Date: Thu, 18 Feb 2021 13:46:14 +0100 From: =?UTF-8?B?VMSzbA==?= Coosemans <tijl@FreeBSD.org> To: Alexander Richardson <arichardson@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: <20210218134614.376e4eaa@FreeBSD.org> In-Reply-To: <CA%2BZ_v8o%2BBX_xMe2bK4ntBzW-FgEPgksY0y%2B87bwfqA1zTaVpHA@mail.gmail.com> References: <202102031604.113G4SQq019037@gitrepo.freebsd.org> <CAHgpiFyAcK5%2BZh0bdOdMw12Yj96RHo08g9qaNh5C50k1jwP1bA@mail.gmail.com> <CA%2BZ_v8o%2BBX_xMe2bK4ntBzW-FgEPgksY0y%2B87bwfqA1zTaVpHA@mail.gmail.com>
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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.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@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/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. > > 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))).
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