1 //===- LoopInvariantCodeMotion.cpp - Code to perform loop fusion-----------===//
2 //
3 // Part of the MLIR Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements loop invariant code motion.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "mlir/Transforms/Passes.h"
14 
15 #include "mlir/IR/Builders.h"
16 #include "mlir/IR/Function.h"
17 #include "mlir/Pass/Pass.h"
18 #include "mlir/Transforms/LoopLikeInterface.h"
19 #include "mlir/Transforms/SideEffectsInterface.h"
20 #include "llvm/ADT/SmallPtrSet.h"
21 #include "llvm/Support/CommandLine.h"
22 #include "llvm/Support/Debug.h"
23 
24 #define DEBUG_TYPE "licm"
25 
26 using namespace mlir;
27 
28 namespace {
29 
30 using SideEffecting = SideEffectsInterface::SideEffecting;
31 
32 /// Loop invariant code motion (LICM) pass.
33 struct LoopInvariantCodeMotion : public OperationPass<LoopInvariantCodeMotion> {
34 public:
35   void runOnOperation() override;
36 };
37 
38 // Checks whether the given op can be hoisted by checking that
39 // - the op and any of its contained operations do not depend on SSA values
40 //   defined inside of the loop (by means of calling definedOutside).
41 // - the op has no side-effects. If sideEffecting is Never, sideeffects of this
42 //   op and its nested ops are ignored.
43 static bool canBeHoisted(Operation *op,
44                          function_ref<bool(Value)> definedOutside,
45                          SideEffecting sideEffecting,
46                          SideEffectsInterface &interface) {
47   // Check that dependencies are defined outside of loop.
48   if (!llvm::all_of(op->getOperands(), definedOutside))
49     return false;
50   // Check whether this op is side-effect free. If we already know that there
51   // can be no side-effects because the surrounding op has claimed so, we can
52   // (and have to) skip this step.
53   auto thisOpIsSideEffecting = sideEffecting;
54   if (thisOpIsSideEffecting != SideEffecting::Never) {
55     thisOpIsSideEffecting = interface.isSideEffecting(op);
56     // If the op always has sideeffects, we cannot hoist.
57     if (thisOpIsSideEffecting == SideEffecting::Always)
58       return false;
59   }
60   // Recurse into the regions for this op and check whether the contained ops
61   // can be hoisted.
62   for (auto &region : op->getRegions()) {
63     for (auto &block : region.getBlocks()) {
64       for (auto &innerOp : block) {
65         if (innerOp.isKnownTerminator())
66           continue;
67         if (!canBeHoisted(&innerOp, definedOutside, thisOpIsSideEffecting,
68                           interface))
69           return false;
70       }
71     }
72   }
73   return true;
74 }
75 
76 static LogicalResult moveLoopInvariantCode(LoopLikeOpInterface looplike,
77                                            SideEffectsInterface &interface) {
78   auto &loopBody = looplike.getLoopBody();
79 
80   // We use two collections here as we need to preserve the order for insertion
81   // and this is easiest.
82   SmallPtrSet<Operation *, 8> willBeMovedSet;
83   SmallVector<Operation *, 8> opsToMove;
84 
85   // Helper to check whether an operation is loop invariant wrt. SSA properties.
86   auto isDefinedOutsideOfBody = [&](Value value) {
87     auto definingOp = value.getDefiningOp();
88     return (definingOp && !!willBeMovedSet.count(definingOp)) ||
89            looplike.isDefinedOutsideOfLoop(value);
90   };
91 
92   // Do not use walk here, as we do not want to go into nested regions and hoist
93   // operations from there. These regions might have semantics unknown to this
94   // rewriting. If the nested regions are loops, they will have been processed.
95   for (auto &block : loopBody) {
96     for (auto &op : block.without_terminator()) {
97       if (canBeHoisted(&op, isDefinedOutsideOfBody,
98                        mlir::SideEffectsDialectInterface::Recursive,
99                        interface)) {
100         opsToMove.push_back(&op);
101         willBeMovedSet.insert(&op);
102       }
103     }
104   }
105 
106   // For all instructions that we found to be invariant, move outside of the
107   // loop.
108   auto result = looplike.moveOutOfLoop(opsToMove);
109   LLVM_DEBUG(looplike.print(llvm::dbgs() << "Modified loop\n"));
110   return result;
111 }
112 
113 } // end anonymous namespace
114 
115 void LoopInvariantCodeMotion::runOnOperation() {
116   SideEffectsInterface interface(&getContext());
117   // Walk through all loops in a function in innermost-loop-first order. This
118   // way, we first LICM from the inner loop, and place the ops in
119   // the outer loop, which in turn can be further LICM'ed.
120   getOperation()->walk([&](Operation *op) {
121     if (auto looplike = dyn_cast<LoopLikeOpInterface>(op)) {
122       LLVM_DEBUG(op->print(llvm::dbgs() << "\nOriginal loop\n"));
123       if (failed(moveLoopInvariantCode(looplike, interface)))
124         signalPassFailure();
125     }
126   });
127 }
128 
129 // Include the generated code for the loop-like interface here, as it otherwise
130 // has no compilation unit. This works as loop-invariant code motion is the
131 // only user of that interface.
132 #include "mlir/Transforms/LoopLikeInterface.cpp.inc"
133 
134 std::unique_ptr<Pass> mlir::createLoopInvariantCodeMotionPass() {
135   return std::make_unique<LoopInvariantCodeMotion>();
136 }
137 
138 static PassRegistration<LoopInvariantCodeMotion>
139     pass("loop-invariant-code-motion",
140          "Hoist loop invariant instructions outside of the loop");
141