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diff --git a/gcc-4.8.1/libstdc++-v3/doc/html/manual/source_design_notes.html b/gcc-4.8.1/libstdc++-v3/doc/html/manual/source_design_notes.html deleted file mode 100644 index 0c36b9820..000000000 --- a/gcc-4.8.1/libstdc++-v3/doc/html/manual/source_design_notes.html +++ /dev/null @@ -1,862 +0,0 @@ -<?xml version="1.0" encoding="UTF-8" standalone="no"?> -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Design Notes</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.77.1" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="appendix_contributing.html" title="Appendix A. Contributing" /><link rel="prev" href="source_code_style.html" title="Coding Style" /><link rel="next" href="appendix_porting.html" title="Appendix B. Porting and Maintenance" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Design Notes</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="source_code_style.html">Prev</a> </td><th width="60%" align="center">Appendix A. - Contributing - -</th><td width="20%" align="right"> <a accesskey="n" href="appendix_porting.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="contrib.design_notes"></a>Design Notes</h2></div></div></div><p> - </p><div class="literallayout"><p><br /> -<br /> - The Library<br /> - -----------<br /> -<br /> - This paper is covers two major areas:<br /> -<br /> - - Features and policies not mentioned in the standard that<br /> - the quality of the library implementation depends on, including<br /> - extensions and "implementation-defined" features;<br /> -<br /> - - Plans for required but unimplemented library features and<br /> - optimizations to them.<br /> -<br /> - Overhead<br /> - --------<br /> -<br /> - The standard defines a large library, much larger than the standard<br /> - C library. A naive implementation would suffer substantial overhead<br /> - in compile time, executable size, and speed, rendering it unusable<br /> - in many (particularly embedded) applications. The alternative demands<br /> - care in construction, and some compiler support, but there is no<br /> - need for library subsets.<br /> -<br /> - What are the sources of this overhead? There are four main causes:<br /> -<br /> - - The library is specified almost entirely as templates, which<br /> - with current compilers must be included in-line, resulting in<br /> - very slow builds as tens or hundreds of thousands of lines<br /> - of function definitions are read for each user source file.<br /> - Indeed, the entire SGI STL, as well as the dos Reis valarray,<br /> - are provided purely as header files, largely for simplicity in<br /> - porting. Iostream/locale is (or will be) as large again.<br /> -<br /> - - The library is very flexible, specifying a multitude of hooks<br /> - where users can insert their own code in place of defaults.<br /> - When these hooks are not used, any time and code expended to<br /> - support that flexibility is wasted.<br /> -<br /> - - Templates are often described as causing to "code bloat". In<br /> - practice, this refers (when it refers to anything real) to several<br /> - independent processes. First, when a class template is manually<br /> - instantiated in its entirely, current compilers place the definitions<br /> - for all members in a single object file, so that a program linking<br /> - to one member gets definitions of all. Second, template functions<br /> - which do not actually depend on the template argument are, under<br /> - current compilers, generated anew for each instantiation, rather<br /> - than being shared with other instantiations. Third, some of the<br /> - flexibility mentioned above comes from virtual functions (both in<br /> - regular classes and template classes) which current linkers add<br /> - to the executable file even when they manifestly cannot be called.<br /> -<br /> - - The library is specified to use a language feature, exceptions,<br /> - which in the current gcc compiler ABI imposes a run time and<br /> - code space cost to handle the possibility of exceptions even when<br /> - they are not used. Under the new ABI (accessed with -fnew-abi),<br /> - there is a space overhead and a small reduction in code efficiency<br /> - resulting from lost optimization opportunities associated with<br /> - non-local branches associated with exceptions.<br /> -<br /> - What can be done to eliminate this overhead? A variety of coding<br /> - techniques, and compiler, linker and library improvements and<br /> - extensions may be used, as covered below. Most are not difficult,<br /> - and some are already implemented in varying degrees.<br /> -<br /> - Overhead: Compilation Time<br /> - --------------------------<br /> -<br /> - Providing "ready-instantiated" template code in object code archives<br /> - allows us to avoid generating and optimizing template instantiations<br /> - in each compilation unit which uses them. However, the number of such<br /> - instantiations that are useful to provide is limited, and anyway this<br /> - is not enough, by itself, to minimize compilation time. In particular,<br /> - it does not reduce time spent parsing conforming headers.<br /> -<br /> - Quicker header parsing will depend on library extensions and compiler<br /> - improvements. One approach is some variation on the techniques<br /> - previously marketed as "pre-compiled headers", now standardized as<br /> - support for the "export" keyword. "Exported" template definitions<br /> - can be placed (once) in a "repository" -- really just a library, but<br /> - of template definitions rather than object code -- to be drawn upon<br /> - at link time when an instantiation is needed, rather than placed in<br /> - header files to be parsed along with every compilation unit.<br /> -<br /> - Until "export" is implemented we can put some of the lengthy template<br /> - definitions in #if guards or alternative headers so that users can skip<br /> - over the full definitions when they need only the ready-instantiated<br /> - specializations.