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diff --git a/contrib/gcc/bugs.html b/contrib/gcc/bugs.html new file mode 100644 index 0000000..a8dab54 --- /dev/null +++ b/contrib/gcc/bugs.html @@ -0,0 +1,698 @@ +<html> + +<head> +<title>GCC Bugs</title> +</head> + +<body> +<h1>GCC Bugs</h1> + +<p>The latest version of this document is always available at +<a href="http://www.gnu.org/software/gcc/bugs.html">http://www.gnu.org/software/gcc/bugs.html</a>.</p> + +<hr /> + +<h2>Table of Contents</h2> +<ul> +<li><a href="#report">Reporting Bugs</a> + <ul> + <li><a href="#need">What we need</a></li> + <li><a href="#dontwant">What we DON'T want</a></li> + <li><a href="#where">Where to post it</a></li> + <li><a href="#detailed">Detailed bug reporting instructions</a></li> + <li><a href="#gnat">Detailed bug reporting instructions for GNAT</a></li> + </ul> +</li> +<li><a href="#manage">Managing Bugs (GNATS and the test-suite)</a></li> +<li><a href="#known">Frequently Reported Bugs in GCC</a> + <ul> + <li><a href="#general">General</a></li> + <li><a href="#fortran">Fortran</a></li> + <li><a href="#c">C</a></li> + <li><a href="#cplusplus">C++</a> + <ul> + <li><a href="#updating">Common problems updating from G++ 2.95 to + G++ 3.0</a></li> + <li><a href="#nonbugs">Non-bugs</a></li> + <li><a href="#missing">Missing features</a></li> + <li><a href="#parsing">Parse errors for "simple" code</a></li> + <li><a href="#-O3">Optimization at <code>-O3</code> takes a + very long time</a></li> + </ul> + </li> + </ul> + </li> +</ul> + +<hr /> + +<h1><a name="report">Reporting Bugs</a></h1> + +<p>Our preferred way of receiving bugs is via the +<a href="gnats.html">GCC GNATS bug reporting system</a>.</p> + +<p>Before you report a bug, please check the +<a href="#known">list of well-known bugs</a> and, <strong>if possible +in any way, try a current development snapshot</strong>. +If you want to report a bug with versions of GCC before 3.1 we strongly +recommend upgrading to the current release first.</p> + +<p>Before reporting that GCC compiles your code incorrectly, please +compile it with <code>gcc -Wall</code> and see whether this shows +anything wrong with your code that could be the cause instead of a bug +in GCC.</p> + +<h2>Summarized bug reporting instructions</h2> + +<p>After this summary, you'll find detailed bug reporting +instructions, that explain how to obtain some of the information +requested in this summary.</p> + +<h3><a name="need">What we need</a></h3> + +Please include in your bug report all of the following items, the first +three of which can be obtained from the output of <code>gcc -v</code>: + +<ul> + <li>the exact version of GCC;</li> + <li>the system type;</li> + <li>the options given when GCC was configured/built;</li> + <li>the complete command line that triggers the bug;</li> + <li>the compiler output (error messages, warnings, etc.); and</li> + <li>the <em>preprocessed</em> file (<code>*.i*</code>) that triggers the + bug, generated by adding <code>-save-temps</code> to the complete + compilation command, or, in the case of a bug report for the GNAT front end, + a complete set of source files (see below).</li> +</ul> + +<h3><a name="dontwant">What we do <strong>not</strong> want</a></h3> + +<ul> + <li>A source file that <code>#include</code>s header files that are left + out of the bug report (see above)</li> + + <li>That source file and a collection of header files.</li> + + <li>An attached archive (tar, zip, shar, whatever) containing all + (or some :-) of the above.</li> + + <li>A code snippet that won't cause the compiler to produce the + exact output mentioned in the bug report (e.g., a snippet with just + a few lines around the one that <b>apparently</b> triggers the bug, + with some pieces replaced with ellipses or comments for extra + obfuscation :-)</li> + + <li>The location (URL) of the package that failed to build (we won't + download it, anyway, since you've already given us what we need to + duplicate the bug, haven't you? :-)</li> + + <li>An error that occurs only some of the times a certain file is + compiled, such that retrying a sufficient number of times results in + a successful compilation; this is a symptom of a hardware problem, + not of a compiler bug (sorry)</li> + + <li>E-mail messages that complement previous, incomplete bug + reports. Post a new, self-contained, full bug report instead, if + possible as a follow-up to the original bug