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Re: RFC: Patch for switch elimination (PR 54742)
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Steve Ellcey <sellcey at mips dot com>
- Cc: Jeff Law <law at redhat dot com>, David Malcolm <dmalcolm at redhat dot com>, Sebastian Pop <sebpop at gmail dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>, james dot greenhalgh at arm dot com, Jakub Jelinek <jakub at redhat dot com>
- Date: Fri, 15 Aug 2014 12:13:41 +0200
- Subject: Re: RFC: Patch for switch elimination (PR 54742)
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On Thu, Aug 14, 2014 at 8:25 PM, Steve Ellcey <sellcey@mips.com> 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'?
Well - as an open-source compiler we have the luxury to not care
about "benchmark compilers" ;) At least that's what our non-existant
sales-team told me.
There are plenty "real" interpreters around which may have states
that deterministically forward to another state. If you optimize
those as well, fine.
Btw - the patch doesn't contain a single testcase ....
With coremark being "secret" what's the real-world testcase this
optimizes? Note that the benchmarks used in SPEC are usually
available and taken from real-world apps. I don't know coremark
at all, but from its name it sounds like sth like nullstone?
Richard.
> Steve Ellcey
> sellcey@mips.com
>