On Wed, 2009-09-23 at 18:39 -0500, Gabriel Dos Reis wrote: > On Wed, Sep 23, 2009 at 6:23 PM, Janis Johnson <janis...@us.ibm.com> wrote: > > On Wed, 2009-09-23 at 16:27 -0500, Gabriel Dos Reis wrote: > >> On Wed, Sep 23, 2009 at 4:11 PM, Janis Johnson <janis...@us.ibm.com> wrote: > >> > On Wed, 2009-09-23 at 10:29 +0200, Richard Guenther wrote: > >> >> On Wed, Sep 23, 2009 at 2:38 AM, Janis Johnson <janis...@us.ibm.com> > >> >> wrote: > >> >> > I've been implementing ISO/IEC TR 24733, "an extension for the > >> >> > programming language C++ to support decimal floating-point > >> >> > arithmetic", > >> >> > in GCC. It might be ready as an experimental feature for 4.5, but I > >> >> > would particularly like to get in the compiler changes that are needed > >> >> > for it. > >> >> > > >> >> > Most of the support for the TR is in new header files in libstdc++ > >> >> > that > >> >> > depend on compiler support for decimal float scalar types. Most of > >> >> > that > >> >> > compiler functionality was already available in G++ via mode > >> >> > attributes. > >> >> > I've made a couple of small fixes and have a couple more to submit, > >> >> > and > >> >> > when those are in I'll starting running dfp tests for C++ as well as > >> >> > C. > >> >> > The suitable tests have already been moved from gcc.dg to > >> >> > c-c++-common. > >> >> > > >> >> > In order to provide interoperability with C, people on the C++ ABI > >> >> > mailing list suggested that a C++ compiler should recognize the new > >> >> > decimal classes defined in the TR and pass arguments of those types > >> >> > the > >> >> > same as scalar decimal float types for a particular target. I had > >> >> > this > >> >> > working in an ugly way using a langhook, but that broke with LTO. I'm > >> >> > looking for the right places to record that an argument or return > >> >> > value > >> >> > should be passed as if it were a different type, but could use some > >> >> > advice about that. > >> >> > >> >> How do we (do we?) handle std::complex<> there? My first shot would > >> >> be to make sure the aggregate type has the proper mode, but I guess > >> >> most target ABIs would already pass them in registers, no? > >> > > >> > std::complex<> is not interoperable with GCC's complex extension, which > >> > is generally viewed as "unfortunate". > >> > >> Could you expand on why std::complex<> is not interoperable with GCC's > >> complex extension. The reason is that I would like to know better where > >> the incompatibilities come from -- I've tried to remove any. > > > > I was just repeating what I had heard from C++ experts. On > > powerpc-linux they are currently passed and mangled differently. > > I've been careful not to define a copy constructor or a destructor > for the specializations of std::complex<T> so that they get treated as PODs, > with the hope that the compiler will do the right thing. At least on > my x86-64 box > running openSUSE, I don't see a difference. I've also left the > copy-n-assignment operator at the discretion of the compiler > > // The compiler knows how to do this efficiently > // complex& operator=(const complex&); > > So, if there is any difference on powerpc-*-linux, then that should be blamed > on > poor ABI choice than anything else intrinsic to std::complex (or C++). > Where possible, we should look into how to fix that. > > In many ways, it is assumed that std::complex<T> is isomorphic to the > GNU extension.
The PowerPC 32-bit ELF ABI says that a struct is passed as a pointer to an object or a copy of the object. Classes are treated the same as classes. Does the C++ ABI have rules about classes like std::complex that would cause them to be treated differently? Janis
