Re: New GCC options for loop vectorization

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Sep 13, 2013 at 5:16 PM, Xinliang David Li <davidxl@google.com> wrote:
> On Fri, Sep 13, 2013 at 1:30 AM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Thu, Sep 12, 2013 at 10:31 PM, Xinliang David Li <davidxl@google.com> wrote:
>>> Currently -ftree-vectorize turns on both loop and slp vectorizations,
>>> but there is no simple way to turn on loop vectorization alone. The
>>> logic for default O3 setting is also complicated.
>>>
>>> In this patch, two new options are introduced:
>>>
>>> 1) -ftree-loop-vectorize
>>>
>>> This option is used to turn on loop vectorization only. option
>>> -ftree-slp-vectorize also becomes a first class citizen, and no funny
>>> business of Init(2) is needed.  With this change, -ftree-vectorize
>>> becomes a simple alias to -ftree-loop-vectorize +
>>> -ftree-slp-vectorize.
>>>
>>> For instance, to turn on only slp vectorize at O3, the old way is:
>>>
>>>      -O3 -fno-tree-vectorize -ftree-slp-vectorize
>>>
>>> With the new change it becomes:
>>>
>>>     -O3 -fno-loop-vectorize
>>>
>>>
>>> To turn on only loop vectorize at O2, the old way is
>>>
>>>     -O2 -ftree-vectorize -fno-slp-vectorize
>>>
>>> The new way is
>>>
>>>     -O2 -ftree-loop-vectorize
>>>
>>>
>>>
>>> 2) -ftree-vect-loop-peeling
>>>
>>> This option is used to turn on/off loop peeling for alignment.  In the
>>> long run, this should be folded into the cheap cost model proposed by
>>> Richard.  This option is also useful in scenarios where peeling can
>>> introduce runtime problems:
>>> http://gcc.gnu.org/ml/gcc/2005-12/msg00390.html  which happens to be
>>> common in practice.
>>>
>>>
>>>
>>> Patch attached. Compiler boostrapped. Ok after testing?
>>
>> I'd like you to split 1) and 2), mainly because I agree on 1) but not on 2).
>
> Ok. Can you also comment on 2) ?

I think we want to decide how granular we want to control the vectorizer
and using which mechanism.  My cost-model re-org makes
ftree-vect-loop-version a no-op (basically removes it), so 2) looks like
a step backwards in this context.

So, can you summarize what pieces (including versioning) of the vectorizer
you'd want to be able to disable separately?  Just disabling peeling for
alignment may get you into the versioning for alignment path (and thus
an unvectorized loop at runtime).  Also it's know that the alignment peeling
code needs some serious TLC (it's outcome depends on the order of DRs,
the cost model it uses leaves to be desired as we cannot distinguish
between unaligned load and store costs).

Richard.

>>
>> I've stopped a quick try doing 1) myself because
>>
>> @@ -1691,6 +1695,12 @@ common_handle_option (struct gcc_options
>>          opts->x_flag_ipa_reference = false;
>>        break;
>>
>> +    case OPT_ftree_vectorize:
>> +      if (!opts_set->x_flag_tree_loop_vectorize)
>> + opts->x_flag_tree_loop_vectorize = value;
>> +      if (!opts_set->x_flag_tree_slp_vectorize)
>> + opts->x_flag_tree_slp_vectorize = value;
>> +      break;
>>
>> doesn't look obviously correct.  Does that handle
>>
>>   -ftree-vectorize -fno-tree-loop-vectorize -ftree-vectorize
>>
>> or
>>
>>   -ftree-loop-vectorize -fno-tree-vectorize
>>
>> properly?  Currently at least
>>
>>   -ftree-slp-vectorize -fno-tree-vectorize
>>
>> doesn't "work".
>
>
> Right -- same is true for -fprofile-use option. FDO enables some
> passes, but can not re-enable them if they are flipped off before.
>
>>
>> That said, the option machinery doesn't handle an option being an alias
>> for two other options, so it's mechanism to contract positives/negatives
>> doesn't work here and the override hooks do not work reliably for
>> repeated options.
>>
>> Or am I wrong here?  Should we care at all?  Joseph?
>
> We should probably just document the behavior. Even better, we should
> deprecate the old option.
>
> thanks,
>
> David
>
>>
>> Thanks,
>> Richard.
>>
>>>
>>> thanks,
>>>
>>> David