report</li> + + <li>Assembly files (<code>*.s</code>) produced by the compiler, or any + binary files, such as object files, executables or core files</li> + + <li>Duplicate bug reports, or reports of bugs already fixed in the + development tree, especially those that have already been reported + as fixed last week :-)</li> + + <li>Bugs in the assembler, the linker or the C library. These are + separate projects, with separate mailing lists and different bug + reporting procedures</li> + + <li>Bugs in releases or snapshots of GCC not issued by the GNU + Project. Report them to whoever provided you with the release</li> + + <li>Questions about the correctness or the expected behavior of + certain constructs that are not GCC extensions. Ask them in forums + dedicated to the discussion of the programming language</li> +</ul> + +<h3><a name="where">Where to post it</a></h3> + +<p>Please submit your bug report directly to the +<a href="gnats.html">GCC GNATS bug database</a>. +Only if this is not possible, mail all information to +<a href="mailto:bug-gcc@gnu.org">bug-gcc@gnu.org</a> or +<a href="mailto:gcc-bugs@gcc.gnu.org">gcc-bugs@gcc.gnu.org</a>.</p> + +<p>The GCC lists have message size limits (200 kbytes) and bug reports +over those limits will currently be bounced. If your bug is larger +than that, please post it using the <a href="gnats.html">GCC GNATS bug +database</a>.</p> + +<h2><a name="detailed">Detailed bug reporting instructions</a></h2> + +<p>Please refer to the <a href="#gnat">next section</a> when reporting +bugs in GNAT, the Ada compiler.</p> + +<p>In general, all the information we need can be obtained by +collecting the command line below, as well as its output and the +preprocessed file it generates.</p> + +<blockquote><code>gcc -v -save-temps <i>all-your-options +source-file</i></code></blockquote> + +<p>Typically the preprocessed file (extension <code>.i</code> for C or +<code>.ii</code> for C++) will be large, so please compress the +resulting file with one of the popular compression programs such as +bzip2, gzip, zip or compress (in +decreasing order of preference). Use maximum compression +(<code>-9</code>) if available. Please include the compressed +preprocessor output in your bug report, even if the source code is +freely available elsewhere; it makes the job of our volunteer testers +much easier.</p> + +<p>The <b>only</b> excuses to not send us the preprocessed sources are +(i) if you've found a bug in the preprocessor, or (ii) if you've +reduced the testcase to a small file that doesn't include any +other file. If you can't post the preprocessed sources because +they're proprietary code, then try to create a small file that +triggers the same problem.</p> + +<p>Since we're supposed to be able to re-create the assembly output +(extension <code>.s</code>), you usually should not include +it in the bug report, although you may want to post parts of it to +point out assembly code you consider to be wrong.</p> + +<p>Whether to use MIME attachments or <code>uuencode</code> is up to +you. In any case, make sure the compiler command line, version and +error output are in plain text, so that we don't have to decode the +bug report in order to tell who should take care of it. A meaningful +subject indicating language and platform also helps.</p> + +<p>Please avoid posting an archive (.tar, .shar or .zip); we generally +need just a single file to reproduce the bug (the .i/.ii preprocessed +file), and, by storing it in an archive, you're just making our +volunteers' jobs harder. Only when your bug report requires multiple +source files to be reproduced should you use an archive. In any case, +make sure the compiler version, error message, etc, are included in +the body of your bug report as plain text, even if needlessly +duplicated as part of an archive.</p> + +<p>If you fail to supply enough information for a bug report to be +reproduced, someone will probably ask you to post additional +information (or just ignore your bug report, if they're in a bad day, +so try to get it right on the first posting :-). In this case, please +post the additional information to the bug reporting mailing list, not +just to the person who requested it, unless explicitly told so. If +possible, please include in this follow-up all the information you had +supplied in the incomplete bug report (including the preprocessor +output), so that the new bug report is self-contained.