<br /> -<br /> - To be precise, this means that certain headers which define<br /> - templates which users normally use only for certain arguments<br /> - can be instrumented to avoid exposing the template definitions<br /> - to the compiler unless a macro is defined. For example, in<br /> - <string>, we might have:<br /> -<br /> - template <class _CharT, ... > class basic_string {<br /> - ... // member declarations<br /> - };<br /> - ... // operator declarations<br /> -<br /> - #ifdef _STRICT_ISO_<br /> - # if _G_NO_TEMPLATE_EXPORT<br /> - # include <bits/std_locale.h> // headers needed by definitions<br /> - # ...<br /> - # include <bits/string.tcc> // member and global template definitions.<br /> - # endif<br /> - #endif<br /> -<br /> - Users who compile without specifying a strict-ISO-conforming flag<br /> - would not see many of the template definitions they now see, and rely<br /> - instead on ready-instantiated specializations in the library. This<br /> - technique would be useful for the following substantial components:<br /> - string, locale/iostreams, valarray. It would *not* be useful or<br /> - usable with the following: containers, algorithms, iterators,<br /> - allocator. Since these constitute a large (though decreasing)<br /> - fraction of the library, the benefit the technique offers is<br /> - limited.<br /> -<br /> - The language specifies the semantics of the "export" keyword, but<br /> - the gcc compiler does not yet support it. When it does, problems<br /> - with large template inclusions can largely disappear, given some<br /> - minor library reorganization, along with the need for the apparatus<br /> - described above.<br /> -<br /> - Overhead: Flexibility Cost<br /> - --------------------------<br /> -<br /> - The library offers many places where users can specify operations<br /> - to be performed by the library in place of defaults. Sometimes<br /> - this seems to require that the library use a more-roundabout, and<br /> - possibly slower, way to accomplish the default requirements than<br /> - would be used otherwise.<br /> -<br /> - The primary protection against this overhead is thorough compiler<br /> - optimization, to crush out layers of inline function interfaces.<br /> - Kuck & Associates has demonstrated the practicality of this kind<br /> - of optimization.<br /> -<br /> - The second line of defense against this overhead is explicit<br /> - specialization. By defining helper function templates, and writing<br /> - specialized code for the default case, overhead can be eliminated<br /> - for that case without sacrificing flexibility. This takes full<br /> - advantage of any ability of the optimizer to crush out degenerate<br /> - code.<br /> -<br /> - The library specifies many virtual functions which current linkers<br /> - load even when they cannot be called. Some minor improvements to the<br /> - compiler and to ld would eliminate any such overhead by simply<br /> - omitting virtual functions that the complete program does not call.<br /> - A prototype of this work has already been done. For targets where<br /> - GNU ld is not used, a "pre-linker" could do the same job.<br /> -<br /> - The main areas in the standard interface where user flexibility<br /> - can result in overhead are:<br /> -<br /> - - Allocators: Containers are specified to use user-definable<br /> - allocator types and objects, making tuning for the container<br /> - characteristics tricky.<br /> -<br /> - - Locales: the standard specifies locale objects used to implement<br /> - iostream operations, involving many virtual functions which use<br /> - streambuf iterators.<br /> -<br /> - - Algorithms and containers: these may be instantiated on any type,<br /> - frequently duplicating code for identical operations.<br /> -<br /> - - Iostreams and strings: users are permitted to use these on their<br /> - own types, and specify the operations the stream must use on these<br /> - types.<br /> -<br /> - Note that these sources of overhead are _avoidable_. The techniques<br /> - to avoid them are covered below.<br /> -<br /> - Code Bloat<br /> - ----------<br /> -<br /> - In the SGI STL, and in some other headers, many of the templates<br /> - are defined "inline" -- either explicitly or by their placement<br /> - in class definitions -- which should not be inline. This is a<br /> - source of code bloat. Matt had remarked that he was relying on<br /> - the compiler to recognize what was too big to benefit from inlining,<br /> - and generate it out-of-line automatically. However, this also can<br /> - result in code bloat except where the linker can eliminate the extra<br /> - copies.<br /> -<br /> - Fixing these cases will require an audit of all inline functions<br /> - defined in the library to determine which merit inlining, and moving<br /> - the rest out of line. This is an issue mainly in chapters 23, 25, and<br /> - 27. Of course it can be done incrementally, and we should generally<br /> - accept patches that move large functions out of line and into ".tcc"<br /> - files, which can later be pulled into a repository. Compiler/linker<br /> - improvements to recognize very large inline functions and move them<br /> - out-of-line, but shared among compilation units, could make this<br /> - work unnecessary.<br /> -<br /> - Pre-instantiating template specializations currently produces large<br /> - amounts of dead code which bloats statically linked programs. The<br /> - current state of the static library, libstdc++.a, is intolerable on<br /> - this account, and will fuel further confused speculation about a need<br /> - for a library "subset". A compiler improvement that treats each<br /> - instantiated function as a separate object file, for linking purposes,<br /> - would be one solution to this problem. An alternative would be to<br /> - split up the manual instantiation files into dozens upon dozens of<br /> - little files, each compiled separately, but an abortive attempt at<br /> - this was done for <string> and, though it is far from complete, it<br /> - is already a nuisance. A better interim solution (just until we have<br /> - "export") is badly needed.