The most useful GCC options and extensions

<i>"If Java had true garbage collection, most programs would delete themselves upon execution." — Robert Sewell</i><p>Nice ;-)

I was definitely not expecting an article about useful GCC options to mention -trigraphs. The -ansi option isn't particularly useful either, imo, since just about every interesting program is going to do something crazy like open a network socket or list the files in a directory.<p>By far the most useful gcc extension wasn't mentioned at all. typeof().

Has anybody used the __builtin_expect feature to good effect? We had been using this extensively in our codebase in places where there is an obvious common "fast path".<p>I tried an experiment where I did a comparison of the __builtin_expect version against a version with our likely/unlikely macros defined to the identity, and I actually saw no difference, which was disappointing. When I did some digging I found this blog post: <a href="http://bitsup.blogspot.com/2008/04/measuring-performance-of-linux-kernel.html" rel="nofollow">http://bitsup.blogspot.com/2008/04/measuring-performance-of-...</a> which suggests the same result for the Linux kernel.<p>FWIW GCC also has a reasonable set of PGO options (not mentioned in this article), and those actually can (anecdotally) make a noticeable difference, although you're subject to the overtraining problem if you're not careful.

Another useful GCC extension is statements within expressions:<p>#define max(a,b) ({ typeof (a) _a = (a); typeof (b) _b = (b); _a &#62; _b ? _a : _b; })

Computed goto: <a href="http://gcc.gnu.org/onlinedocs/gcc-3.4.1/gcc/Labels-as-Values.html" rel="nofollow">http://gcc.gnu.org/onlinedocs/gcc-3.4.1/gcc/Labels-as-Values...</a>

A few others I use are -Wsign-compare and -Wformat-nonliteral. Sign compare warns on possible bugs/vulnerabilities due to comparing signed and unsigned values. Using nonliteral formats can lead to vulnerabilities (such as the famous attacks on FTP servers <a href="http://seclists.org/bugtraq/1999/Sep/328" rel="nofollow">http://seclists.org/bugtraq/1999/Sep/328</a>)

Sometimes when compiling and trying to use system calls gcc will complain about the functions not existing. This is usually because of some preprocessor guard not being defined. An easy way to see what macros have been defined is to run gcc with the "-E -dM" flags.

-fmudflap always interested me, but I never seem to remember it when I'm actually confronting a hard to debug pointer issue.

My favorite is the "weak" attribute. <a href="http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html" rel="nofollow">http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html</a><p>Makes separate compilation for languages that compile to C <i>much</i> easier.

Is it me, or is this statement from the article wrong:<p><i>"All warnings can be enabled with -w."</i><p>man g++ says:<p><i>"-w Inhibit all warning messages."</i><p>Inhibit means to prevent (not to enable).

No love for --coverage?