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Re: IRA for GCC 4.4
On Thu, Apr 24, 2008 at 10:42:49AM -0400, Vladimir Makarov wrote:
> FX wrote:
> >> The best way to test IRA is to build and use the branch. It is easy to
> >>compare the old RA (which is the default on the branch) and IRA (-fira
> >>option switches IRA on). I'd recommend to try the following option sets:
> >> -fira
> >> -fira -fira-algorithm=CB
> >>
> >
> >OK, I've done that and I see a 40% to 60% increase in compilation time
> >for the first (Fortran) testcase I tried, is that expected?
> >
> >
> Yes, that is known problem for -O0. The old allocator does not use
> global allocator at -O0, IRA is used always even for -O0. The correct
> comparison would be at -O2. There are several solution of the problem:
> o We could make only the reload working for -O0. In this case, the
> time will be the same.
> o We could prevent regional allocation at -O0. In this case, the
> slow down would be 20% (I guess).
> o Use a very fast and simple local allocation.
> o Or just ignore this.
>
> I'd prefer the second solution.
Ignoring it isn't an option; developers who spend most of their
time in the compile/debug/test/fix loop would see a 40-60% decrease at -O0
as a very severe regression. Even 20% would get a lot of complaints.