Date: Thu, 18 Feb 2021 14:21:06 +0000 From: Alexander Richardson <arichardson@freebsd.org> To: =?UTF-8?Q?T=C4=B3l_Coosemans?= <tijl@freebsd.org> Cc: Ryan Libby <rlibby@freebsd.org>, John Baldwin <jhb@freebsd.org>, src-committers <src-committers@freebsd.org>, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 8fa6abb6f4f6 - main - Expose clang's alignment builtins and use them for roundup2/rounddown2 Message-ID: <CA%2BZ_v8o4oQCFA9tGcs2Nhywq5xthJmnWXHLBhtJN0-x-ppFx9A@mail.gmail.com> In-Reply-To: <20210218134614.376e4eaa@FreeBSD.org> References: <202102031604.113G4SQq019037@gitrepo.freebsd.org> <CAHgpiFyAcK5%2BZh0bdOdMw12Yj96RHo08g9qaNh5C50k1jwP1bA@mail.gmail.com> <CA%2BZ_v8o%2BBX_xMe2bK4ntBzW-FgEPgksY0y%2B87bwfqA1zTaVpHA@mail.gmail.com> <20210218134614.376e4eaa@FreeBSD.org>
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On Thu, 18 Feb 2021 at 12:46, T=C4=B3l Coosemans <tijl@freebsd.org> wrote: > > On Wed, 17 Feb 2021 21:01:05 +0000 Alexander Richardson > <arichardson@freebsd.org> wrote: > > On Wed, 17 Feb 2021 at 20:46, Ryan Libby <rlibby@freebsd.org> wrote: > >> On Wed, Feb 3, 2021 at 8:04 AM Alex Richardson <arichardson@freebsd.or= g> wrote: > >>> The branch main has been updated by arichardson: > >>> > >>> URL: https://cgit.FreeBSD.org/src/commit/?id=3D8fa6abb6f4f64f4f23e292= 0e2aea7996566851a4 > >>> > >>> commit 8fa6abb6f4f64f4f23e2920e2aea7996566851a4 > >>> Author: Alex Richardson <arichardson@FreeBSD.org> > >>> AuthorDate: 2021-02-03 15:27:17 +0000 > >>> Commit: Alex Richardson <arichardson@FreeBSD.org> > >>> CommitDate: 2021-02-03 16:02:54 +0000 > >>> > >>> Expose clang's alignment builtins and use them for roundup2/round= down2 > >>> > >>> This makes roundup2/rounddown2 type- and const-preserving and all= ows > >>> 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 gener= ation. > >>> When using it with integer values there should be no change other= than > >>> the compiler checking that the alignment value is a valid power-o= f-two. > >>> > >>> I originally implemented these builtins for CHERI a few years ago= and > >>> they have been very useful for CheriBSD. However, they are also u= seful > >>> 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 alignme= nt > >>> of pointers and integers. These builtins can be used to avoid rel= ying > >>> on implementation-defined behavior of arithmetic on integers deri= ved > >>> from pointers. Additionally, these builtins retain type informati= on > >>> and, unlike bitwise arithmetic, they can perform semantic checkin= g 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=3D98641 > >>> > >>> 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 gen= eration. */ > >>> +/* 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)) =3D=3D 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 t= o > >> 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))=3D=3D0) > >>> > >>> /* 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 +=3D 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. > > > > 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. > > The comma operator might work here: typeof(((void)0,(x))). Thanks, I hadn't considered that. It looks like GCC is happy with that for both pointers and bitfields: https://godbolt.org/z/KvMEYs
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