r207433 - [driver] Disable the slp vectorizer at -O0, -O1, and -Oz. This mirrors the

[Eric Christopher]

On Mon, Apr 28, 2014 at 1:39 PM, Chandler Carruth <chandlerc at google.com> wrote:
> I think the real question is -Oz. My understanding is that the SLP
> vectorizer should always make code smaller.
>
> I don't think matching the loop vectorizer is important. I think the
> expectations of -O flags are not about the vectorization of code, but about
> the trade-offs between user experiences. While I've mentioned it on other
> threads:
>
> O1: quickly optimize what you can
> O2: our general ideal tradeoff between compile time and optimizations
> O3: (i wish we didn't have it) burn more compile time and/or binary size to
> try and dig out some optimizations.
> Os: optimize, but try to make the resulting code smaller where possible.
> Oz: do anything necessary to make code smaller, even things that hurt
> performance
>
> My impression was that SLP vectorization made sense at least for 2, 3, s,
> and z. If the compile time is low enough, maybe for O1 as well.
>

This makes sense to me. Partially my problem for not looking at what
Chad wrote enough to go more than "at O0? why?" and forgot to look at
the rest.

-eric

>
>
> On Mon, Apr 28, 2014 at 1:09 PM, Chad Rosier <mcrosier at codeaurora.org>
> wrote:
>>
>> One argument for matching the behavior of the loop vectorizer is that it's
>> probably what the user expects.
>>
>> > Hi Chandler,
>> > The discussion was in the PR as well as via IRC.  I'm certainly not
>> > against continuing the discussion.
>> >
>> > I believe everyone strongly agrees this shouldn't be on at -O0;  Let me
>> > know if you feel otherwise.  The decision to match the behavior of the
>> > loop vectorizer at -O1 and -Oz was mostly arbitrary and I have no strong
>> > feelings either way.
>> >
>> > What are your thoughts?
>> >
>> >  Chad
>> >
>> >> On Mon, Apr 28, 2014 at 12:30 PM, Chad Rosier
>> >> <mcrosier at codeaurora.org>wrote:
>> >>
>> >>> -O1, and -Oz
>> >>
>> >>
>> >> Wait, why? Based on what discussion?
>> >>
>> >> This is a pretty significant change, I'd like to understand why it was
>> >> made.
>> >>
>> >
>> >
>> > --
>> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>> > hosted by The Linux Foundation
>> >
>>
>>
>> --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>> hosted by The Linux Foundation
>>
>
>
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