r189218 - Simplify now that -O4 just maps to -O3 and -O is an alias of -O2.

[Hal Finkel]

----- Original Message -----
> 
> On Sep 3, 2013, at 9:53 AM, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
> 
> >>> This is something Chris requested, IIRC. I'm not sure I remember
> >>> the
> >>> motivation beyond that.
> >> 
> >> Two reasons:
> >> 
> >> 1) -O1 doesn't actually mean anything.  It is the optimization
> >> level least understood by both the GCC folks and us.  Recently
> >> (last 5 years?) there has been a move to try to make this
> >> "optimize without messing up debug info", but this is a new
> >> movement.
> > 
> > Another thing that is making -O1 better defined is asan. It is the
> > optimization level that makes asan fast but still provides useful
> > backtraces :-)
> 
> Ok, makes sense.  Those uses can use an explicit -O1 though :-)
> 
> > 
> >> 2) Users who specify -O generally don't know it maps onto -O1.
> >>  They almost certainly don't want whatever -O1 provides.  In my
> >> experience, most are coming from Sun, HP or other compilers,
> >> where -O was a generally useful flag.
> >> 
> >> 3) There are some benchmarks that pass -O (because of #2), which
> >> is ridiculous, but reflects some reality that people use -O.  I
> >> don't recall what these benchmarks were.
> >> 
> >> These are reasons that I suggested the change.  I really don't
> >> like -O1 :-)
> > 
> > I haven't seen -O being used in wild, so I OK with keeping it
> > mapping to -O2.
> 
> I'd be curious to know how many files in a linux distro are built
> with -O.  I wouldn't be surprised if it is  5-10%.

FWIW, IBM's xlc compiler (at least the version on the BG/Q machines), have several -O<level>, but specifically omit -O1. -O is an alias to -O2.

 -Hal

> 
> -Chris
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory