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Re: S/390: -march= and -mcpu= options
- From: Richard Earnshaw <rearnsha at arm dot com>
- To: Alexandre Oliva <aoliva at redhat dot com>
- Cc: Ulrich Weigand <weigand at immd1 dot informatik dot uni-erlangen dot de>, dje at watson dot ibm dot com, drow at mvista dot com, hpenner at de dot ibm dot com, gcc-patches at gcc dot gnu dot org, Richard dot Earnshaw at arm dot com
- Date: Tue, 07 Jan 2003 12:08:33 +0000
- Subject: Re: S/390: -march= and -mcpu= options
- Organization: ARM Ltd.
- Reply-to: Richard dot Earnshaw at arm dot com
> On Dec 23, 2002, Ulrich Weigand <weigand@immd1.informatik.uni-erlangen.de> wrote:
>
> >> Second, the people who participated in the earlier discussion
> >> agreed that there should be a shorthand.
>
> > Why shouldn't -march=z900 be enough?
>
> It should. IMHO, any sane implementation of -march/-mtune should tune
> for the specified arch unless an -mtune argument is present, which
> renders -mcpu (which is an alias for -march -mtune) totally useless.
>
> The only exception I can think of is in case the default tuning is
> something actually more recent than the default arch, in which case
> someone might want to downgrade the arch without downgrading tuning.
> This probably happens seldom enough, if ever, that arranging for
> -march to be the -mtune default is perfectly reasonable.
>
The problem here is that tuning for an architecture (ie an abstract ISA)
can be meaningless. There can be multiple implementations of an ISA with
different trade-offs. You just can't guess and give good results.
R.