<br /> -<br /> - When building a shared library, the current compiler/linker cannot<br /> - automatically generate the instantiations needed. This creates a<br /> - miserable situation; it means any time something is changed in the<br /> - library, before a shared library can be built someone must manually<br /> - copy the declarations of all templates that are needed by other parts<br /> - of the library to an "instantiation" file, and add it to the build<br /> - system to be compiled and linked to the library. This process is<br /> - readily automated, and should be automated as soon as possible.<br /> - Users building their own shared libraries experience identical<br /> - frustrations.<br /> -<br /> - Sharing common aspects of template definitions among instantiations<br /> - can radically reduce code bloat. The compiler could help a great<br /> - deal here by recognizing when a function depends on nothing about<br /> - a template parameter, or only on its size, and giving the resulting<br /> - function a link-name "equate" that allows it to be shared with other<br /> - instantiations. Implementation code could take advantage of the<br /> - capability by factoring out code that does not depend on the template<br /> - argument into separate functions to be merged by the compiler.<br /> -<br /> - Until such a compiler optimization is implemented, much can be done<br /> - manually (if tediously) in this direction. One such optimization is<br /> - to derive class templates from non-template classes, and move as much<br /> - implementation as possible into the base class. Another is to partial-<br /> - specialize certain common instantiations, such as vector<T*>, to share<br /> - code for instantiations on all types T. While these techniques work,<br /> - they are far from the complete solution that a compiler improvement<br /> - would afford.<br /> -<br /> - Overhead: Expensive Language Features<br /> - -------------------------------------<br /> -<br /> - The main "expensive" language feature used in the standard library<br /> - is exception support, which requires compiling in cleanup code with<br /> - static table data to locate it, and linking in library code to use<br /> - the table. For small embedded programs the amount of such library<br /> - code and table data is assumed by some to be excessive. Under the<br /> - "new" ABI this perception is generally exaggerated, although in some<br /> - cases it may actually be excessive.<br /> -<br /> - To implement a library which does not use exceptions directly is<br /> - not difficult given minor compiler support (to "turn off" exceptions<br /> - and ignore exception constructs), and results in no great library<br /> - maintenance difficulties. To be precise, given "-fno-exceptions",<br /> - the compiler should treat "try" blocks as ordinary blocks, and<br /> - "catch" blocks as dead code to ignore or eliminate. Compiler<br /> - support is not strictly necessary, except in the case of "function<br /> - try blocks"; otherwise the following macros almost suffice:<br /> -<br /> - #define throw(X)<br /> - #define try if (true)<br /> - #define catch(X) else if (false)<br /> -<br /> - However, there may be a need to use function try blocks in the<br /> - library implementation, and use of macros in this way can make<br /> - correct diagnostics impossible. Furthermore, use of this scheme<br /> - would require the library to call a function to re-throw exceptions<br /> - from a try block. Implementing the above semantics in the compiler<br /> - is preferable.<br /> -<br /> - Given the support above (however implemented) it only remains to<br /> - replace code that "throws" with a call to a well-documented "handler"<br /> - function in a separate compilation unit which may be replaced by<br /> - the user. The main source of exceptions that would be difficult<br /> - for users to avoid is memory allocation failures, but users can<br /> - define their own memory allocation primitives that never throw.<br /> - Otherwise, the complete list of such handlers, and which library<br /> - functions may call them, would be needed for users to be able to<br /> - implement the necessary substitutes. (Fortunately, they have the<br /> - source code.)<br /> -<br /> - Opportunities<br /> - -------------<br /> -<br /> - The template capabilities of C++ offer enormous opportunities for<br /> - optimizing common library operations, well beyond what would be<br /> - considered "eliminating overhead". In particular, many operations<br /> - done in Glibc with macros that depend on proprietary language<br /> - extensions can be implemented in pristine Standard C++. For example,<br /> - the chapter 25 algorithms, and even C library functions such as strchr,<br /> - can be specialized for the case of static arrays of known (small) size.<br /> -<br /> - Detailed optimization opportunities are identified below where<br /> - the component where they would appear is discussed. Of course new<br /> - opportunities will be identified during implementation.<br /> -<br /> - Unimplemented Required Library Features<br /> - ---------------------------------------<br /> -<br /> - The standard specifies hundreds of components, grouped broadly by<br /> - chapter. These are listed in excruciating detail in the CHECKLIST<br /> - file.<br /> -<br /> - 17 general<br /> - 18 support<br /> - 19 diagnostics<br /> - 20 utilities<br /> - 21 string<br /> - 22 locale<br /> - 23 containers<br /> - 24 iterators<br /> - 25 algorithms<br /> - 26 numerics<br /> - 27 iostreams<br /> - Annex D backward compatibility<br /> -<br /> - Anyone participating in implementation of the library should obtain<br /> - a copy of the standard, ISO 14882. People in the U.S. can obtain an<br /> - electronic copy for US$18 from ANSI's web site. Those from other<br /> - countries should visit http://www.iso.org/ to find out the location<br /> - of their country's representation in ISO, in order to know who can<br /> - sell them a copy.