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Date: Mon, 25 Jan 2016 16:00:05 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Bits deduplication: current situation
On Mon, Jan 25, 2016 at 11:22:13AM -0800, Dan Gohman wrote:
> Concerning stdint.h, there are a few details beyond just 32-bit vs 64-bit.
> For example, int64_t can be either "long" or "long long" on an LP64 target.
> The difference usually doesn't matter, but there are things which end up
> noticing, like C++ name mangling and C format-string checking.
I'm pretty sure int64_t is long on all LP64 targets we support. Are
there others that differ?
> GCC >= 4.5 and clang predefine macros providing almost everything stdint.h
> (and inttypes.h) needs. For example, see the attached file. Would you be
> interested in a patch which refactors stdint.h to use this approach by
> default, with a mechanism to support older compilers if needed?
No, the intent is that the public headers be compatible with basically
all compilers honoring the ABI, not just gcc and compatible ones.
There are a very small number of things (documented in the outdated
manual) that need extensions in the public headers, mainly _Complex_I,
tgmath.h, and stdarg.h, and in those cases we use the conventions that
gcc and other existing compilers have created.
Also it's musl's intent to be explicit with definitions, and this is
actually helpful with the C++ types issue. IMO it's much better to get
an error that a new compiler you're trying has the ABI wrong than to
silently use different types.
Rich
P.S. Could you follw up replies below the quoted text (if any) when
replying to the list rather than top-posting?
> On Sun, Jan 24, 2016 at 7:59 PM, Rich Felker <dalias@...c.org> wrote:
>
> > I'm about to try starting the bits deduplication, but before getting
> > started, I took a quick survey of the current bits headers we have:
> >
> >
> > endian.h: We could have generic ones for little and big, but each arch
> > that has subarchs with both endians needs its own custom version that
> > tests the psABI-defined macro.
> >
> > errno.h: Almost all archs can share a generic errno.h. Those that
> > don't might be able to share sub subset (thus benefiting from a more
> > elaborate bits-header-gen system) but only a couple ugly archs are
> > affected anyway.
> >
> > fcntl.h: Not sure how much these differ or how much they could share.
> > Almost all archs' versions are unique now, but some may only have
> > cosmetic differences.
> >
> > fenv.h: We can have a generic softfloat/no-fenv version, but each arch
> > with hard float basically needs its own version.
> >
> > float.h: Only 3 generic versions should need to exist: ld64, ld80, and
> > ld128(ieeequad).
> >
> > io.h: Most archs can use a generic empty file.
> >
> > ioctl.h: Varies highly but it may be possible to have generic versions
> > (perhaps one 32-bit and one 64-bit) for the clean archs to share.
> >
> > ipc.h: Lots of trivial variations to account for kernel bugs in
> > type/padding/etc. Not sure if they can be unified.
> >
> > limits.h: Varies by page size and 32/64-bit. Not sure if it makes
> > sense to have generic versions; the logic to pick which one would be
> > as large as the file. It would be nice to get the #ifdefs out of it
> > though.
> >
> > mman.h: Seems to vary but differences may be mostly cosmetic; not
> > sure.
> >
> > msg.h: Same deal as ipc.h.
> >
> > poll.h: Empty except for mips; generic definitions are in top-level
> > poll.h now. With bits dedup we could move them to a generic bits file
> > so that top-level doesn't have a nasty #ifndef.
> >
> > posix.h: Only 2 versions: ILP32 and LP64. They can be generic.
> >
> > reg.h: Completely arch-specific except in the case of multiple logical
> > archs for the same ISA (x32).
> >
> > resource.h: Same deal as poll.h.
> >
> > sem.h: Same deal as ipc.h.
> >
> > setjmp.h: Arch-specific, same as reg.h.
> >
> > shm.h: Same deal as ipc.h.
> >
> > signal.h: Arch-specific, and currently omits siginfo_t which is
> > gratuitously different on mips (and thus broken). Moving siginfo_t
> > into it would add A LOT of duplication and maintenance burden unless
> > we have an elaborate bits generation system that can piece these
> > headers together from multiple parts so the siginfo_t part can be
> > shared by all but mips.
> >
> > socket.h: The main difference is that workarounds for bogus kernel
> > definitions of msghdr and cmsghdr are needed on 64-bit archs. A few
> > archs also have their own definitions of some constants which override
> > the top-level file's.
> >
> > stat.h: It varies a lot on current archs, but in principle there's a
> > generic stat/stat64 that should be used for all new archs on the
> > kernel side, so perhaps we could have a generic one for that.
> >
> > statfs.h: Mostly generic, but mips and x32 have quirks.
> >
> > stdarg.h: Not even used except with ancient/broken compilers. Same on
> > all archs but i386 where the invalid legacy defs are provided.
> > Probably should be dropped entirely.
> >
> > stdint.h: Purely a matter of 32 vs 64 bit, otherwise totally generic.
> >
> > syscall.h: Arch-specific except new kernel archs should use the
> > generic one, which we can do as a generic.
> >
> > termios.h: Generic except for wacky archs (mips and powerpc).
