Re: OpenACC dimension range propagation optimization

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

On Wed, Nov 4, 2015 at 2:56 PM, Nathan Sidwell <nathan@acm.org> wrote:
> On 11/04/15 05:26, Richard Biener wrote:
>>
>> On Tue, Nov 3, 2015 at 7:11 PM, Nathan Sidwell <nathan@acm.org> wrote:
>>>
>>> Richard,
>>> this patch implements VRP for the 2 openacc axis internal fns I've added.
>>> We know the position within  a dimension cannot exceed that dimensions
>>> extend. Further, if the extend is dynamic, the target backend may well
>>> know
>>> there's a hardware-mandated maximum value.
>>>
>>> Hence, added a new target hook to allow the backend to specify that upper
>>> bound, and added smarts to extract_range_basic to process the two
>>> internal
>>> functions.
>>>
>>> Incidentally, this was the bit I was working on at the cauldron, which
>>> caused me to work on the min/max range combining.
>>>
>>> ok for trunk?
>>
>>
>> +         /* Optimizing these two internal functions helps the loop
>> +            optimizer eliminate outer comparisons.  Size is [1,N]
>> +            and pos is [0,N-1].  */
>> +         {
>> +           bool is_pos = ifn_code == IFN_GOACC_DIM_POS;
>> +           tree attr = get_oacc_fn_attrib (current_function_decl);
>> +           tree arg = gimple_call_arg (stmt, 0);
>> +           int axis = TREE_INT_CST_LOW (arg);
>> +           tree dims = TREE_VALUE (attr);
>> +
>> +           for (int ix = axis; ix--;)
>> +             dims = TREE_CHAIN (dims);
>> +           int size = TREE_INT_CST_LOW (TREE_VALUE (dims));
>> +
>> +           if (!size)
>> +             /* If it's dynamic, the backend might know a hardware
>> +                limitation.  */
>> +             size = targetm.goacc.dim_limit (axis);
>>
>> this all seems a little bit fragile and relying on implementation details?
>> Is the attribute always present?
>
>
> Yes.  The ifns are inserted by a pass (execute_oacc_device_lower) that only
> operates on such functions. (I considered adding  an assert, but figured the
> resulting segfault  would be loud enough)
>
>> Is the call argument always a constant
>> that fits in a HOST_WIDE_INT (or even int here)?
>
>
> Yes.
>
>
> Are there always enough
>>
>> 'dims' in the tree list?
>
>
> Yes.  The oacc_device_lower pass has already validated and canonicalized the
> dimensions.
>
>
> Is the 'dim' value always an INTEGER_CST that
>>
>> fits a HOST_WIDE_INT (or even an int here)?
>
>
> Yes.  That's part of the validation.  see set_oacc_fn_attrib (omp-low.c)
>
>> I hope all these constraints (esp. 'int' fitting) are verified by the
>> parser.
>
>
> It's an internal function not visible the user.  Generation is entirely
> within the compiler
>
>> If so I'd like to see helper functions to hide these implementation
>> details
>> from generic code like this.
>
>
> ok.
>
>>
>> You miss to provide the known lower bound to VRP when size is 0
>> in the end.  Just unconditioonally do
>>
>>    tree type = TREE_TYPE (gimple_call_lhs (stmt));
>>    set_value_range (vr, VR_RANGE,
>>                             build_int_cst (type, is_pos ? 0 : 1),
>>                             size
>>                             ? build_int_cst (type, size - is_pos)
>>                             : vrp_val_max (type), NULL);
>
>
> I'm confused.  If size is zero, we never execute that path, and IIUC
> therefore never specify a range.  What you suggest looks like an additional
> improvement though.  Is that what you meant?

Yes.

> nathan