On Wed, Apr 11, 2012 at 2:43 AM, Richard Guenther <richard.guent...@gmail.com> wrote: > On Wed, Apr 11, 2012 at 4:24 AM, Lawrence Crowl <cr...@google.com> wrote: >> On 4/10/12, Jakub Jelinek <ja...@redhat.com> wrote: >>> That when stepping through code in the debugger you keep >>> enterring/exiting these one liner inlines, most of them really >>> should be at least by default considered just as normal statements >>> (e.g. glibc heavily uses artificial attribute for those, still >>> gdb doesn't hide those by default). >> >> You do want to step into those inline functions, except when you do. >> In the short term, we can make the debugger behave as though they did >> not exist. In the longer term, we really want debugging tools that >> help C++ programmers. One way to get there is to use C++ ourselves. > > Fix the debugger first please. > >>> > The above is just quickly cooked up examples. A carefully >>> > designed C++ based API can be self documenting and make the >>> > client code very readable. It is hard to believe that there is >>> > no room for improvement in GCC. >>> >>> Do you have examples? E.g. I haven't touched gold, because, >>> while it is a new C++ codebase, looks completely unreadable to >>> me, similarly libdw C++ stuff. A carefully designed C based API >>> can be self documenting and make the code very readable as well, >>> often more so. >> >> If you just look at any decently sized code base, it'll look pretty >> much unreadable. The question is how quickly can someone who learns >> the base vocabulary can produce reasonable modifications. >> >> There are many places where C++ can help substantially. For example: >> >> () The C++ postfix member function call syntax means that following >> a chain of attributes is a linear read of the expression. With C >> function call syntax, you need to read the expression inside out. > > It's a matter of what you are used to (consider LISP). > >> () C++ has both overloaded functions and member functions, so you can >> use the same verb to talk about several different kinds of objects. >> With C function names, we have to invent a new function name for >> each type. Such names are longer and burden both the author and >> the reader of the code. > > Agreed. Function overloading is one of the nice things that does not > automatically make the code-base look "partial C++". Likewise > operator overloading can make things like > > bit_offset = double_int_add (bit_offset, > tree_to_double_int > (DECL_FIELD_BIT_OFFSET (field))); > > be just > > bit_offset = bit_offset + DECL_FIELD_BIT_OFFSET (field); > > it still looks like C but with some C++ "magic". >
Function overloading is both bless and curse. It makes code look better, but may reduce debuggability. >> () Standard C++ idioms enable mashing program components with ease. >> The C++ standard library is based on mixing and matching algorithms >> and data structures, via the common idiom of iterators. > > Sort-of agreed. Though iterator-style (and more so functor style) was never > one of my favorite. > >> () The overloadable operator new means that memory can be >> _implicitly_ allocated in the right place. > > Implicit allocation is bad. In a compiler you want to _see_ where you > spend memory. overload operator new per class allows memory management easier -- many different allocation policies (e.g pool based) can be easily implemented. > >> () Constructors and destructors reduce the number of places in the >> code where you need to do explicit memory management. Without garbage >> collection, leaks are less frequent. With garbage collection, you >> have much less active garbage, and can run longer between collection >> runs. Indeed, a conservative collector would be sufficient. > > Time will tell. > >> () Constructors and destructors also neatly handle actions that >> must occur in pairs. The classic example is mutex lock and unlock. >> Within GCC, timevar operations need to happen in pairs. > > Agreed. > >> () Class hierarchies (even without virtual functions) can directly >> represent type relationships, which means that a debugger dump of >> a C++ type has little unnecessary information, as opposed to the >> present union of structs approach with GCC trees. > > In GCC trees only the "base" is a union, and it is so as implementation > detail. That gdb does not grok a 'tree' well is because gdb is stupid. > All the information is there. > >> () Class hierarchies also mean that programmers can distinguish >> in the pointer types that a function needs a decl parameter, >> without having to say 'all trees' versus 'a very specific tree'. >> The static type checking avoids run-time bugs. > > True. In a very limited set of cases. C++ is not powerful enough > to express pointer-to-everything-that-would-be-considered-a-gimple-val. > Maybe C++ is not the right choice after all? (I suppose C++ concepts > would have helped here? pointer-to-tree-that-fulfils-is_gimple_val ... > (though is_gimple_val is not be a static property). > >> I have written compilers in both C and C++. I much prefer the >> latter. > > Did you ever try to convert an existing large C codebase to C++? > I would not expect a very good result and rather start from scratch. > So I don't see that we ever arrive (or want to arrive) at a pure C++-style > GCC. Instead I expect we end up (and desire to end up) with GCC > compiled with a C++ compiler that uses C++ features to make the > existing style more readable and maintainable. I like your proposal (from my reading) about keeping core APIs in C, while the rest can be migrated (gradually). thanks, David > > Richard. > >> -- >> Lawrence Crowl