<br /> -<br /> - The emphasis in the following sections is on unimplemented features<br /> - and optimization opportunities.<br /> -<br /> - Chapter 17 General<br /> - -------------------<br /> -<br /> - Chapter 17 concerns overall library requirements.<br /> -<br /> - The standard doesn't mention threads. A multi-thread (MT) extension<br /> - primarily affects operators new and delete (18), allocator (20),<br /> - string (21), locale (22), and iostreams (27). The common underlying<br /> - support needed for this is discussed under chapter 20.<br /> -<br /> - The standard requirements on names from the C headers create a<br /> - lot of work, mostly done. Names in the C headers must be visible<br /> - in the std:: and sometimes the global namespace; the names in the<br /> - two scopes must refer to the same object. More stringent is that<br /> - Koenig lookup implies that any types specified as defined in std::<br /> - really are defined in std::. Names optionally implemented as<br /> - macros in C cannot be macros in C++. (An overview may be read at<br /> - <http://www.cantrip.org/cheaders.html>). The scripts "inclosure"<br /> - and "mkcshadow", and the directories shadow/ and cshadow/, are the<br /> - beginning of an effort to conform in this area.<br /> -<br /> - A correct conforming definition of C header names based on underlying<br /> - C library headers, and practical linking of conforming namespaced<br /> - customer code with third-party C libraries depends ultimately on<br /> - an ABI change, allowing namespaced C type names to be mangled into<br /> - type names as if they were global, somewhat as C function names in a<br /> - namespace, or C++ global variable names, are left unmangled. Perhaps<br /> - another "extern" mode, such as 'extern "C-global"' would be an<br /> - appropriate place for such type definitions. Such a type would<br /> - affect mangling as follows:<br /> -<br /> - namespace A {<br /> - struct X {};<br /> - extern "C-global" { // or maybe just 'extern "C"'<br /> - struct Y {};<br /> - };<br /> - }<br /> - void f(A::X*); // mangles to f__FPQ21A1X<br /> - void f(A::Y*); // mangles to f__FP1Y<br /> -<br /> - (It may be that this is really the appropriate semantics for regular<br /> - 'extern "C"', and 'extern "C-global"', as an extension, would not be<br /> - necessary.) This would allow functions declared in non-standard C headers<br /> - (and thus fixable by neither us nor users) to link properly with functions<br /> - declared using C types defined in properly-namespaced headers. The<br /> - problem this solves is that C headers (which C++ programmers do persist<br /> - in using) frequently forward-declare C struct tags without including<br /> - the header where the type is defined, as in<br /> -<br /> - struct tm;<br /> - void munge(tm*);<br /> -<br /> - Without some compiler accommodation, munge cannot be called by correct<br /> - C++ code using a pointer to a correctly-scoped tm* value.<br /> -<br /> - The current C headers use the preprocessor extension "#include_next",<br /> - which the compiler complains about when run "-pedantic".<br /> - (Incidentally, it appears that "-fpedantic" is currently ignored,<br /> - probably a bug.) The solution in the C compiler is to use<br /> - "-isystem" rather than "-I", but unfortunately in g++ this seems<br /> - also to wrap the whole header in an 'extern "C"' block, so it's<br /> - unusable for C++ headers. The correct solution appears to be to<br /> - allow the various special include-directory options, if not given<br /> - an argument, to affect subsequent include-directory options additively,<br /> - so that if one said<br /> -<br /> - -pedantic -iprefix $(prefix) \<br /> - -idirafter -ino-pedantic -ino-extern-c -iwithprefix -I g++-v3 \<br /> - -iwithprefix -I g++-v3/ext<br /> -<br /> - the compiler would search $(prefix)/g++-v3 and not report<br /> - pedantic warnings for files found there, but treat files in<br /> - $(prefix)/g++-v3/ext pedantically. (The undocumented semantics<br /> - of "-isystem" in g++ stink. Can they be rescinded? If not it<br /> - must be replaced with something more rationally behaved.)<br /> -<br /> - All the C headers need the treatment above; in the standard these<br /> - headers are mentioned in various chapters. Below, I have only<br /> - mentioned those that present interesting implementation issues.<br /> -<br /> - The components identified as "mostly complete", below, have not been<br /> - audited for conformance. In many cases where the library passes<br /> - conformance tests we have non-conforming extensions that must be<br /> - wrapped in #if guards for "pedantic" use, and in some cases renamed<br /> - in a conforming way for continued use in the implementation regardless<br /> - of conformance flags.<br /> -<br /> - The STL portion of the library still depends on a header<br /> - stl/bits/stl_config.h full of #ifdef clauses. This apparatus<br /> - should be replaced with autoconf/automake machinery.<br /> -<br /> - The SGI STL defines a type_traits<> template, specialized for<br /> - many types in their code including the built-in numeric and<br /> - pointer types and some library types, to direct optimizations of<br /> - standard functions. The SGI compiler has been extended to generate<br /> - specializations of this template automatically for user types,<br /> - so that use of STL templates on user types can take advantage of<br /> - these optimizations. Specializations for other, non-STL, types<br /> - would make more optimizations possible, but extending the gcc<br /> - compiler in the same way would be much better. Probably the next<br /> - round of standardization will ratify this, but probably with<br /> - changes, so it probably should be renamed to place it in the<br /> - implementation namespace.