Re: RFC: LRA for x86/x86-64 [0/9]

[Vladimir Makarov <vmakarov at redhat dot com>]

On 10/01/2012 03:19 PM, David Miller wrote:
> From: Ian Lance Taylor <iant@google.com>
> Date: Mon, 1 Oct 2012 11:55:56 -0700
>
> > Steven is correct in saying that there is a tendency to move on and
> > never address GCC bugs.  However, there is also a counter-vailing
> > tendency to fix GCC bugs.  Anyhow I'm certainly not saying that in all
> > cases it's OK to accept a merge with regressions; I'm saying that in
> > this specific case it is OK.
> I think it's more important in this case to recognize Steven's real
> point, which is that for an identical situation (IRA), and with an
> identical patch author, we had similar bugs.  They were promised to be
> worked on, and yet some of those regressions are still very much with
> us.
That is not true.  I worked on many compiler time regression bugs. I 
remeber one serious degradation of compilation time on 
all_cp2k_gfortran.f90.  I solved the problem and make IRA working faster 
and generating much better code than the old RA.

http://blog.gmane.org/gmane.comp.gcc.patches/month=20080501/page=15

About other two mentioned PRs by Steven:

PR26854.  I worked on this bug even when IRA was on the branch and make 
again GCC with IRA 5% faster on this test than GCC with the old RA.

PR 54146 is 3 months old.  There were a lot work on other optimizations 
before IRA became important.  It happens only 2 months ago. I had no 
time to work on it but I am going to.

People sometimes see that RA takes a lot of compilation time but it is 
in the nature of RA.  I'd recommend first to check how the old RA 
behaves and then call it a degradation.

And please, don't listen just one side.
> The likelyhood of a repeat is therefore very real.
>
> I really don't have a lot of confidence given what has happened in
> the past.  I also don't understand what's so evil about sorting this
> out on a branch.  It's the perfect carrot to get the compile time
> regressions fixed.
Wrong assumptions result in wrong conclusions.