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Message-ID: <20140504050213.GA20625@brightrain.aerifal.cx> Date: Sun, 4 May 2014 01:02:13 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH] add definition of max_align_t to stddef.h On Sun, May 04, 2014 at 04:36:03AM +0200, Paweł Dziepak wrote: > 2014-04-30 23:42 GMT+02:00 Szabolcs Nagy <nsz@...t70.net>: > > * Pawel Dziepak <pdziepak@...rnos.org> [2014-04-30 22:23:01 +0200]: > >> > >> +TYPEDEF union { long double ld; long long ll; } max_align_t; > > > > this is wrong > > > > - ld and ll identifiers are not reserved for the implementation > > (you could name them _ld, _ll or __ld, __ll etc) > > I will fix that. However, I must admit I don't see why members of the > union (or struct) have to use identifiers reserved for the > implementation. It's not like they can conflict with anything, isn't > it? #define ll 0 #include <stddef.h> > > and see previous max_align_t discussion > > http://www.openwall.com/lists/musl/2014/04/28/8 > > > > - compiler implementations are non-conforming on some platforms > > (_Alignof returns inconsistent results for the same object type so > > reasoning about alignments is problematic, there are exceptions > > where this is allowed in c++11 but not in c11) > > > > - max_align_t is part of the abi and your solution is incompatible > > with gcc and clang (your definition gives 4 byte _Alignof(max_align_t) > > on i386 instead of 8) > > The behavior of _Alignof on x86 is indeed quite surprising. I actually It's also wrong. The correct alignment for max_align_t on i386 is 4, not 8. It's a bug that GCC ever returns 8 for alignof on i386. We really need to file a bug against GCC and explain this clearly, because I have a feeling they're going to be opposed to fixing it... > don't see why 8 is the right value and 4 isn't - System V ABI for x86 > doesn't mention any type with alignment 8. Anyway, I agree that it You're completely right; GCC is wrong. > would be a good thing to mach the definition gcc and clang use, i.e. > something like that: > > union max_align_t { > alignas(long long) long long _ll; > alignas(long double) long double _ld; > }; This should not give results different from omitting the "alignas". The only reason it does give different results is a bug in GCC, so we should not be copying this confusing mess that's a no-op for a correct compiler. (Applying alignas(T) to type T is always a no-op.) Rich
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