</p> + +<h2><a name="gnat">Detailed bug reporting instructions for GNAT</a></h2> + +<p>See the <a href="#detailed">previous section</a> for bug reporting +instructions for GCC language implementations other than Ada.</p> + +<p>Bug reports have to contain at least the following information in +order to be useful:</p> + +<ul> +<li>the exact version of GCC, as shown by "<code>gcc -v</code>";</li> +<li>the system type;</li> +<li>the options when GCC was configured/built;</li> +<li>the exact command line passed to the <code>gcc</code> program +triggering the bug +(not just the flags passed to <code>gnatmake</code>, but +<code>gnatmake</code> prints the parameters it passed to <code>gcc</code>)</li> +<li>a collection of source files for reproducing the bug, +preferably a minimal set (see below);</li> +<li>a description of the expected behavior;</li> +<li>a description of actual behavior.</li> +</ul> + +<p>If your code depends on additional source files (usually package +specifications), submit the source code for these compilation units in +a single file that is acceptable input to <code>gnatchop</code>, +i.e. contains no non-Ada text. If the compilation terminated +normally, you can usually obtain a list of dependencies using the +"<code>gnatls -d <i>main_unit</i></code>" command, where +<code><i>main_unit</i></code> is the file name of the main compilation +unit (which is also passed to <code>gcc</code>).</p> + +<p>If you report a bug which causes the compiler to print a bug box, +include that bug box in your report, and do not forget to send all the +source files listed after the bug box along with your report.</p> + +<p>If you use <code>gnatprep</code>, be sure to send in preprocessed +sources (unless you have to report a bug in <code>gnatprep</code>).</p> + +<p>When you have checked that your report meets these criteria, please +submit it accoding to our <a href="#where">generic instructions</a>. +(If you use a mailing list for reporting, please include an +"<code>[Ada]</code>" tag in the subject.)</p> + +<h1><a name="manage">Managing Bugs (GNATS and the test-suite)</a></h1> + +<p>This section contains information mostly intended for GCC +contributors.</p> + +<p>If you find a bug, but you are not fixing it (yet):</p> +<ol> +<li>Create a (minimal) test-case.</li> +<li>Add the test-case to our test-suite, marking it as XFAIL unless +the bug is a regression.</li> +<li>Add a bug report referencing the test-case to GNATS.</li> +</ol> + +<p>If you fix a bug for which there is already a GNATS entry:</p> +<ol> +<li>Remove the XFAIL on the test-case.</li> +<li>Close the bug report in GNATS.</li> +</ol> + +<p>If you find a bug, and you are fixing it right then:</p> +<ol> +<li>Create a (minimal) test-case.</li> +<li>Add the test-case to our test-suite, marking it as PASS.</li> +<li>Check in your fixes.</li> +</ol> + +<hr /> + +<h1><a name="known">Frequently Reported Bugs in GCC</a></h1> + +<h2><a name="fortran">Fortran</a></h2> + +<p>Fortran bugs are documented in the G77 manual rather than +explicitly listed here. Please see +<a href="http://gcc.gnu.org/onlinedocs/g77/Trouble.html">Known Causes of +Trouble with GNU Fortran</a> in the G77 manual.</p> + +<hr /> + +<h2><a name="c">C</a></h2> + +<p>The following are not bugs in the C compiler, but are reported +often enough to warrant a mention here.</p> + +<dl> +<dt>Cannot initialize a static variable with <code>stdin</code>.</dt> +<dd><p>This has nothing to do with GCC, but people ask us about it a +lot. Code like this:</p> + +<blockquote><pre> +#include <stdio.h> + +FILE *yyin = stdin; +</pre></blockquote> + +<p>will not compile with GNU libc (GNU/Linux libc6), because +<code>stdin</code> is not a constant. This was done deliberately, to make +it easier to maintain binary compatibility when the type <code>FILE</code> +needs to be changed. It is surprising for people used to traditional Unix +C libraries, but it is permitted by the C standard.</p> + +<p>This construct commonly occurs in code generated by old versions of +lex or yacc. We suggest you try regenerating the parser with a +current version of flex or bison, respectively. In your own code, the +appropriate fix is to move the initialization to the beginning of +main.</p> + +<p>There is a common misconception that the GCC developers are +responsible for GNU libc. These are in fact two entirely separate +projects; please check the +<a href="http://www.gnu.org/software/glibc/">GNU libc web pages</a> +for details. +</p></dd> + +<dt>Cannot use preprocessor directive in macro arguments.