> >
> > user.h: Highly arch-specific.
> >
> >
> > The good news is that there are not a lot of places where there's
> > value in doing anything elaborate with the deduplication. Just having
> > a fixed ordered list of include dirs to search while building, and
> > installation rules to pick the first matching one and install it in
> > $(includedir)/bits, would probably work.
> >
> > It's possible that we could eliminate some bits headers entirely by
> > having features.h (via a new bits/features.h) expose some parameters
> > like endianness, ILP32-vs-LP64, etc. which the top-level headers could
> > then use to define things in a non-arch-specific way. I'm not sure
> > whether I like doing that though. It simplifies porting and header
> > maintenance work, but at the cost of some explicitness whereby you can
> > just open the header file (or the bits header file) and see how
> > something is defined right away.
> >
> > A possible compromise is to highly abstract these things at the musl
> > source level, but generate flat bits files to install, or even flatten
> > the headers completely to remove bits so that all definitions are
> > inline and explicit in the top-level headers.
> >
> > Ideas/requests/preferences/etc.?
> >
> > Rich
> >
> typedef __INT8_TYPE__ int8_t;
> typedef __INT16_TYPE__ int16_t;
> typedef __INT32_TYPE__ int32_t;
> typedef __INT64_TYPE__ int64_t;
> typedef __UINT8_TYPE__ uint8_t;
> typedef __UINT16_TYPE__ uint16_t;
> typedef __UINT32_TYPE__ uint32_t;
> typedef __UINT64_TYPE__ uint64_t;
>
> typedef __INT_FAST8_TYPE__ int_fast8_t;
> typedef __INT_FAST16_TYPE__ int_fast16_t;
> typedef __INT_FAST32_TYPE__ int_fast32_t;
> typedef __INT_FAST64_TYPE__ int_fast64_t;
> typedef __UINT_FAST8_TYPE__ uint_fast8_t;
> typedef __UINT_FAST16_TYPE__ uint_fast16_t;
> typedef __UINT_FAST32_TYPE__ uint_fast32_t;
> typedef __UINT_FAST64_TYPE__ uint_fast64_t;
>
> #define INT_FAST8_MIN (-__UINT_FAST8_MAX__ - 1)
> #define INT_FAST16_MIN (-__UINT_FAST16_MAX__ - 1)
> #define INT_FAST32_MIN (-__UINT_FAST32_MAX__ - 1)
> #define INT_FAST64_MIN (-__UINT_FAST64_MAX__ - 1)
>
> #define INT_FAST8_MAX __UINT_FAST8_MAX__
> #define INT_FAST16_MAX __UINT_FAST16_MAX__
> #define INT_FAST32_MAX __UINT_FAST32_MAX__
> #define INT_FAST64_MAX __UINT_FAST64_MAX__
>
> #define UINT_FAST8_MAX __UINT_FAST8_MAX
> #define UINT_FAST16_MAX __UINT_FAST16_MAX
> #define UINT_FAST32_MAX __UINT_FAST32_MAX
> #define UINT_FAST64_MAX __UINT_FAST64_MAX
>
> typedef __INT_LEAST8_TYPE__ int_least8_t;
> typedef __INT_LEAST16_TYPE__ int_least16_t;
> typedef __INT_LEAST32_TYPE__ int_least32_t;
> typedef __INT_LEAST64_TYPE__ int_least64_t;
> typedef __UINT_LEAST8_TYPE__ uint_least8_t;
> typedef __UINT_LEAST16_TYPE__ uint_least16_t;
> typedef __UINT_LEAST32_TYPE__ uint_least32_t;
> typedef __UINT_LEAST64_TYPE__ uint_least64_t;
>
> #define INT_LEAST8_MIN (-__UINT_LEAST8_MAX__ - 1)
> #define INT_LEAST16_MIN (-__UINT_LEAST16_MAX__ - 1)
> #define INT_LEAST32_MIN (-__UINT_LEAST32_MAX__ - 1)
> #define INT_LEAST64_MIN (-__UINT_LEAST64_MAX__ - 1)
>
> #define INT_LEAST8_MAX __UINT_LEAST8_MAX__
> #define INT_LEAST16_MAX __UINT_LEAST16_MAX__
> #define INT_LEAST32_MAX __UINT_LEAST32_MAX__
> #define INT_LEAST64_MAX __UINT_LEAST64_MAX__
>
> #define UINT_LEAST8_MAX __UINT_LEAST8_MAX
> #define UINT_LEAST16_MAX __UINT_LEAST16_MAX
> #define UINT_LEAST32_MAX __UINT_LEAST32_MAX
> #define UINT_LEAST64_MAX __UINT_LEAST64_MAX
>
> #define INTPTR_MIN (-__INTPTR_MAX__ - 1)
> #define INTPTR_MAX __INTPTR_MAX__
> #define UINTPTR_MAX __UINTPTR_MAX__
> #define PTRDIFF_MIN (-__PTRDIFF_MAX__ - 1)
> #define PTRDIFF_MAX __PTRDIFF_MAX__
> #define SIZE_MAX __SIZE_MAX__
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