Date: Sun, 6 Aug 2017 21:39:18 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Bruce Evans <brde@optusnet.com.au>, Hans Petter Selasky <hps@selasky.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r321920 - head/sys/sys Message-ID: <20170806213334.V1366@besplex.bde.org> In-Reply-To: <20170805074243.GX1700@kib.kiev.ua> References: <201708021014.v72AEHEk061037@repo.freebsd.org> <f7a4a90c-f1d8-b381-27fe-3cf76b574a29@selasky.org> <37abc595-c80e-a8da-04a8-815f42c634de@selasky.org> <20170802135455.GG1700@kib.kiev.ua> <20170803122015.Q1093@besplex.bde.org> <20170803075747.GJ1700@kib.kiev.ua> <20170803180419.R2314@besplex.bde.org> <20170803120729.GO1700@kib.kiev.ua> <20170805112924.W1055@besplex.bde.org> <20170805074243.GX1700@kib.kiev.ua>
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On Sat, 5 Aug 2017, Konstantin Belousov wrote: > On Sat, Aug 05, 2017 at 12:26:03PM +1000, Bruce Evans wrote: >>> +#define minor(x) ((int)(x)) >> >> Another nice simplification. Strictly, it should be (int)(dev_t)(x) since >> the pseudo-prototype says that the arg is converted to dev_t, but yesterday >> I couldn't see any differences even for exotic x and exotic arches. >> >> Today I can see some difference for exotic x and perverse implementations. >> E.g., x with extra bits in a large signed integer type to invoke >> implementation-defined behaviour, and a perverse implementation that >> truncates to 0 for direct conversion to int but does the right thing >> for indirect conversion). But we depend on the implementation doing >> the right thing for other casts to int. >> >> Also, if dev_t == uintptr_t, it is valid for the caller to keep dev_t's >> in void *'s internally but not to pass void * to minor() or major() >> according to the prototype. However, casting in the macros breaks the >> warning for this. I think pure macros should not cast their args or >> pretend to have prototypes, but require callers to pass args of supported >> types. The old dev_t macros were closer to this -- they have expressions >> like ((x) << 8) which will fail if x is not an integral type or give >> wrong results ix x has extra bits. > So you are arguing to keep the & 0xffffffff operation ? No, I don't want to go that deep. > I think this is not needed, since your note about cast being equivalent > holds for all supported architectures and I do not see anywhere a contract > for the operations to work on non-dev_t. The pseudo-prototypes look like they provide such a contract. They must be read sort of backwards as saying that it is the caller's responsibility to pass args of the type in the pseudo-prototype, like for a K&R function or a C90 function with prototype is in scope. > So are you fine with the posted patch to sys/types.h ? OK. Bruce
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