On May 15, 2017 6:56:53 PM GMT+02:00, Steve Ellcey <sell...@cavium.com> wrote:
>On Sat, 2017-05-13 at 08:18 +0200, Richard Biener wrote:
>> On May 12, 2017 10:42:34 PM GMT+02:00, Steve Ellcey <sellcey@cavium.c
>> om> wrote:
>> >
>> > (Short version of this email, is there a way to recalculate/rebuild
>> > virtual
>> > phi nodes after modifying the CFG.)
>> >
>> > I have a question about duplicating loops and virtual phi nodes.
>> > I am trying to implement the following optimization as a pass:
>> >
>> > Transform:
>> >
>> > for (i = 0; i < n; i++) {
>> > A[i] = A[i] + B[i];
>> > C[i] = C[i-1] + D[i];
>> > }
>> >
>> > Into:
>> >
>> > if (noalias between A&B, A&C, A&D)
>> > for (i = 0; i < 100; i++)
>> > A[i] = A[i] + B[i];
>> > for (i = 0; i < 100; i++)
>> > C[i] = C[i-1] + D[i];
>> > else
>> > for (i = 0; i < 100; i++) {
>> > A[i] = A[i] + B[i];
>> > C[i] = C[i-1] + D[i];
>> > }
>> >
>> > Right now the vectorizer sees that 'C[i] = C[i-1] + D[i];' cannot
>be
>> > vectorized so it gives up and does not vectorize the loop. If we
>split
>> > up the loop into two loops then the vector add with A[i] could be
>> > vectorized
>> > even if the one with C[i] could not.
>> Loop distribution does this transform but it doesn't know about
>> versioning for unknown dependences.
>>
>
>Yes, I looked at loop distribution. But it only works with global
>arrays and not with pointer arguments where it doesn't know the size of
>the array being pointed at. I would like to be able to have it work
>with pointer arguments. If I call a function with 2 or
>more integer pointers, and I have a loop that accesses them with
>offsets between 0 and N where N is loop invariant then I should have
>enough information (at runtime) to determine if there are overlapping
>memory accesses through the pointers and determine whether or not I can
>distribute the loop.
Not sure where you got that from. Loop distribution works with our data
reference / dependence analysis. The cost model might be more restricted but
that can be fixed.
>The loop splitting code seemed like a better template since it already
>knows how to split a loop based on a runtime determined condition. That
>part seems to be working for me, it is when I try to
>distribute/duplicate one of those loops (under the unaliased condition)
>that I am running into the problem with virtual PHIs.
There's mark_virtual*for_renaming (sp?).
But as said you are performing loop distribution so please enhance the existing
pass rather than writing a new one.
Richard.
>Steve Ellcey
>sell...@cavium.com