</dt> +<dd><p>Let me guess... you wrote code that looks something like this:</p> +<blockquote><pre> + memcpy(dest, src, +#ifdef PLATFORM1 + 12 +#else + 24 +#endif + ); +</pre></blockquote> +<p>and you got a whole pile of error messages:</p> +<blockquote><code> + +test.c:11: warning: preprocessing directive not recognized within +macro arg<br /> +test.c:11: warning: preprocessing directive not recognized within +macro arg<br /> +test.c:11: warning: preprocessing directive not recognized within +macro arg<br /> +test.c: In function `foo':<br /> +test.c:6: undefined or invalid # directive<br /> +test.c:8: undefined or invalid # directive<br /> +test.c:9: parse error before `24'<br /> +test.c:10: undefined or invalid # directive<br /> +test.c:11: parse error before `#'<br /> +</code></blockquote> + +<p><strong>Update:</strong> As of GCC 3.2 this kind of construct is +always accepted and CPP will probably do what you expect, but see the +manual for detailed semantics.</p> + +<p>However, versions of GCC prior to 3.2 did not allow you to put +<code>#ifdef</code> (or any other directive) inside the arguments of a +macro. Your C library's <code><string.h></code> happens to +define <code>memcpy</code> as a macro - this is perfectly legitimate. +The code therefore would not compile.</p> + +<p>This kind of code is not portable. It is "undefined behavior" +according to the C standard; that means different compilers will do +different things with it. It is always possible to rewrite code which +uses conditionals inside macros so that it doesn't. You could write +the above example</p> +<blockquote><pre> +#ifdef PLATFORM1 + memcpy(dest, src, 12); +#else + memcpy(dest, src, 24); +#endif +</pre></blockquote> +<p>This is a bit more typing, but I personally think it's better style +in addition to being more portable.</p> + +<p>In recent versions of glibc, <code>printf</code> is among the +functions which are implemented as macros.</p></dd> +</dl> + +<hr /> + +<h2><a name="cplusplus">C++</a></h2> + +<p>This is the list of bugs (and non-bugs) in g++ (aka GNU C++) that +are reported very often, but not yet fixed. While it is certainly +better to fix bugs instead of documenting them, this document might +save people the effort of writing a bug report when the bug is already +well-known. <a href="#report">How to report bugs</a> tells you how to +report a bug.</p> + +<p>There are many reasons why reported bugs don't get fixed. It might +be difficult to fix, or fixing it might break compatibility. Often, +reports get a low priority when there is a simple work-around. In +particular, bugs caused by invalid C++ code have a simple work-around, +<em>fix the code</em>. Now that there is an agreed ISO/ANSI standard +for C++, the compiler has a definitive document to adhere to. Earlier +versions might have accepted source code that is <em>no longer</em> +C++. This means that code which might have `worked' in a previous +version, is now rejected. You should update your code to be C++.</p> + +<p>You should try to use the latest stable release of the GNU C++ +compiler.</p> + +<h3><a name="updating">Common problems updating from G++ 2.95 to G++ +3.0</a></h3> + +<p>G++ 3.0 conforms much closer to the ISO C++ standard (available at +<a href="http://www.ncits.org/cplusplus.htm">http://www.ncits.org/cplusplus.htm</a>).</p> + +<p>We have also implemented some of the core and library defect reports +(available at +<a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html">http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_defects.html</a> +& +<a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html"> +http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html</a> +respectively).</p> + +<ul> + +<li>The ABI has changed. This means that both class layout and name +mangling is different. You <em>must</em> recompile all c++ libraries (if +you don't you will get link errors).</li> + +<li>The standard library is much more conformant, and uses the +<code>std::</code> namespace.</li> + +<li><code>std::</code> is now a real namespace, not an alias for +<code>::</code>.</li> + +<li>The standard header files for the c library don't end with +<code>.h</code>, but begin with <code>c</code> (i.e. +<code><cstdlib></code> rather than <code><stdlib.h></code>). +The <code>.h</code> names are still available, but are deprecated.