Re: Target CPU default selection on alpha

[David Edelsohn <dje at watson dot ibm dot com>]

>>>>> Jeffrey A Law writes:

Jeff> In message <19981218203058.A23463@dot.cygnus.com>you write:
>> On Fri, Dec 18, 1998 at 10:26:21PM +0100, Rainer Orth wrote:
>> > While this is convenient for a single machine, I suppose it may
>> > cause problems when the resulting gcc is run on a machine with a differen
>> t
>> > cpu type: e.g. configured for a 21164/ev5, but run on a 21064/ev4.
>> 
>> Not that particular example, but for example ev56 won't run on ev5.
>> 
>> > but its more in line with the treatment of other processors to leave
>> > this choice to the user (via --with-cpu=).
>> 
>> Hum..  I hadn't really given it much thought, since the alpha
>> port has "always" done it that way.  I suppose it would be 
>> better, since there would be less in the way of user error.

Jeff> There isn't any kind of consistency in this area.  For example the ppc
Jeff> defaults to "common" mode.

Jeff> It's not 100% clear that either choice is right.  There's arguments for
Jeff> and against defaulting to the least common denominator for code generation.

	All operating systems other than AIX default to a particular
architecture.  IBM's XLC compiler for AIX defaults to "common" mode and a
substantial number of users were getting confused when GCC did not behave
the same way.  Also, the POWER and PowerPC architectures are fixed and not
changing with each processor revision.  And neither POWER nor PowerPC are
a true subset of the other architecture.  It did not make sense for the
architecture to be decided arbitrarily based on the particular system on
which GCC was built in a multi-architecture site.

David