On Wed, Dec 03, 2008 at 04:26:11PM +0100, Joel Carnat wrote: > Hello, > > I read that Slackware Linux compiles most of its packages with > "-march=i486 -mtune=i686". As far as I understood gcc's manpage, this > means "keep code compatible with 486 CPU but allow the use pentiumpro > special features", right ? No. It produces binaries using the instruction set from the i486 with a scheduling optimised for the i686. The way CPUs work, the order in which you feed it the instructions matters a lot. > Still from the man page, MMX is supported since "pentium2 (or pentium-mmx > only)", SSE since "pentium3", SSE2 since "pentium-m", SSE3 since > "prescott". > > I tweaked my mk.conf to compile pkgsrc (on a domU) using "-O2 -march=486 > -mtune=686" but I'm now wondering if any on those compiled programs will > use MMX or SSE when running on my laptop or intel-based servers (intel > core2duo). They won't. When you use -march, gcc will produce code that can run on that architecture. > Then second question, in case the previous thought is correct, if I use > "-march=i486 -mtune=nocona", is there a way to see which instruction the > program uses ? For example, if I compile xorg/firefox/mplayer and then run > it in a special manner, can I see that the core2duo uses MMX/SSE* > instruction while the C3 only use MMX instruction (from the same binary) ? Those applications use assembly (either external or inline) to achieve that. It's not gcc's choice. -- Quentin Garnier - cube%cubidou.net@localhost - cube%NetBSD.org@localhost "See the look on my face from staying too long in one place [...] every time the morning breaks I know I'm closer to falling" KT Tunstall, Saving My Face, Drastic Fantastic, 2007.
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