-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 It looks like the redundancy analyzer in gcc is broken. My first hint was that after 6 hours of work I finally narrowed down some interesting performance measurements using nbench[1]* to Partial Redundancy Elimination; my second was popping into #gcc on OFTC and being told the RA is "stupid" and doesn't work so hot on x86.
It turns out that -O2 turns -ftree-pre on by default, and -Os turns it off; so certain operations were quite awesomely faster with -Os (and others slower). Fiddling with tons of optimizations I eventually noticed the SAME hit -O2 takes in ONE optimization; I disabled this during an -O2 compile of nbench and lo and behold the numbers looked a lot better. Details on the Wiki at [2]. Somebody should plug some real-world application into this, like rendering a JPEG image, Web page, or Ogg Vorbis file and measuring how long this takes in real time using both '-O2' and '-O2 -fno-tree-pre'. Some code seems slightly slower, while most code seems significantly faster. [1]http://www.tux.org/~mayer/linux/bmark.html [2]http://wiki.laptop.org/go/User:Bluefoxicy/gcc_optimizations *nbench is highly CPU intensive, it does no IO benchmarks. The CPU will get hot. I compiled it on a tmpfs mounted on /tmp/x to avoid touching flash. - -- All content of all messages exchanged herein are left in the Public Domain, unless otherwise explicitly stated. Creative brains are a valuable, limited resource. They shouldn't be wasted on re-inventing the wheel when there are so many fascinating new problems waiting out there. -- Eric Steven Raymond We will enslave their women, eat their children and rape their cattle! -- Bosc, Evil alien overlord from the fifth dimension -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iQIVAwUBRR79XQs1xW0HCTEFAQL2Uw/9G7aNVyFBn6v3kVM9gykqD+vhy89CL7pc WjjwDJ1YOMlZdsmcW+hrbykCgL7vZYKqFKN6MdK7Z4lM9z5FIfU8QGdAIkWq/N1a D1f7eJbCuL/FcMxNMmcG+alJDh6RQW6QBmafFtzTsezPkCSovaGTcY0kn85593nV S+wxOb2N+EwDi0QFkji4U8CM2P6OjrE/1BQkyL2EskWzvHoRuu7HKdwcXWFqIQMc kxqktyTVTLjJnXuWse1hiPuy7gN+LcTEHTLqUEWBZLUyXa1dr0u1MG4i+JI4xRpM cE6BUPw27Fg4LcRM2F8nda+cHhjcrRt+xJAAMR/driNQfehFCYXX0GMJEFprH9Zv HQtHS2wzjmwUu4WjbH4gdiJcIFTVfUpNm3coa8Z/PzBV0oQ5Atv+L3S4qbJZKD0B 2ZdtljBXWPZIDl9CS2g0jF1nVqKH/jszu9grix6Pl4P0rhEz6YE5zRJe5IZaK2q9 aGX5htZqmxRDAFOBU6fYhPs/nk1JrhhBRjqsCsp2v/CEJNxz0+GCMMbcKCOzoHPK yhDXFbcceCeh83wwIpzYdffrHj3EallIkNcW8I5rvjpOZ9yzatkfxod3+ovRIB6n ojjH8IuIqliiPjO07NqoxLItJbUn3wPbDBnr7oGojEf7LRL/9Kugulkqk9euVB5g +nUB5IG65hk= =Y0Xu -----END PGP SIGNATURE----- _______________________________________________ Devel mailing list [email protected] http://mailman.laptop.org/mailman/listinfo/devel
