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From: Alexander Skwar <listen@alexander.skwar.name>
Subject: [gentoo-user] Re: Cannot compile texinfo: Illegal instruction -> Wrong -march and -mtune flags?
Date: Thu, 02 Aug 2007 23:36:28 +0200
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=C2=B7 Florian Philipp <f.philipp@addcom.de>:
> You see, they are not compatible and even if some code works I wouldn't=
bet=20
> multimedia apps will perform well.
>=20
> With -mtune the instruction set stays the same. It is just "rearranged".
Hm. Allright. When using just -mtune (ie. without -march), the
docs at http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/i386-and-x86_002d64-O=
ptions.html
say:
| While picking a specific cpu-type will schedule things appropriately
| for that particular chip, the compiler will not generate any code that
| does not run on the i386 without the -march=3Dcpu-type option being use=
d. =20
If -mtune=3Dathlon-xp is used, code is generated which may make
use of 3dNOW!. 3dNOW! is, of course, not to be found on 386 :)
If the instruction set stays the same, code generated with
-mtune=3Dathlon-xp would not be executable on 386 machines, if
I understand you correctly.
Hm. With -mtune, the set of available instructions (ie.
stuff like 3dNOW!, I suppose?) is NOT changed from the default
of i386, is it? Or what does "Tune to cpu-type everything applicable
about the generated code, except for the ABI and the set of available
instructions." mean - especially note the "except for [...] the set of=20
available instructions" part.
So with "-mtune=3Dpentium-m -march=3Dathlon-xp" I'm making the compiler
generate code which is "ordered" the way it's best for pentium-m
machines while allowing it to use athlon-xp instruction set? Is
that what I'm doing?
If so, then it seems you're right - code will run, but maybe not
so well.
Is that understanding correct? If so, then I really should think
twice about using "-mtune=3Dpentium-m -march=3Dathlon-xp", shouldn't
I?
Curious,
Alexander Skwar
--=20
No matter how many resources you have, it is never enough.=20
-- Murphy's Computer Laws n=B01
--=20
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