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Re: RFC: Improving GCC8 default option settings


On Thu, Sep 14, 2017 at 12:18 PM, Martin Liška <mliska@suse.cz> wrote:
> On 09/14/2017 12:07 PM, Markus Trippelsdorf wrote:
>> On 2017.09.14 at 11:57 +0200, Richard Biener wrote:
>>> On Wed, Sep 13, 2017 at 6:11 PM, Nikos Chantziaras <realnc@gmail.com> wrote:
>>>> On 12/09/17 16:57, Wilco Dijkstra wrote:
>>>>>
>>>>> [...] As a result users are
>>>>> required to enable several additional optimizations by hand to get good
>>>>> code.
>>>>> Other compilers enable more optimizations at -O2 (loop unrolling in LLVM
>>>>> was
>>>>> mentioned repeatedly) which GCC could/should do as well.
>>>>> [...]
>>>>>
>>>>> I'd welcome discussion and other proposals for similar improvements.
>>>>
>>>>
>>>> What's the status of graphite? It's been around for years. Isn't it mature
>>>> enough to enable these:
>>>>
>>>> -floop-interchange -ftree-loop-distribution -floop-strip-mine -floop-block
>>>>
>>>> by default for -O2? (And I'm not even sure those are the complete set of
>>>> graphite optimization flags, or just the "useful" ones.)
>>>
>>> It's not on by default at any optimization level.  The main issue is the
>>> lack of maintainance and a set of known common internal compiler errors
>>> we hit.  The other issue is that there's no benefit of turning those on for
>>> SPEC CPU benchmarking as far as I remember but quite a bit of extra
>>> compile-time cost.
>>
>> Not to mention the numerous wrong-code bugs. IMHO graphite should
>> deprecated as soon as possible.
>>
>
> For wrong-code bugs we've got and I recently went through, I fully agree with this
> approach and I would do it for GCC 8. There are PRs where order of simple 2 loops
> is changed, causing wrong-code as there's a data dependence.
>
> Moreover, I know that Bin was thinking about selection whether to use classical loop
> optimizations or Graphite (depending on options provided). This would simplify it ;)

I don't think removing graphite is warranted, I still think it is the
approach to use when
handling non-perfect nests.

Richard.

> Martin


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