[cfe-dev] Driver: Default CPUs

R P Herrold herrold at owlriver.com
Wed Oct 12 03:20:53 PDT 2011

On Wed, 12 Oct 2011, James Molloy wrote:

> The driver, currently, *always* sets the "-target-cpu" option to cc1. The
> way it finds a default CPU for an architecture is hardcoded and nasty, and
> I'd love to get rid of it.

Part of the problem here is that some of the RPM based Linux 
porting folks are 'coining' their own tuple definitions to fit 
'political' branding decisions as to hardware FP and such.  I 
don't see HOW one avoids 'nasty' when one has to interface to 
that layer of the stack

> My personal opinion is that there should be no need to set a default CPU all
> the time. The target triple should suffice,

But for the reasons above, this shifts the load of chasing 
political decisions to the wrong place ... it is all fine, and 
well and good for people to make what are essentially 
'branding' decisions in their tuple, but it should not 
transfer the load for supporting such (sorry, but my feeling:) 
insanity into code

and if the user wants
> performance tuned to a specific CPU they should mention that on the
> commandline with -mcpu= or -mtune=. If no CPU is specified, I feel that a
> "blended" scheduling should be done such that the resultant code runs well
> on all chips with the given architecture/triple.

But 'blended' just disguises the issue of whose ox is 
being gored by abstracting it behind another level of hiding 
... The argument could be made that 'blended' should be that 
capable of running on the largest number of platforms (and 
thus at the expense of code bload carrying around run time 
maths code, not needed when HW FP _is_ present

> What do people think? Do people care?

It is a discussion worth having, I think

- Russ herrold

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