[gentoo-user] -march=native is *EXTREMELY* conservative

[Walter Dnes]

Here's the setup of my secondary machine..  It's a Dell Inspiron
Desktop 530, with 2 gigs of ram, assembled and shipped August 2007.  It
has an Intel Core2 (*NOT* a "Core2 Duo") cpu.  From /proc/cpuinfo...

vendor_id       : GenuineIntel
cpu family      : 6
model           : 15
model name      : Genuine Intel(R) CPU            2140  @ 1.60GHz
stepping        : 2
cpu MHz         : 1595.660
cache size      : 1024 KB

  Here are the cpu flags...

waltdnes@d530 ~ $ grep flags /proc/cpuinfo
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm
constant_tsc arch_perfmon pebs bts aperfmperf pni dtes64 monitor ds_cpl
est tm2 ssse3 cx16 xtpr pdcm lahf_lm dts

  Then I ran "gcc -march=native -Q --help=target" and got a major shock.
It's a long output listing of what -march=native thinks about my cpu.
Here are some relevant items...

-march=                               core2
-mmmx                                 [disabled]
-msse                                 [disabled]
-msse2                                [disabled]
-msse3                                [disabled]
-mssse3                               [disabled]

  It properly identified the cpu as "core2".  But mmx, sse, sse2, sse3
(aka pni), and ssse3 are disabled!!! 

  The reason I dug into this was that I was trying to 1920x1080i
streaming video from my HDHomerun TV tuner box.  I originally used

CFLAGS="-O2 -march=native -fomit-frame-pointer -pipe"

when setting up the system.  It could not handle 1920x1080i streaming TV
video.

  After findinding out about "-native", I changed CFLAGS to...

CFLAGS="-O2 -march=native -mmmx -msse -msse2 -msse3 -mssse3 -mfpmath=sse -fomit-frame-pointer -pipe"

...and emerged system and world and rebuilt the kernel.  Now the onboard
Intel GPU handles 1920x1080i flawlessly.  As a matter of fact, it
handles 1080p flawlessly.

-- 
Walter Dnes <waltdnes@××××××××.org>