typeof()), which can
> lead to significant code bloat after preprocessing if something passes a
> compound expression into READ_ONCE().
> The compiler works it out eventually, but we've seen an actual slowdown
> in compile speed from this recently, especially on clang.
> 
> I think if you move the
> 
>         typeof(&(x)) __x = &(x);
> 
> line first, all other instances can use typeof(*__x) instead of typeof(x)
> and avoid this problem.
Cheers, I was only thinking about side-effects when I wrote this, but
bloating built time is very unpopular, so I'll go with your suggestion.
> Once we make gcc-4.9 the minimum version,
> this could be further improved to
> 
>        __auto_type __x = &(x);
Is anybody working on moving to 4.9? I've seen the mails from Linus
championing it, but I thought there was a RHEL in support that people
might care about?

Will

On Wed, Jul 1, 2020 at 12:19 PM Will Deacon <will at kernel.org>
wrote:
> On Tue, Jun 30, 2020 at 09:25:03PM +0200, Arnd Bergmann wrote:
> > On Tue, Jun 30, 2020 at 7:39 PM Will Deacon <will at kernel.org>
wrote:
> > Once we make gcc-4.9 the minimum version,
> > this could be further improved to
> >
> >        __auto_type __x = &(x);
>
> Is anybody working on moving to 4.9? I've seen the mails from Linus
> championing it, but I thought there was a RHEL in support that people
> might care about?
I don't think there was a serious discussion about it so far, and
we only just moved to gcc-4.8.

I think moving to gnu11 (gcc-4.9 or clang) instead of gnu99 has other
benefits as well, so we may well want to do it anyway when something
else comes up.

For __auto_type(), we could do it like

#if (clang or gcc-4.9+)
#define auto_typeof(x) __auto_type
#else
#define auto_typeof(x) typeof(x)
#endif

which could be used in a lot of macros.

     Arnd