[flang] Inline hlfir.matmul[_transpose]. (#122821)

Inlining `hlfir.matmul` as `hlfir.eval_in_mem` does not allow
to get rid of a temporary array in many cases, but it may still be
much better allowing to:
  * Get rid of any overhead related to calling runtime MATMUL
    (such as descriptors creation).
  * Use CPU-specific vectorization cost model for matmul loops,
    which Fortran runtime cannot currently do.
  * Optimize matmul of known-size arrays by complete unrolling.

One of the drawbacks of `hlfir.eval_in_mem` inlining is that
the ops inside it with store memory effects block the current
MLIR CSE, so I decided to run this inlining late in the pipeline.
There is a source commen explaining the CSE issue in more detail.

Straightforward inlining of `hlfir.matmul` as an `hlfir.elemental`
is not good for performance, and I got performance regressions
with it comparing to Fortran runtime implementation. I put it
under an enigneering option for experiments.

At the same time, inlining `hlfir.matmul_transpose` as `hlfir.elemental`
seems to be a good approach, e.g. it allows getting rid of a temporay
array in cases like: `A(:)=B(:)+MATMUL(TRANSPOSE(C(:,:)),D(:))`.

This patch improves performance of galgel and tonto a little bit.

Author: vzakhari

flang/include/flang/Optimizer/Builder/FIRBuilder.h

@@ -804,6 +804,15 @@ elideLengthsAlreadyInType(mlir::Type type, mlir::ValueRange lenParams);
 /// Get the address space which should be used for allocas
 uint64_t getAllocaAddressSpace(mlir::DataLayout *dataLayout);
 
+/// The two vectors of MLIR values have the following property:
+///   \p extents1[i] must have the same value as \p extents2[i]
+/// The function returns a new vector of MLIR values that preserves
+/// the same property vs \p extents1 and \p extents2, but allows
+/// more optimizations. For example, if extents1[j] is a known constant,
+/// and extents2[j] is not, then result[j] is the MLIR value extents1[j].
+llvm::SmallVector<mlir::Value> deduceOptimalExtents(mlir::ValueRange extents1,
+                                                    mlir::ValueRange extents2);
+
 } // namespace fir::factory
 
 #endif // FORTRAN_OPTIMIZER_BUILDER_FIRBUILDER_H

flang/include/flang/Optimizer/Builder/HLFIRTools.h

@@ -508,6 +508,11 @@ genTypeAndKindConvert(mlir::Location loc, fir::FirOpBuilder &builder,
                       hlfir::Entity source, mlir::Type toType,
                       bool preserveLowerBounds);
 
+/// A shortcut for loadTrivialScalar(getElementAt()),
+/// which designates and loads an element of an array.
+Entity loadElementAt(mlir::Location loc, fir::FirOpBuilder &builder,
+                     Entity entity, mlir::ValueRange oneBasedIndices);
+
 } // namespace hlfir
 
 #endif // FORTRAN_OPTIMIZER_BUILDER_HLFIRTOOLS_H

flang/include/flang/Optimizer/HLFIR/Passes.td

@@ -43,6 +43,17 @@ def LowerHLFIROrderedAssignments : Pass<"lower-hlfir-ordered-assignments", "::ml
 
 def SimplifyHLFIRIntrinsics : Pass<"simplify-hlfir-intrinsics"> {
   let summary = "Simplify HLFIR intrinsic operations that don't need to result in runtime calls";
+  let options = [Option<"allowNewSideEffects", "allow-new-side-effects", "bool",
+                        /*default=*/"false",
+                        "If enabled, then the HLFIR operations simplification "
+                        "may introduce operations with side effects. "
+                        "For example, hlfir.matmul may be inlined as "
+                        "and hlfir.eval_in_mem with hlfir.assign inside it."
+                        "The hlfir.assign has a write effect on the memory "
+                        "argument of hlfir.eval_in_mem, which may block "
+                        "some existing MLIR transformations (e.g. CSE) "
+                        "that otherwise would have been possible across "
+                        "the hlfir.matmul.">];
 }
 
 def InlineElementals : Pass<"inline-elementals"> {

flang/lib/Optimizer/Builder/FIRBuilder.cpp

@@ -1740,3 +1740,17 @@ uint64_t fir::factory::getAllocaAddressSpace(mlir::DataLayout *dataLayout) {
       return mlir::cast<mlir::IntegerAttr>(addrSpace).getUInt();
   return 0;
 }
+
+llvm::SmallVector<mlir::Value>
+fir::factory::deduceOptimalExtents(mlir::ValueRange extents1,
+                                   mlir::ValueRange extents2) {
+  llvm::SmallVector<mlir::Value> extents;
+  extents.reserve(extents1.size());
+  for (auto [extent1, extent2] : llvm::zip(extents1, extents2)) {
+    if (!fir::getIntIfConstant(extent1) && fir::getIntIfConstant(extent2))
+      extents.push_back(extent2);
+    else
+      extents.push_back(extent1);
+  }
+  return extents;
+}

flang/lib/Optimizer/Builder/HLFIRTools.cpp

@@ -939,8 +939,10 @@ llvm::SmallVector<mlir::Value> hlfir::genLoopNestWithReductions(
       doLoop = builder.create<fir::DoLoopOp>(loc, one, ub, one, isUnordered,
                                              /*finalCountValue=*/false,
                                              parentLoop.getRegionIterArgs());
-      // Return the results of the child loop from its parent loop.
-      builder.create<fir::ResultOp>(loc, doLoop.getResults());
+      if (!reductionInits.empty()) {
+        // Return the results of the child loop from its parent loop.
+        builder.create<fir::ResultOp>(loc, doLoop.getResults());
+      }
     }
 
     builder.setInsertionPointToStart(doLoop.getBody());
@@ -955,7 +957,8 @@ llvm::SmallVector<mlir::Value> hlfir::genLoopNestWithReductions(
   reductionValues =
       genBody(loc, builder, oneBasedIndices, parentLoop.getRegionIterArgs());
   builder.setInsertionPointToEnd(parentLoop.getBody());
-  builder.create<fir::ResultOp>(loc, reductionValues);
+  if (!reductionValues.empty())
+    builder.create<fir::ResultOp>(loc, reductionValues);
   builder.setInsertionPointAfter(outerLoop);
   return outerLoop->getResults();
 }
@@ -1410,3 +1413,11 @@ void hlfir::computeEvaluateOpIn(mlir::Location loc, fir::FirOpBuilder &builder,
     builder.clone(op, mapper);
   return;
 }
+
+hlfir::Entity hlfir::loadElementAt(mlir::Location loc,
+                                   fir::FirOpBuilder &builder,
+                                   hlfir::Entity entity,
+                                   mlir::ValueRange oneBasedIndices) {
+  return loadTrivialScalar(loc, builder,
+                           getElementAt(loc, builder, entity, oneBasedIndices));
+}