<br /> -<br /> - The SGI STL also defines a large number of extensions visible in<br /> - standard headers. (Other extensions that appear in separate headers<br /> - have been sequestered in subdirectories ext/ and backward/.) All<br /> - these extensions should be moved to other headers where possible,<br /> - and in any case wrapped in a namespace (not std!), and (where kept<br /> - in a standard header) girded about with macro guards. Some cannot be<br /> - moved out of standard headers because they are used to implement<br /> - standard features. The canonical method for accommodating these<br /> - is to use a protected name, aliased in macro guards to a user-space<br /> - name. Unfortunately C++ offers no satisfactory template typedef<br /> - mechanism, so very ad-hoc and unsatisfactory aliasing must be used<br /> - instead.<br /> -<br /> - Implementation of a template typedef mechanism should have the highest<br /> - priority among possible extensions, on the same level as implementation<br /> - of the template "export" feature.<br /> -<br /> - Chapter 18 Language support<br /> - ----------------------------<br /> -<br /> - Headers: <limits> <new> <typeinfo> <exception><br /> - C headers: <cstddef> <climits> <cfloat> <cstdarg> <csetjmp><br /> - <ctime> <csignal> <cstdlib> (also 21, 25, 26)<br /> -<br /> - This defines the built-in exceptions, rtti, numeric_limits<>,<br /> - operator new and delete. Much of this is provided by the<br /> - compiler in its static runtime library.<br /> -<br /> - Work to do includes defining numeric_limits<> specializations in<br /> - separate files for all target architectures. Values for integer types<br /> - except for bool and wchar_t are readily obtained from the C header<br /> - <limits.h>, but values for the remaining numeric types (bool, wchar_t,<br /> - float, double, long double) must be entered manually. This is<br /> - largely dog work except for those members whose values are not<br /> - easily deduced from available documentation. Also, this involves<br /> - some work in target configuration to identify the correct choice of<br /> - file to build against and to install.<br /> -<br /> - The definitions of the various operators new and delete must be<br /> - made thread-safe, which depends on a portable exclusion mechanism,<br /> - discussed under chapter 20. Of course there is always plenty of<br /> - room for improvements to the speed of operators new and delete.<br /> -<br /> - <cstdarg>, in Glibc, defines some macros that gcc does not allow to<br /> - be wrapped into an inline function. Probably this header will demand<br /> - attention whenever a new target is chosen. The functions atexit(),<br /> - exit(), and abort() in cstdlib have different semantics in C++, so<br /> - must be re-implemented for C++.<br /> -<br /> - Chapter 19 Diagnostics<br /> - -----------------------<br /> -<br /> - Headers: <stdexcept><br /> - C headers: <cassert> <cerrno><br /> -<br /> - This defines the standard exception objects, which are "mostly complete".<br /> - Cygnus has a version, and now SGI provides a slightly different one.<br /> - It makes little difference which we use.<br /> -<br /> - The C global name "errno", which C allows to be a variable or a macro,<br /> - is required in C++ to be a macro. For MT it must typically result in<br /> - a function call.<br /> -<br /> - Chapter 20 Utilities<br /> - ---------------------<br /> - Headers: <utility> <functional> <memory><br /> - C header: <ctime> (also in 18)<br /> -<br /> - SGI STL provides "mostly complete" versions of all the components<br /> - defined in this chapter. However, the auto_ptr<> implementation<br /> - is known to be wrong. Furthermore, the standard definition of it<br /> - is known to be unimplementable as written. A minor change to the<br /> - standard would fix it, and auto_ptr<> should be adjusted to match.<br /> -<br /> - Multi-threading affects the allocator implementation, and there must<br /> - be configuration/installation choices for different users' MT<br /> - requirements. Anyway, users will want to tune allocator options<br /> - to support different target conditions, MT or no.<br /> -<br /> - The primitives used for MT implementation should be exposed, as an<br /> - extension, for users' own work. We need cross-CPU "mutex" support,<br /> - multi-processor shared-memory atomic integer operations, and single-<br /> - processor uninterruptible integer operations, and all three configurable<br /> - to be stubbed out for non-MT use, or to use an appropriately-loaded<br /> - dynamic library for the actual runtime environment, or statically<br /> - compiled in for cases where the target architecture is known.<br /> -<br /> - Chapter 21 String<br /> - ------------------<br /> - Headers: <string><br /> - C headers: <cctype> <cwctype> <cstring> <cwchar> (also in 27)<br /> - <cstdlib> (also in 18, 25, 26)<br /> -<br /> - We have "mostly-complete" char_traits<> implementations. Many of the<br /> - char_traits<char> operations might be optimized further using existing<br /> - proprietary language extensions.<br /> -<br /> - We have a "mostly-complete" basic_string<> implementation. The work<br /> - to manually instantiate char and wchar_t specializations in object<br /> - files to improve link-time behavior is extremely unsatisfactory,<br /> - literally tripling library-build time with no commensurate improvement<br /> - in static program link sizes. It must be redone. (Similar work is<br /> - needed for some components in chapters 22 and 27.)<br /> -<br /> - Other work needed for strings is MT-safety, as discussed under the<br /> - chapter 20 heading.<br /> -<br /> - The standard C type mbstate_t from <cwchar> and used in char_traits<><br /> - must be different in C++ than in C, because in C++ the default constructor<br /> - value mbstate_t() must be the "base" or "ground" sequence state.