</li> + +<li><code><strstream></code> is deprecated, use +<code><sstream></code> instead.</li> + +<li><code>streambuf::seekoff</code> & +<code>streambuf::seekpos</code> are private, instead use +<code>streambuf::pubseekoff</code> & +<code>streambuf::pubseekpos</code> respectively.</li> + +<li>If <code>std::operator << (std::ostream &, long long)</code> +doesn't exist, you need to recompile libstdc++ with +<code>--enable-long-long</code>.</li> + +</ul> + +This means you may get lots of errors about things like +<code>strcmp</code> not being found. You've most likely forgotton to +tell the compiler to look in the <code>std::</code> namespace. There are +several ways to do this, + +<ul> + +<li>Say, <code>std::strcmp</code> at the call. This is the most explicit +way of saying what you mean.</li> + +<li>Say, <code>using std::strcmp;</code> somewhere before the call. You +will need to do this for each function or type you wish to use from the +standard library.</li> + +<li>Say, <code>using namespace std;</code> somewhere before the call. +This is the quick-but-dirty fix. This brings the <em>whole</em> of the +<code>std::</code> namespace into scope. <em>Never</em> do this in a +header file, as you will be forcing users of your header file to do the +same.</li> + +</ul> + +<h3><a name="abi">ABI bugs</a></h3> + +<p>3.0 had a new ABI, which affected class layout, function mangling and +calling conventions. We had intended it to be complete, unfortunately +some issues came to light, too late to fix in the 3.0 series. +The ABI should not change in dot releases, so we addressed most issues +in GCC 3.1. +</p> + +<dl> + +<dt>Covariant return types</dt> + +<dd>We do not implement non-trivial covariant returns. We also generate +incorrect virtual function tables for trivial covariance. Although +trivial covariance will work, it is incompatible with the ABI. GNATS PR +3706 tracks this problem.</dd> + +</dl> + +<h3><a name="nonbugs">Non-bugs</a></h3> + +<p>Here are some features that have been reported as bugs, but are +not.</p> + +<dl> + +<dt>Nested classes can access private types of the containing +class.</dt> +<dd><p>G++ now implements type access control on member types. Defect +report 45 clarifies that nested classes are members of the class they +are nested in, and so are granted access to private members of that +class.</p></dd> + +<dt>Classes in exception specifiers must be complete types.</dt> +<dd><p>[15.4]/1 tells you that you cannot have an incomplete type, or +pointer to incomplete (other than <code><i>cv</i> void *</code>) in +an exception specification.</p></dd> + +<dt>G++ emits two copies of constructors and destructors.</dt> + +<dd><p>In general there are <em>three</em> types of constructors (and +destructors).</p> +<ol> +<li>The complete object constructor/destructor.</li> +<li>The base object constructor/destructor.</li> +<li>The allocating destructor/deallocating destructor.</li> +</ol> +<p>The first two are different, when virtual base classes are involved. +In some cases we can do better, and this is logged in GNATS.</p></dd> + +<dt>Exceptions don't work in multithreaded applications.</dt> + +<dd><p>You need to rebuild g++ and libstdc++ with +<code>--enable-threads</code>. Remember, c++ exceptions are not like +hardware interrupts. You cannot throw an exception in one thread and +catch it in another. You cannot throw an exception from a signal +handler, and catch it in the main thread.</p></dd> + +<dt>Global destructors are not run in the correct order.</dt> + +<dd><p>Global destructors should be run in the reverse order of their +constructors <em>completing</em>. In most cases this is the same as +the reverse order of constructors <em>starting</em>, but sometimes it +is different, and that is important. You need to compile and link your +programs with <code>--use-cxa-atexit</code>. We have not turned this +switch on by default, as it requires a <code>cxa</code> aware runtime +library (<code>libc</code>, <code>glibc</code>, or +equivalent).</p></dd> + +<dt>Problems with floating point computations.</dt> +<dd><p>In a number of cases, GCC appears to perform floating point +computations incorrectly. For example, the program</p> +<blockquote><code> +#include <iostream><br /> +<br /> +int main() {<br /> +<br /> + double min = 0.0;<br /> + double max = 0.5;<br /> + double width = 0.01;<br /> + std::cout << (int)(((max - min) / width) - 1) << + std::endl;<br /> +<br /> +}<br /> +</code></blockquote> +<p>might print 50 on some systems and optimization levels, and 51 on +others.