[flang] add DoLoopHelper

[schweitz]

Summary

This diff adds the helper class for creating fir.do_loop operations.

A fir.do_loop models a Fortran DO loop. It is used to capture Fortran looping constructs for the high-level optimizer.

[kiranchandramohan]

Looks clear to me. A couple of questions before i can approve.

flang/lib/Lower/CMakeLists.txt

13

Is this is for a future patch?

Why is there no FIR dependency here?

flang/lib/Lower/DoLoopHelper.cpp

22

What is the story for down counting loops? Where are these handled? can the FIR loop dow count?

[schweitz]

flang/lib/Lower/CMakeLists.txt

13

I was seeing some build failures locally when trying to slurp in an appropriate sized hunk of code, so it seemed like a good idea to just grab these now. So, yes, these will be needed.

(More to come...)

flang/lib/Lower/DoLoopHelper.cpp

22

fir.do_loop handles count down loops as one would expect. (Step can be a signed integer value, constant or otherwise.)

Code gen (not part of this patch) also correctly lowers fir.do_loop. (The bug has been fixed.)

fir.do_loop thus has different semantics than scf.for.

[schweitz]

Add the reference to FIR ops table gen.
Update one of the MLIR table gen dependences (recent name change).

[kiranchandramohan]

LGTM.

flang/include/flang/Lower/DoLoopHelper.h

+//===-- Lower/DoLoopHelper.h -- gen fir.do_loop ops -------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FORTRAN_LOWER_DOLOOPHELPER_H
+#define FORTRAN_LOWER_DOLOOPHELPER_H
+
+#include "flang/Lower/FIRBuilder.h"
+
+namespace Fortran::lower {
+
+/// Helper to build fir.do_loop Ops.
+class DoLoopHelper {
+public:
+  explicit DoLoopHelper(FirOpBuilder &builder, mlir::Location loc)
+      : builder(builder), loc(loc) {}
+  DoLoopHelper(const DoLoopHelper &) = delete;
+
+  /// Type of a callback to generate the loop body.
+  using BodyGenerator = std::function<void(FirOpBuilder &, mlir::Value)>;
+
+  /// Build loop [\p lb, \p ub] with step \p step.
+  /// If \p step is an empty value, 1 is used for the step.
+  void createLoop(mlir::Value lb, mlir::Value ub, mlir::Value step,
+                  const BodyGenerator &bodyGenerator);
+
+  /// Build loop [\p lb,  \p ub] with step 1.
+  void createLoop(mlir::Value lb, mlir::Value ub,
+                  const BodyGenerator &bodyGenerator);
+
+  /// Build loop [0, \p count) with step 1.
+  void createLoop(mlir::Value count, const BodyGenerator &bodyGenerator);
+
+private:
+  FirOpBuilder &builder;
+  mlir::Location loc;
+};
+
+} // namespace Fortran::lower
+
+#endif // FORTRAN_LOWER_DOLOOPHELPER_H

flang/lib/Lower/CMakeLists.txt

 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error -Wno-unused-parameter")
 
 add_flang_library(FortranLower
-  ConvertType.cpp
   ComplexExpr.cpp
+  ConvertType.cpp
+  DoLoopHelper.cpp
   OpenMP.cpp
   PFTBuilder.cpp
 
+  DEPENDS
+  MLIROpAsmInterfacesIncGen
+  MLIRControlFlowInterfaces
+  MLIRLinalgStructuredOpsInterfaceIncGen
+
   LINK_COMPONENTS
   Support
 )

flang/lib/Lower/DoLoopHelper.cpp

+//===-- DoLoopHelper.cpp --------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang/Lower/DoLoopHelper.h"
+
+//===----------------------------------------------------------------------===//
+// DoLoopHelper implementation
+//===----------------------------------------------------------------------===//
+
+void Fortran::lower::DoLoopHelper::createLoop(
+    mlir::Value lb, mlir::Value ub, mlir::Value step,
+    const BodyGenerator &bodyGenerator) {
+  auto lbi = builder.convertToIndexType(loc, lb);
+  auto ubi = builder.convertToIndexType(loc, ub);
+  assert(step && "step must be an actual Value");
+  auto inc = builder.convertToIndexType(loc, step);
+  auto loop = builder.create<fir::LoopOp>(loc, lbi, ubi, inc);
+  auto insertPt = builder.saveInsertionPoint();
+  builder.setInsertionPointToStart(loop.getBody());
+  auto index = loop.getInductionVar();
+  bodyGenerator(builder, index);
+  builder.restoreInsertionPoint(insertPt);
+}
+
+void Fortran::lower::DoLoopHelper::createLoop(
+    mlir::Value lb, mlir::Value ub, const BodyGenerator &bodyGenerator) {
+  createLoop(lb, ub,
+             builder.createIntegerConstant(loc, builder.getIndexType(), 1),
+             bodyGenerator);
+}
+
+void Fortran::lower::DoLoopHelper::createLoop(
+    mlir::Value count, const BodyGenerator &bodyGenerator) {
+  auto indexType = builder.getIndexType();
+  auto zero = builder.createIntegerConstant(loc, indexType, 0);
+  auto one = builder.createIntegerConstant(loc, count.getType(), 1);
+  auto up = builder.create<mlir::SubIOp>(loc, count, one);
+  createLoop(zero, up, one, bodyGenerator);
+}