On 28 May 2009, at 11:17, Daniel Iliev wrote:
...
Not the same thing. "-march=" instructs gcc to produce a binary
designed to run only on the given CPU architecture, while
USE flags instruct the build system to include or not support for a
given feature ...
Actually, I _think_ in mplayer the difference is that the USE flags
enable the use of hand-written assembler code which takes advantage of
the MMX or whatever.
I'm not 100% sure about this at all, but what's the difference in the
architectures of the Pentium 2 & the Pentium 3, if not for the support
for SSE? (and / or whatever)
Sure they are designed differently, but surely the instruction sets of
all 32-bit processors from the earliest Pentiums even to Core Duos
(not Core2s) are all basically x86. Some have these extended
instruction sets (EIS, I think you called them earlier in the thread)
and those are the additions from one generation to the next. So if you
choose -march=pentium3 it will not run on a Pentium 2 because Pentium
2s do not have SSSE, and it's the *compiler* that added the code that
utilises SSE.
I believe that mplayer is one of those exceptional packages which uses
some assembler optimisations. These don't make any sense for most
programs, because it takes too much time to write & much maintenance
is loads of hassle, and assembler will make little difference. Most
programs are one minute opening a new window, and the next minute are
waiting for the mail server to reply and are doing lots of different
tasks which are not worth hand optimising. But once mplayer has
started decoding a video, then it's doing the same thing time & time
again, decoding each frame of the picture - it does exactly the same
thing 25 times per second for an hour or two at a time, so that may be
worth optimising for if the compiler is found to be less clever than
someone like John Carmack.
I'm not absolutely sure that the USE flags enable hand-written
assembler code, but what I do know is that:
1) ASM stands for assembler. See http://en.wikipedia.org/wiki/Asm
2) Google "mplayer asm" returns a bunch of hits.
3) MMX was marketed as "MultiMedia eXtension"
4) mplayer is a multimedia player.
5) MMX doesn't have floating point support, but SSE adds that and has
dedicated functions for multiplying floating point numbers [1], which
is just the sort of thing that one are useful in decoding mpegs &
playing back video.
Stroller.
[1] http://en.wikipedia.org/wiki/Streaming_SIMD_Extensions
