git: 8fa6abb6f4f6 - main - Expose clang's alignment builtins and use them for roundup2/rounddown2

[Alexander Richardson]

On Wed, 17 Feb 2021 at 20:46, Ryan Libby <rlibby at freebsd.org> wrote:
>
> On Wed, Feb 3, 2021 at 8:04 AM Alex Richardson <arichardson at freebsd.org> wrote:
> >
> > The branch main has been updated by arichardson:
> >
> > URL: https://cgit.FreeBSD.org/src/commit/?id=8fa6abb6f4f64f4f23e2920e2aea7996566851a4
> >
> > commit 8fa6abb6f4f64f4f23e2920e2aea7996566851a4
> > Author:     Alex Richardson <arichardson at FreeBSD.org>
> > AuthorDate: 2021-02-03 15:27:17 +0000
> > Commit:     Alex Richardson <arichardson at FreeBSD.org>
> > CommitDate: 2021-02-03 16:02:54 +0000
> >
> >     Expose clang's alignment builtins and use them for roundup2/rounddown2
> >
> >     This makes roundup2/rounddown2 type- and const-preserving and allows
> >     using it on pointer types without casting to uintptr_t first. Not
> >     performing pointer-to-integer conversions also helps the compiler's
> >     optimization passes and can therefore result in better code generation.
> >     When using it with integer values there should be no change other than
> >     the compiler checking that the alignment value is a valid power-of-two.
> >
> >     I originally implemented these builtins for CHERI a few years ago and
> >     they have been very useful for CheriBSD. However, they are also useful
> >     for non-CHERI code so I was able to upstream them for Clang 10.0.
> >
> >     Rationale from the clang documentation:
> >     Clang provides builtins to support checking and adjusting alignment
> >     of pointers and integers. These builtins can be used to avoid relying
> >     on implementation-defined behavior of arithmetic on integers derived
> >     from pointers. Additionally, these builtins retain type information
> >     and, unlike bitwise arithmetic, they can perform semantic checking on
> >     the alignment value.
> >
> >     There is also a feature request for GCC, so GCC may also support it in
> >     the future: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98641
> >
> >     Reviewed By:    brooks, jhb, imp
> >     Differential Revision: https://reviews.freebsd.org/D28332
> > ---
> >  sys/sys/cdefs.h | 19 +++++++++++++++++++
> >  sys/sys/param.h |  4 ++--
> >  2 files changed, 21 insertions(+), 2 deletions(-)
> >
> > diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
> > index 75bedd4b8128..72ef942084f2 100644
> > --- a/sys/sys/cdefs.h
> > +++ b/sys/sys/cdefs.h
> > @@ -884,4 +884,23 @@
> >  #define        __guarded_by(x)         __lock_annotate(guarded_by(x))
> >  #define        __pt_guarded_by(x)      __lock_annotate(pt_guarded_by(x))
> >
> > +/* Alignment builtins for better type checking and improved code generation. */
> > +/* Provide fallback versions for other compilers (GCC/Clang < 10): */
> > +#if !__has_builtin(__builtin_is_aligned)
> > +#define __builtin_is_aligned(x, align) \
> > +       (((__uintptr_t)x & ((align) - 1)) == 0)
> > +#endif
> > +#if !__has_builtin(__builtin_align_up)
> > +#define __builtin_align_up(x, align)   \
> > +       ((__typeof__(x))(((__uintptr_t)(x)+((align)-1))&(~((align)-1))))
> > +#endif
> > +#if !__has_builtin(__builtin_align_down)
> > +#define __builtin_align_down(x, align) \
> > +       ((__typeof__(x))((x)&(~((align)-1))))
> > +#endif
> > +
> > +#define __align_up(x, y) __builtin_align_up(x, y)
> > +#define __align_down(x, y) __builtin_align_down(x, y)
> > +#define __is_aligned(x, y) __builtin_is_aligned(x, y)
> > +
>
> Since these are only valid for powers of 2, I think it would be good to
> indicate that in the names (__align_up2() etc).
>
> >  #endif /* !_SYS_CDEFS_H_ */
> > diff --git a/sys/sys/param.h b/sys/sys/param.h
> > index 079357a19d47..d6f1eb21dcd2 100644
> > --- a/sys/sys/param.h
> > +++ b/sys/sys/param.h
> > @@ -305,9 +305,9 @@
> >  #endif
> >  #define        nitems(x)       (sizeof((x)) / sizeof((x)[0]))
> >  #define        rounddown(x, y) (((x)/(y))*(y))
> > -#define        rounddown2(x, y) ((x)&(~((y)-1)))          /* if y is power of two */
> > +#define        rounddown2(x, y) __align_down(x, y) /* if y is power of two */
> >  #define        roundup(x, y)   ((((x)+((y)-1))/(y))*(y))  /* to any y */
> > -#define        roundup2(x, y)  (((x)+((y)-1))&(~((y)-1))) /* if y is powers of two */
> > +#define        roundup2(x, y)  __align_up(x, y) /* if y is powers of two */
> >  #define powerof2(x)    ((((x)-1)&(x))==0)
> >
> >  /* Macros for min/max. */
>
> This broke the gcc build:
> https://ci.freebsd.org/job/FreeBSD-main-amd64-gcc6_build/3200/console
> 00:40:30 --- all_subdir_firewire ---
> 00:40:30 In file included from /workspace/src/sys/sys/types.h:43:0,
> 00:40:30                  from /workspace/src/sys/sys/param.h:99,
> 00:40:30                  from /workspace/src/sys/dev/firewire/fwohci.c:40:
> 00:40:30 /workspace/src/sys/dev/firewire/fwohci.c: In function
> 'fwohci_get_plen':
> 00:40:30 /workspace/src/sys/dev/firewire/fwohci.c:2699:17: error:
> 'typeof' applied to a bit-field
> 00:40:30    r += roundup2(fp->mode.wreqb.len, sizeof(uint32_t));
>
> We could:
>  - Drop the cast for the fallback.
>  - Cast with (__typeof__(+(x))) which unfortunately promotes e.g. char
>    to int but otherwise I think behaves okay, and the promotion was
>    previously happening for roundup2/rounddown2 anyway.
>  - Punt the casting burden to callers of roundup2/rounddown2.
>
> Ryan

Hi Ryan,

https://reviews.freebsd.org/D28599 fixes this specific issue. One
reason for the typeof() is to allow using it with pointer types.
Currently there are no such uses, but we have some in CheriBSD.
Unfortunately +(x) would break that. (x)+0 should work for most cases,
but does break void*.
I believe the current approach only breaks with bitfields (which is
hopefully rare), so I think adding casts for GCC when roundup2() in
those cases might be the simplest solution.

Thanks,
Alex