<br /> - (According to the likely resolution of a recently raised Core issue,<br /> - this may become unnecessary. However, there are other reasons to<br /> - use a state type not as limited as whatever the C library provides.)<br /> - If we might want to provide conversions from (e.g.) internally-<br /> - represented EUC-wide to externally-represented Unicode, or vice-<br /> - versa, the mbstate_t we choose will need to be more accommodating<br /> - than what might be provided by an underlying C library.<br /> -<br /> - There remain some basic_string template-member functions which do<br /> - not overload properly with their non-template brethren. The infamous<br /> - hack akin to what was done in vector<> is needed, to conform to<br /> - 23.1.1 para 10. The CHECKLIST items for basic_string marked 'X',<br /> - or incomplete, are so marked for this reason.<br /> -<br /> - Replacing the string iterators, which currently are simple character<br /> - pointers, with class objects would greatly increase the safety of the<br /> - client interface, and also permit a "debug" mode in which range,<br /> - ownership, and validity are rigorously checked. The current use of<br /> - raw pointers as string iterators is evil. vector<> iterators need the<br /> - same treatment. Note that the current implementation freely mixes<br /> - pointers and iterators, and that must be fixed before safer iterators<br /> - can be introduced.<br /> -<br /> - Some of the functions in <cstring> are different from the C version.<br /> - generally overloaded on const and non-const argument pointers. For<br /> - example, in <cstring> strchr is overloaded. The functions isupper<br /> - etc. in <cctype> typically implemented as macros in C are functions<br /> - in C++, because they are overloaded with others of the same name<br /> - defined in <locale>.<br /> -<br /> - Many of the functions required in <cwctype> and <cwchar> cannot be<br /> - implemented using underlying C facilities on intended targets because<br /> - such facilities only partly exist.<br /> -<br /> - Chapter 22 Locale<br /> - ------------------<br /> - Headers: <locale><br /> - C headers: <clocale><br /> -<br /> - We have a "mostly complete" class locale, with the exception of<br /> - code for constructing, and handling the names of, named locales.<br /> - The ways that locales are named (particularly when categories<br /> - (e.g. LC_TIME, LC_COLLATE) are different) varies among all target<br /> - environments. This code must be written in various versions and<br /> - chosen by configuration parameters.<br /> -<br /> - Members of many of the facets defined in <locale> are stubs. Generally,<br /> - there are two sets of facets: the base class facets (which are supposed<br /> - to implement the "C" locale) and the "byname" facets, which are supposed<br /> - to read files to determine their behavior. The base ctype<>, collate<>,<br /> - and numpunct<> facets are "mostly complete", except that the table of<br /> - bitmask values used for "is" operations, and corresponding mask values,<br /> - are still defined in libio and just included/linked. (We will need to<br /> - implement these tables independently, soon, but should take advantage<br /> - of libio where possible.) The num_put<>::put members for integer types<br /> - are "mostly complete".<br /> -<br /> - A complete list of what has and has not been implemented may be<br /> - found in CHECKLIST. However, note that the current definition of<br /> - codecvt<wchar_t,char,mbstate_t> is wrong. It should simply write<br /> - out the raw bytes representing the wide characters, rather than<br /> - trying to convert each to a corresponding single "char" value.<br /> -<br /> - Some of the facets are more important than others. Specifically,<br /> - the members of ctype<>, numpunct<>, num_put<>, and num_get<> facets<br /> - are used by other library facilities defined in <string>, <istream>,<br /> - and <ostream>, and the codecvt<> facet is used by basic_filebuf<><br /> - in <fstream>, so a conforming iostream implementation depends on<br /> - these.<br /> -<br /> - The "long long" type eventually must be supported, but code mentioning<br /> - it should be wrapped in #if guards to allow pedantic-mode compiling.<br /> -<br /> - Performance of num_put<> and num_get<> depend critically on<br /> - caching computed values in ios_base objects, and on extensions<br /> - to the interface with streambufs.<br /> -<br /> - Specifically: retrieving a copy of the locale object, extracting<br /> - the needed facets, and gathering data from them, for each call to<br /> - (e.g.) operator<< would be prohibitively slow. To cache format<br /> - data for use by num_put<> and num_get<> we have a _Format_cache<><br /> - object stored in the ios_base::pword() array. This is constructed<br /> - and initialized lazily, and is organized purely for utility. It<br /> - is discarded when a new locale with different facets is imbued.<br /> -<br /> - Using only the public interfaces of the iterator arguments to the<br /> - facet functions would limit performance by forbidding "vector-style"<br /> - character operations. The streambuf iterator optimizations are<br /> - described under chapter 24, but facets can also bypass the streambuf<br /> - iterators via explicit specializations and operate directly on the<br /> - streambufs, and use extended interfaces to get direct access to the<br /> - streambuf internal buffer arrays. These extensions are mentioned<br /> - under chapter 27. These optimizations are particularly important<br /> - for input parsing.<br /> -<br /> - Unused virtual members of locale facets can be omitted, as mentioned<br /> - above, by a smart linker.<br /> -<br /> - Chapter 23 Containers<br /> - ----------------------<br /> - Headers: <deque> <list> <queue> <stack> <vector> <map> <set> <bitset><br /> -<br /> - All the components in chapter 23 are implemented in the SGI STL.<br /> - They are "mostly complete"; they include a large number of<br /> - nonconforming extensions which must be wrapped. Some of these<br /> - are used internally and must be renamed or duplicated.