On Mon, Nov 23, 2015 at 7:22 PM, Patrick Palka <patr...@parcs.ath.cx> wrote: > On Mon, Nov 23, 2015 at 3:53 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >> On Mon, Nov 23, 2015 at 1:57 AM, Richard Biener >> <richard.guent...@gmail.com> wrote: >>> On Sat, Nov 21, 2015 at 12:46 AM, H.J. Lu <hjl.to...@gmail.com> wrote: >>>> On Fri, Nov 20, 2015 at 2:17 PM, Jason Merrill <ja...@redhat.com> wrote: >>>>> On 11/20/2015 01:52 PM, H.J. Lu wrote: >>>>>> >>>>>> On Tue, Nov 17, 2015 at 4:22 AM, Richard Biener >>>>>> <richard.guent...@gmail.com> wrote: >>>>>>> >>>>>>> On Tue, Nov 17, 2015 at 12:01 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >>>>>>>> >>>>>>>> Empty record should be returned and passed the same way in C and C++. >>>>>>>> This patch adds LANG_HOOKS_EMPTY_RECORD_P for C++ empty class, which >>>>>>>> defaults to return false. For C++, LANG_HOOKS_EMPTY_RECORD_P is >>>>>>>> defined >>>>>>>> to is_really_empty_class, which returns true for C++ empty classes. >>>>>>>> For >>>>>>>> LTO, we stream out a bit to indicate if a record is empty and we store >>>>>>>> it in TYPE_LANG_FLAG_0 when streaming in. get_ref_base_and_extent is >>>>>>>> changed to set bitsize to 0 for empty records. Middle-end and x86 >>>>>>>> backend are updated to ignore empty records for parameter passing and >>>>>>>> function value return. Other targets may need similar changes. >>>>>>> >>>>>>> >>>>>>> Please avoid a new langhook for this and instead claim a bit in >>>>>>> tree_type_common >>>>>>> like for example restrict_flag (double-check it is unused for >>>>>>> non-pointers). >>>>>> >>>>>> >>>>>> There is no bit in tree_type_common I can overload. restrict_flag is >>>>>> checked for non-pointers to issue an error when it is used on >>>>>> non-pointers: >>>>>> >>>>>> >>>>>> /export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/template/qualttp20.C:19:38: >>>>>> error: ‘__restrict__’ qualifiers cannot be applied to ‘AS::L’ >>>>>> typedef typename T::L __restrict__ r;// { dg-error "'__restrict__' >>>>>> qualifiers cannot" "" } >>>>> >>>>> >>>>> The C++ front end only needs to check TYPE_RESTRICT for this purpose on >>>>> front-end-specific type codes like TEMPLATE_TYPE_PARM; cp_type_quals could >>>>> handle that specifically if you change TYPE_RESTRICT to only apply to >>>>> pointers. >>>>> >>>> >>>> restrict_flag is also checked in this case: >>>> >>>> [hjl@gnu-6 gcc]$ cat x.i >>>> struct dummy { }; >>>> >>>> struct dummy >>>> foo (struct dummy __restrict__ i) >>>> { >>>> return i; >>>> } >>>> [hjl@gnu-6 gcc]$ gcc -S x.i -Wall >>>> x.i:4:13: error: invalid use of ‘restrict’ >>>> foo (struct dummy __restrict__ i) >>>> ^ >>>> x.i:4:13: error: invalid use of ‘restrict’ >>>> [hjl@gnu-6 gcc]$ >>>> >>>> restrict_flag can't also be used to indicate `i' is an empty record. >>> >>> I'm sure this error can be done during parsing w/o relying on TYPE_RESTRICT. >>> >>> But well, use any other free bit (but do not enlarge >>> tree_type_common). Eventually >>> you can free up a bit by putting sth into type_lang_specific currently >>> using bits >>> in tree_type_common. >> >> There are no bits in tree_type_common I can move. Instead, >> this patch overloads side_effects_flag in tree_base. Tested on >> Linux/x86-64. OK for trunk? >> > > Hi, > > Coincidentally a few months ago I was experimenting with making > empty-struct function arguments zero-cost (and thus making them behave > the same way as in GNU C). My approach (patch attached) was to assign > empty-struct arguments to a virtual register (instead of on the stack > or to a hard register) during RTL call expansion. These > virtual-register assignments would then be trivially DCE'd later. > This approach seemed to work surprisingly well with minimal code > changes. I wonder what > your thoughts are on this approach..
I don't think it works for C++ class. empty_record_or_union_type_p missed: for (binfo = TYPE_BINFO (type), i = 0; BINFO_BASE_ITERATE (binfo, i, base_binfo); ++i) if (!is_really_empty_class (BINFO_TYPE (base_binfo))) return false; Does it work with variable argument list? Did you run GCC testsuite for both i686 and x86-64? -- H.J.