On Sun, Feb 7, 2016 at 12:52 PM, H.J. Lu <hjl.to...@gmail.com> wrote: > On Sun, Feb 7, 2016 at 12:48 PM, Florian Weimer <f...@deneb.enyo.de> wrote: >> * H. J. Lu: >> >>>> I tested GCC 5.3.1 and Clang 3.5.0. >>>> >>>> GCC Clang >>>> s0 non-empty non-empty >>>> s1 non-empty empty >>>> s2 non-empty empty >>>> s3 empty empty >>>> s4 empty empty >>>> s5 non-empty empty >>>> >>>> I believe s3, s4, s5 are non-empty according to your rules. Why put >>>> both compilers in the wrong? That doesn't make sense to me. >>> >>> Please try testcases in >>> >>> https://llvm.org/bugs/show_bug.cgi?id=26337 >>> >>> with clang on both ia32 and x86-64. Clang isn't consistent between >>> ia32 and x86-64. >> >> Okay, with -m32, I get non-empty passing for s5 from Clang as well. >> But I still don't think it makes sense to change the ABI for s3 and >> s4, and even for s5, the Clang/x86_64 behavior is arguably more >> correct. > > Not really. For > > struct dummy0 { }; > struct dummy { struct dummy0 d[PAD_SIZE]; }; > > clang changes behavior when PAD_SIZE > 16 on x86-64 and PAD_SIZE > 1 > on ia32. > >>>> Jason already indicated he intends GCC to move towards more empty >>>> arguments, not fewer. >>>> >>>>>> How do existing C++ compilers implement empty array members (an >>>>>> extension)? Does the type of such members affect whether a class is a >>>>>> standard-layout class? >>>> >>>>> Are they "POD for the purpose of layout"? If yes, they are covered here. >>>> >>>> The C++ standard does not define this. >>> >>> GCC has >>> >>> * Nonzero means that this class type is not POD for the purpose of layout >>> (as defined in the ABI). This is different from the language's POD. */ >>> #define CLASSTYPE_NON_LAYOUT_POD_P(NODE) \ >>> >>> We can use this definition for ia32, x86-64 and IA MCU psABIs. >> >> It still has to be spelled out in non-GCC terms, IMHO. > > Sure. Do you care to propose a wording for "POD for the purpose of layout"? >
The standard-layout POD is well defined: https://en.wikipedia.org/wiki/C%2B%2B11#Modification_to_the_definition_of_plain_old_data Here is the updated proposal for Intel386, x86-64 and IA MCU psABIs: 1. "collection". A collection is a structure, union or C++ class. 2. "empty collection". An empty collection is: a. A collection without member. Or b. A collection with only empty collections. Or c. An array of empty collections. 3. "empty record". An empty record is Plain Old Data (POD) for the purpose of standard-layout and a. A collection without member. Or b. A collection with only empty collections. 4. No memory slot nor register should be used to pass or return an object of empty record. -- H.J.