<br /> -<br /> - The SGI components are optimized for large-memory environments. For<br /> - embedded targets, different criteria might be more appropriate. Users<br /> - will want to be able to tune this behavior. We should provide<br /> - ways for users to compile the library with different memory usage<br /> - characteristics.<br /> -<br /> - A lot more work is needed on factoring out common code from different<br /> - specializations to reduce code size here and in chapter 25. The<br /> - easiest fix for this would be a compiler/ABI improvement that allows<br /> - the compiler to recognize when a specialization depends only on the<br /> - size (or other gross quality) of a template argument, and allow the<br /> - linker to share the code with similar specializations. In its<br /> - absence, many of the algorithms and containers can be partial-<br /> - specialized, at least for the case of pointers, but this only solves<br /> - a small part of the problem. Use of a type_traits-style template<br /> - allows a few more optimization opportunities, more if the compiler<br /> - can generate the specializations automatically.<br /> -<br /> - As an optimization, containers can specialize on the default allocator<br /> - and bypass it, or take advantage of details of its implementation<br /> - after it has been improved upon.<br /> -<br /> - Replacing the vector iterators, which currently are simple element<br /> - pointers, with class objects would greatly increase the safety of the<br /> - client interface, and also permit a "debug" mode in which range,<br /> - ownership, and validity are rigorously checked. The current use of<br /> - pointers for iterators is evil.<br /> -<br /> - As mentioned for chapter 24, the deque iterator is a good example of<br /> - an opportunity to implement a "staged" iterator that would benefit<br /> - from specializations of some algorithms.<br /> -<br /> - Chapter 24 Iterators<br /> - ---------------------<br /> - Headers: <iterator><br /> -<br /> - Standard iterators are "mostly complete", with the exception of<br /> - the stream iterators, which are not yet templatized on the<br /> - stream type. Also, the base class template iterator<> appears<br /> - to be wrong, so everything derived from it must also be wrong,<br /> - currently.<br /> -<br /> - The streambuf iterators (currently located in stl/bits/std_iterator.h,<br /> - but should be under bits/) can be rewritten to take advantage of<br /> - friendship with the streambuf implementation.<br /> -<br /> - Matt Austern has identified opportunities where certain iterator<br /> - types, particularly including streambuf iterators and deque<br /> - iterators, have a "two-stage" quality, such that an intermediate<br /> - limit can be checked much more quickly than the true limit on<br /> - range operations. If identified with a member of iterator_traits,<br /> - algorithms may be specialized for this case. Of course the<br /> - iterators that have this quality can be identified by specializing<br /> - a traits class.<br /> -<br /> - Many of the algorithms must be specialized for the streambuf<br /> - iterators, to take advantage of block-mode operations, in order<br /> - to allow iostream/locale operations' performance not to suffer.<br /> - It may be that they could be treated as staged iterators and<br /> - take advantage of those optimizations.<br /> -<br /> - Chapter 25 Algorithms<br /> - ----------------------<br /> - Headers: <algorithm><br /> - C headers: <cstdlib> (also in 18, 21, 26))<br /> -<br /> - The algorithms are "mostly complete". As mentioned above, they<br /> - are optimized for speed at the expense of code and data size.<br /> -<br /> - Specializations of many of the algorithms for non-STL types would<br /> - give performance improvements, but we must use great care not to<br /> - interfere with fragile template overloading semantics for the<br /> - standard interfaces. Conventionally the standard function template<br /> - interface is an inline which delegates to a non-standard function<br /> - which is then overloaded (this is already done in many places in<br /> - the library). Particularly appealing opportunities for the sake of<br /> - iostream performance are for copy and find applied to streambuf<br /> - iterators or (as noted elsewhere) for staged iterators, of which<br /> - the streambuf iterators are a good example.<br /> -<br /> - The bsearch and qsort functions cannot be overloaded properly as<br /> - required by the standard because gcc does not yet allow overloading<br /> - on the extern-"C"-ness of a function pointer.<br /> -<br /> - Chapter 26 Numerics<br /> - --------------------<br /> - Headers: <complex> <valarray> <numeric><br /> - C headers: <cmath>, <cstdlib> (also 18, 21, 25)<br /> -<br /> - Numeric components: Gabriel dos Reis's valarray, Drepper's complex,<br /> - and the few algorithms from the STL are "mostly done". Of course<br /> - optimization opportunities abound for the numerically literate. It<br /> - is not clear whether the valarray implementation really conforms<br /> - fully, in the assumptions it makes about aliasing (and lack thereof)<br /> - in its arguments.<br /> -<br /> - The C div() and ldiv() functions are interesting, because they are the<br /> - only case where a C library function returns a class object by value.<br /> - Since the C++ type div_t must be different from the underlying C type<br /> - (which is in the wrong namespace) the underlying functions div() and<br /> - ldiv() cannot be re-used efficiently. Fortunately they are trivial to<br /> - re-implement.<br /> -<br /> - Chapter 27 Iostreams<br /> - ---------------------<br /> - Headers: <iosfwd> <streambuf> <ios> <ostream> <istream> <iostream><br /> - <iomanip> <sstream> <fstream><br /> - C headers: <cstdio> <cwchar> (also in 21)<br /> -<br /> - Iostream is currently in a very incomplete state. <iosfwd>, <iomanip>,<br /> - ios_base, and basic_ios<> are "mostly complete". basic_streambuf<> and<br /> - basic_ostream<> are well along, but basic_istream<> has had little work<br /> - done. The standard stream objects, <sstream> and <fstream> have been<br /> - started; basic_filebuf<> "write" functions have been implemented just<br /> - enough to do "hello, world".