[llvm-dev] Loop Opt WG - using MLIR

Michael Kruse via llvm-dev llvm-dev at lists.llvm.org
Wed Aug 14 12:50:09 PDT 2019

Am Mi., 14. Aug. 2019 um 12:49 Uhr schrieb Gary Elsesser via llvm-dev
<llvm-dev at lists.llvm.org>:
> MLIR  is sufficiently expressive to capture general multidimensional
> array references without lose of information.  Fortran pointers
> may require speicial handling, as non-contiguous memory may be
> described - forward or backward in each dimension.
>     p => A(I1:I2:-3, J1:j2, K1:k2:K3)

Can you explain what this does?

> Fortran also permits empty arrays: call S(A(1:0)).

This should not be a problem since it will never be accessed.

> The BIG design chooses include:
>   1. Augment LLVM IR to provide a more general GEP; continue loop restructuring
>       work in LLVM; MLIR may be used for additional high-level optimization.
> OR
>    2. Create a middle-LLVM that works on MLIR; do all loop restructuring here;
>        phase out existing LLVM loop restructuring .
> OR
>    3. adopt MLIR in LLVM ... (a bridge too far).

LLVM is not going to add a dependency on MLIR. I think their goals are
different. Regarding (2.), flang is going to generate MLIR and some
transformations can be done their. LLVM will still do loop
optimizations if only for languages that do not generate MLIR, or have
semantics that are not high-level enough to be represented by MLIR
high-level constructs such as "affine.for". Unless you add some
raising analysis (e.g for-loop to affine.for; delinearization), you
are still stuck on the lower level constructs without gaining
anything. Same is true if you do LLVM->MLIR->LLVM.

Having said that, I believe a loop-centric  (in contrast to
control-flow) representation on which loop transformation work is a
good idea, which I am working on.

> In my mind, delinearization is a side issue.  The central problem is retaining
> subscript information needed for loop restructuring.  Broad agreement is
> clearly needed up front.

Agreed, it is preferable to get this information from the front-end
(if available) than trying to recover it.


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