[Polly] Generalize the pattern matching to the case of tensor contractions

The pattern matching optimization of Polly detects and optimizes dense general
matrix-matrix multiplication. The generated code is close to high performance
implementations of matrix-matrix multiplications, which are contained in
manually tuned libraries. The described pattern matching optimization is
a particular case of tensor contraction optimization, which was
introduced in [1].

This patch generalizes the pattern matching to the case of tensor contractions
using the form of data dependencies and memory accesses produced by tensor
contractions [1].

Optimization of tensor contractions will be added in the next patch. Following
the ideas introduced in [2], it will logically represent tensor contraction
operands as matrix multiplication operands and use an approach for
optimization of matrix-matrix multiplications.

[1] - Gareev R., Grosser T., Kruse M. High-Performance Generalized Tensor
Op­erations: A Compiler-Oriented Approach // ACM Transactions on
Architec­ture and Code Optimization (TACO). 2018. Vol. 15, no. 3.
P. 34:1–34:27. DOI: 10.1145/3235029.

[2] - Matthews D. High-Performance Tensor Contraction without BLAS // SIAM
Journal on Scientific Computing. 2018. Vol. 40, no. 1. P. C 1—C 24.
DOI: 110.1137/16m108968x.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D114336

Author: gareevroman

polly/include/polly/Support/ISLTools.h

@@ -523,6 +523,11 @@ isl::set subtractParams(isl::set Set, isl::set Params);
 /// value. Otherwise, return NaN.
 isl::val getConstant(isl::pw_aff PwAff, bool Max, bool Min);
 
+/// If the relation @p PwAff lies on a hyperplane where the given
+/// dimension @p Pos with the type @p Dim has a fixed value, then
+/// return that value. Otherwise return NaN.
+isl::val getConstant(isl::map Map, isl::dim Dim, int Pos);
+
 /// Check that @p End is valid and return an iterator from @p Begin to @p End
 ///
 /// Use case example:

polly/lib/Support/ISLTools.cpp

@@ -263,6 +263,18 @@ isl::map polly::shiftDim(isl::map Map, isl::dim Dim, int Pos, int Amount) {
   }
 }
 
+isl::val polly::getConstant(isl::map Map, isl::dim Dim, int Pos) {
+  unsigned NumDims = unsignedFromIslSize(Map.dim(Dim));
+  if (Pos < 0)
+    Pos = NumDims + Pos;
+  assert(unsigned(Pos) < NumDims && "Dimension index must be in range");
+  // TODO: The isl_map_plain_get_val_if_fixed function is not robust, since its
+  // result is different depending on the internal representation.
+  // Replace it with a different implementation.
+  return isl::manage(isl_map_plain_get_val_if_fixed(
+      Map.get(), static_cast<enum isl_dim_type>(Dim), Pos));
+}
+
 isl::union_map polly::shiftDim(isl::union_map UMap, isl::dim Dim, int Pos,
                                int Amount) {
   isl::union_map Result = isl::union_map::empty(UMap.ctx());