Re: + gcc-disable-warray-bounds-for-gcc-9.patch added to mm-hotfixes-unstable branch

[Yury Norov <yury.norov@gmail.com>]

On Fri, May 31, 2024 at 03:29:28PM +0200, Arnd Bergmann wrote:
>  "Alexander Lobakin" <aleksander.lobakin@intel.com>
> Subject: Re: + gcc-disable-warray-bounds-for-gcc-9.patch added to mm-hotfixes-unstable
>  branch
> Content-Type: text/plain
> Status: RO
> Content-Length: 3582
> Lines: 85
> 
> On Fri, May 31, 2024, at 03:18, Yury Norov wrote:
> > On Wed, May 29, 2024 at 04:39:45PM +0200, Arnd Bergmann wrote:
> >> On Fri, May 24, 2024, at 05:00, Andrew Morton wrote:
> >
> >> I think this is mixing up
> >> a couple of independent issues, and makes it harder to
> >> ever enable the warning again.
> >> 
> >> The bitmap.h code looks suspicious to me, and if gcc is
> >> unable to analyze this as a false positive, it probably
> >> also can't optimize it correctly,
> >
> > In the b44759705f7d Alexander says:
> >
> >   bloat-o-meter shows no difference in vmlinux and -2 bytes for
> >   gpio-pca953x.ko, which says the optimization didn't suffer due to
> >   that change. The converted helpers have the value width embedded
> >   and always compile-time constant and that helps a lot.
> >
> > Bloat-o-meter itself is not a measure of how effective the code is,
> > but it's a good hit that code generation before/after is at least
> > on par. Have you an evidence that the patch makes code generation
> > worse?
> 
> So far, it's only a suspicion based on the gcc warning:
> this is in the same category as some other warnings that
> depend on gcc analyzing code across function boundaries.
> Another example is -Wmaybe-uninitialized.
> 
> It's usually good at this, but if the code gets too
> complex it fails to track the state correctly, resulting
> both in missed optimizations and false-postiive warnings.
> 
> This often happens in combination with sanitizers, as they
> add more complexity and turn off some optimization steps
> that are required here.
>
> >> so it may be better
> >> to either not have this as an inline function at all,
> >> or find an implementation that gcc can optimize better.
> >
> > The functions look bulky but it boild to one or at max two words fetch
> > plus shifts. Inlining helps to generate better code, particularly in
> > the bitmap_get/set_value8 case because masks and offsets generation is
> > done at compile time.
> >
> > We had quite a few cycles back then... Alexander, can you please
> > share on code generation, particularly inline vs outline versions?
> 
> Right, maybe all we need here is marking it __always_inline
> then. Something I've seen before is functions that
> are meant to become trivial after inlining, but that
> (before inlining) appear to have too many statements for
> gcc to consider them candidates. It may then try to generate
> a specialized version of the function that optimizes for
> certain constant arguments, but is then confused about which
> arguments are constant or not.
> 
> Do you still see gcc-9 false-positive warnings with the
> trivial patch below?

KASAN was not enabled. I disabled all CONFIG_*SAN and CONFIG_KFENCE,
but the warning is still there. __always_inline doesn't improve as well.

Thanks,
Yury
 
> 
>      Arnd
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 8c4768c44a01..08df806df3d2 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -737,7 +737,7 @@ static inline void bitmap_from_u64(unsigned long *dst, u64 mask)
>   * @map memory region. For @nbits = 0 and @nbits > BITS_PER_LONG the return
>   * value is undefined.
>   */
> -static inline unsigned long bitmap_read(const unsigned long *map,
> +static __always_inline unsigned long bitmap_read(const unsigned long *map,
>                                         unsigned long start,
>                                         unsigned long nbits)
>  {
> @@ -772,7 +772,7 @@ static inline unsigned long bitmap_read(const unsigned long *map,
>   *
>   * For @nbits == 0 and @nbits > BITS_PER_LONG no writes are performed.
>   */
> -static inline void bitmap_write(unsigned