[cfe-dev] Driver: Default CPUs

[James Molloy]

Hi,

I'm kicking off this discussion as it came out of my previous thread on ARM
Driver cleanup that people had different opinions on the use of default
CPUs.

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.

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

The fact that LLVM doesn't have many processor itineraries (especially for
ARM) and can't currently do this shouldn't really factor in to this
discussion - I'd like this discussion to be more what we *should* do, not
what we *can* do with the current constraints. We can then take the outcomes
of it and see concretely what we may need to change.

Also, I'm not considering backwards compatibility for Darwin here - any
solution will take "Darwin as a special case" into account.

The arguments I've heard so far are:
  *  Mine, that there should be no default CPU selected if the user doesn't
select it himself. I feel it adds a hidden option that the user is unaware
of, and depending on what that default is may cause inferior performance on
the CPU the user actually runs it on.
  * Jim's (? I forget who responded ?), that there *should* be a default CPU
all the time, but perhaps some pseudo-CPUs could be added that exhibit a
blended scheduling. For example for ARM, a "v7" pseudo-cpu could be added
that has an itinerary that performs well on all v7 cores. By the way, I
think ARM is the only high-visibility target with such a difference between
cores that this matters. I assume MIPS and PowerPC are similar though?
  * Keep the current behaviour and select a default CPU for the given
architecture.
  * ???

Out of all of these, the only one I really dislike is keeping the current
behaviour.

What do people think? Do people care?

Cheers,

James