On Tue, 11 Oct 2011, Benjamin Kosnik wrote: >> Many users still won't have GCC 4.6 deployed yet, so I think it's >> still worth it. > Ouch. I see this is not in, and I though I checked in the draft months > ago. > > Please check this in immediately!!!
Done last evening, and made some further tweaks. For reference hre is the full patch that's now live on the system. Gerald Index: porting_to.html =================================================================== RCS file: porting_to.html diff -N porting_to.html --- /dev/null 1 Jan 1970 00:00:00 -0000 +++ porting_to.html 12 Oct 2011 16:16:54 -0000 1.3 @@ -0,0 +1,150 @@ +<html> + +<head> +<title>Porting to GCC 4.6</title> +</head> + +<body> +<h1>Porting to GCC 4.6</h1> + +<p> +The GCC 4.6 release series differs from previous GCC releases in more +than the usual list of +<a href="http://gcc.gnu.org/gcc-4.6/changes.html";>changes</a>. Some of +these are a result of bug fixing, and some old behaviors have been +intentionally changed in order to support new standards, or relaxed +instandards-conforming ways to facilitate compilation or runtime +performance. Some of these changes are not visible to the naked eye +and will not cause problems when updating from older versions. +</p> + +<p> +However, some of these changes are visible, and can cause grief to +users porting to GCC 4.6. This document is an effort to identify major +issues and provide clear solutions in a quick and easily searched +manner. Additions and suggestions for improvement are welcome. +</p> + +<h2>C language issues</h2> + +<h3>New warnings for unused variables and parameters</h3> + +<p> +The behavior of <code>-Wall</code> has changed and now includes the +new warning flags <code>-Wunused-but-set-variable</code> and +(with <code>-Wall +-Wextra</code>) <code>-Wunused-but-set-parameter</code>. This may +result in new warnings in code that compiled cleanly with previous +versions of GCC. +</p> + +<p>For example,</p> +<pre> + void fn (void) + { + int foo; + foo = bar (); /* foo is never used. */ + } +</pre> +<p>Gives the following diagnostic:</p> +<pre> +warning: variable "foo" set but not used [-Wunused-but-set-variable] +</pre> + +<p>Although these warnings will not result in compilation failure, +often <code>-Wall</code> is used in conjunction with +<code>-Werror</code> and as a result, new warnings are turned into +new errors.</p> + +<p>To fix, first see if the unused variable or parameter can be removed +without changing the result or logic of the surrounding code. If not, +annotate it with <code>__attribute__((__unused__))</code>.</p> + +<p>As a workaround, remove <code>-Werror</code> until the new warnings +are fixed. For conversion warnings add +<code>-Wno-unused-but-set-variable</code> or +<code>-Wno-unused-but-set-parameter</code>.</p> + +<h3>Strict overflow warnings</h3> + +<p>Using the <code>-Wstrict-overflow</code> flag with +<code>-Werror</code> and optmization flags above <code>-O2</code> +may result in compile errors when using glibc optimizations +for <code>strcmp</code>.</p> + +<p>For example,</p> +<pre> +#include <string.h> +void do_rm_rf (const char *p) { if (strcmp (p, "/") == 0) return; } +</pre> +<p>Results in the following diagnostic:</p> +<pre> +error: assuming signed overflow does not occur when changing X +- C1 cmp C2 to X cmp C1 +- C2 [-Werror=strict-overflow] +</pre> + +<p>To work around this, use <code>-D__NO_STRING_INLINES</code>.</p> + +<h2>C++ language issues</h2> + +<h3>Header dependency changes</h3> + +<p> +Many of the standard C++ library include files have been edited to no +longer include <cstddef> to get <code>namespace std</code> +-scoped versions of <code>size_t</code> and <code>ptrdiff_t</code>. +</p> + +<p> +As such, C++ programs that used the macros <code>NULL</code> +or <code>offsetof</code> without including <cstddef> will no +longer compile. The diagnostic produced is similar to: +</p> + +<pre> +error: 'ptrdiff_t' does not name a type +</pre> + +<pre> +error: 'size_t' has not been declared +</pre> + +<pre> +error: 'NULL' was not declared in this scope +</pre> + +<pre> +error: there are no arguments to 'offsetof' that depend on a template +parameter, so a declaration of 'offsetof' must be available +</pre> + +<p> +Fixing this issue is easy: just include <cstddef>. +</p> + +<!-- +<h3>Java issues</h3> +--> + +<h3>Links</h3> + +<p> +Jakub Jelinek, + <a href="http://lists.fedoraproject.org/pipermail/devel/2011-February/148523.html";>GCC +4.6 related common package rebuild failures (was Re: mass rebuild status)</a> +</p> + +<p> +Matthias Klose, +<a href="http://lists.debian.org/debian-devel-announce/2011/02/msg00012.html";>prepare +to fix build failures with new GCC versions</a> +</p> + +<p> +Jim Meyering, + <a href="http://lists.fedoraproject.org/pipermail/devel/2011-March/149355.html";>gcc-4.6.0-0.12.fc15.x86_64 breaks strcmp?</a> +</p> + +</body> +</html> + +
