Re: RFC: Patch for switch elimination (PR 54742)

[Ramana Radhakrishnan <ramana dot radhakrishnan at arm dot com>]

On 14/08/14 19:25, Steve Ellcey wrote:
> On Thu, 2014-08-14 at 10:21 -0600, Jeff Law wrote:
> > On 08/14/14 10:12, David Malcolm wrote:
> > > On Thu, 2014-08-14 at 09:56 -0600, Jeff Law wrote:
> > > > On 08/14/14 04:32, Richard Biener wrote:
> > > > > > You'll note in a separate thread Steve and I discussed this during Cauldron
> > > > > > and it was at my recommendation Steve resurrected his proof of concept
> > > > > > plugin and started beating it into shape.
> > > > >
> > > > > But do we really want a pass just to help coremark?
> > > > And that's the biggest argument against Steve's work.  In theory it
> > > > should be applicable to other FSMs, but nobody's come forth with
> > > > additional testcases from real world applications.
> > >
> > > Maybe a regex library?  Perhaps:
> > > http://vcs.pcre.org/viewvc/code/trunk/pcre_dfa_exec.c?revision=1477 ?
> > The key is that at least some states tell you at compile time what state
> > you'll be in during the next loop iteration.  Thus instead of coming
> > around the loop, evaluating the switch condition, then doing the
> > multi-way branch, we just directly jump to the case for the next iteration.
> >
> > I've never looked at the PCRE code to know if it's got cases like that.
> >
> > jeff
>
> I compiled PCRE but it never triggered this optimization (even if I
> bumped up the parameters for instruction counts and paths).
>
> I understand the desire not to add optimizations just for benchmarks but
> we do know other compilers have added this optimization for coremark
> (See
> http://community.arm.com/groups/embedded/blog/2013/02/21/coremark-and-compiler-performance) and the 13 people on the CC list for this bug certainly shows interest in having it even if it is just for a benchmark.  Does 'competing against other compilers' sound better then 'optimizing for a benchmark'?
>
> Steve Ellcey
> sellcey@mips.com

I've been told and have seen empirical evidence of this triggering in 
another well-known popular embedded benchmark suite.

Why not try this on SPEC2k(6) and see if it triggers with bumped up 
parameters to see if it applies elsewhere ?


regards
Ramana