<br /> -<br /> - Most of the istream and ostream operators << and >> (with the exception<br /> - of the op<<(integer) ones) have not been changed to use locale primitives,<br /> - sentry objects, or char_traits members.<br /> -<br /> - All these templates should be manually instantiated for char and<br /> - wchar_t in a way that links only used members into user programs.<br /> -<br /> - Streambuf is fertile ground for optimization extensions. An extended<br /> - interface giving iterator access to its internal buffer would be very<br /> - useful for other library components.<br /> -<br /> - Iostream operations (primarily operators << and >>) can take advantage<br /> - of the case where user code has not specified a locale, and bypass locale<br /> - operations entirely. The current implementation of op<</num_put<>::put,<br /> - for the integer types, demonstrates how they can cache encoding details<br /> - from the locale on each operation. There is lots more room for<br /> - optimization in this area.<br /> -<br /> - The definition of the relationship between the standard streams<br /> - cout et al. and stdout et al. requires something like a "stdiobuf".<br /> - The SGI solution of using double-indirection to actually use a<br /> - stdio FILE object for buffering is unsatisfactory, because it<br /> - interferes with peephole loop optimizations.<br /> -<br /> - The <sstream> header work has begun. stringbuf can benefit from<br /> - friendship with basic_string<> and basic_string<>::_Rep to use<br /> - those objects directly as buffers, and avoid allocating and making<br /> - copies.<br /> -<br /> - The basic_filebuf<> template is a complex beast. It is specified to<br /> - use the locale facet codecvt<> to translate characters between native<br /> - files and the locale character encoding. In general this involves<br /> - two buffers, one of "char" representing the file and another of<br /> - "char_type", for the stream, with codecvt<> translating. The process<br /> - is complicated by the variable-length nature of the translation, and<br /> - the need to seek to corresponding places in the two representations.<br /> - For the case of basic_filebuf<char>, when no translation is needed,<br /> - a single buffer suffices. A specialized filebuf can be used to reduce<br /> - code space overhead when no locale has been imbued. Matt Austern's<br /> - work at SGI will be useful, perhaps directly as a source of code, or<br /> - at least as an example to draw on.<br /> -<br /> - Filebuf, almost uniquely (cf. operator new), depends heavily on<br /> - underlying environmental facilities. In current releases iostream<br /> - depends fairly heavily on libio constant definitions, but it should<br /> - be made independent. It also depends on operating system primitives<br /> - for file operations. There is immense room for optimizations using<br /> - (e.g.) mmap for reading. The shadow/ directory wraps, besides the<br /> - standard C headers, the libio.h and unistd.h headers, for use mainly<br /> - by filebuf. These wrappings have not been completed, though there<br /> - is scaffolding in place.<br /> -<br /> - The encapsulation of certain C header <cstdio> names presents an<br /> - interesting problem. It is possible to define an inline std::fprintf()<br /> - implemented in terms of the 'extern "C"' vfprintf(), but there is no<br /> - standard vfscanf() to use to implement std::fscanf(). It appears that<br /> - vfscanf but be re-implemented in C++ for targets where no vfscanf<br /> - extension has been defined. This is interesting in that it seems<br /> - to be the only significant case in the C library where this kind of<br /> - rewriting is necessary. (Of course Glibc provides the vfscanf()<br /> - extension.) (The functions related to exit() must be rewritten<br /> - for other reasons.)<br /> -<br /> -<br /> - Annex D<br /> - -------<br /> - Headers: <strstream><br /> -<br /> - Annex D defines many non-library features, and many minor<br /> - modifications to various headers, and a complete header.<br /> - It is "mostly done", except that the libstdc++-2 <strstream><br /> - header has not been adopted into the library, or checked to<br /> - verify that it matches the draft in those details that were<br /> - clarified by the committee. Certainly it must at least be<br /> - moved into the std namespace.<br /> -<br /> - We still need to wrap all the deprecated features in #if guards<br /> - so that pedantic compile modes can detect their use.<br /> -<br /> - Nonstandard Extensions<br /> - ----------------------<br /> - Headers: <iostream.h> <strstream.h> <hash> <rbtree><br /> - <pthread_alloc> <stdiobuf> (etc.)<br /> -<br /> - User code has come to depend on a variety of nonstandard components<br /> - that we must not omit. Much of this code can be adopted from<br /> - libstdc++-v2 or from the SGI STL. This particularly includes<br /> - <iostream.h>, <strstream.h>, and various SGI extensions such<br /> - as <hash_map.h>. Many of these are already placed in the<br /> - subdirectories ext/ and backward/. (Note that it is better to<br /> - include them via "<backward/hash_map.h>" or "<ext/hash_map>" than<br /> - to search the subdirectory itself via a "-I" directive.<br /> - </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="source_code_style.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="appendix_contributing.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="appendix_porting.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Coding Style </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Appendix B. - Porting and Maintenance - -</td></tr></table></div></body></html>
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