</p> + +<p>The is the result of <em>rounding</em>: The computer cannot +represent all real numbers exactly, so it has to use +approximations. When computing with approximation, the computer needs +to round to the nearest representable number.</p> + +<p>This is not a bug in the compiler, but an inherent limitation of +the float and double types. Please study +<a href="http://www.validlab.com/goldberg/paper.ps">this paper</a> +for more information.</p></dd> + +<dt>Templates, scoping, and digraphs.</dt> + +<dd><p>If you have a class in global namespace, say named +<code>X</code>, and want to give it as a template argument to some +other class, say <code>std::vector</code>, then this here fails with a +parser error: <code>std::vector<::X></code>. +</p> + +<p> +The reason is that the standard mandates that the sequence +<code><:</code> is treated as if it were the token +<code>[</code>, and the parser then reports a parse error before the +character <code>:</code> (by which it means the second +colon). There are several such combinations of characters, and +they are called <em>digraphs</em>. +</p> + +<p> +The simplest way to avoid this is to write <code>std::vector< +::X></code>, i.e. place a space between the opening angle bracket +and the scope operator. +</p></dd> + + +</dl> + +<h3><a name="missing">Missing features</a></h3> +<p>We know some things are missing from G++.</p> + +<dl> + +<dt>The <code>export</code> keyword is not implemented.</dt> +<dd><p>Most C++ compilers (G++ included) do not yet implement +<code>export</code>, which is necessary for separate compilation of +template declarations and definitions. Without <code>export</code>, a +template definition must be in scope to be used. The obvious +workaround is simply to place all definitions in the header +itself. Alternatively, the compilation unit containing template +definitions may be included from the header.</p></dd> + +<dt>Two stage lookup in templates is not implemented.</dt> +<dd><p>[14.6] specifies how names are looked up inside a template. G++ +does not do this correctly, but for most templates this will not be +noticeable.</p></dd> + +</dl> + +<h3><a name="parsing">Parse errors for "simple" code</a></h3> + +Up to and including GCC 3.0, the compiler will give "parse error" for +seemingly simple code, such as + +<pre> +struct A{ + A(); + A(int); + void func(); +}; + +struct B{ + B(A); + B(A,A); + void func(); +}; + +void foo(){ + B b(A(),A(1)); //Variable b, initialized with two temporaries + B(A(2)).func(); //B temporary, initialized with A temporary +} +</pre> +The problem is that GCC starts to parse the declaration of +<code>b</code> as a function <code>b</code> returning <code>B</code>, +taking a function returning <code>A</code> as an argument. When it +sees the 1, it is too late. The work-around in these cases is to add +additional parentheses around the expressions that are mistaken as +declarations: +<pre> + (B(A(2))).func(); +</pre> +Sometimes, even that is not enough; to show the compiler that this +should be really an expression, a comma operator with a dummy argument +can be used: +<pre> + B b((0,A()),A(1)); +</pre> +<p> +Another example is the parse error for the <code>return</code> +statement in</p> +<pre> +struct A{}; + +struct B{ + A a; + A f1(bool); +}; + +A B::f1(bool b) +{ + if (b) + return (A()); + return a; +} +</pre> +<p>The problem is that the compiler interprets <code>A()</code> as a +function (taking no arguments, returning <code>A</code>), and +<code>(A()</code>) as a cast - with a missing expression, hence the +parse error. The work-around is to omit the parentheses:</p> +<pre> + if (b) + return A(); +</pre> +<p>This problem occurs in a number of variants; in <code>throw</code> +statements, people also frequently put the object in parentheses. The +exact error also somewhat varies with the compiler version. The +work-arounds proposed do not change the semantics of the program at +all; they make them perhaps less readable.</p> + +<h3><a name="-O3">Optimization at <code>-O3</code> takes a +very long time</a></h3> +<p>At <code>-O3</code>, all functions are candidates for inlining. The +heuristic used has some deficiencies which show up when allowed such +freedom. This is g++ specific, as it has an earlier inliner than +gcc.</p> + +</body> +</html> |