1 //===- SimpleLoopUnswitch.h - Hoist loop-invariant control flow -*- C++ -*-===// 2 // 3 // Part of the LLVM 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 #ifndef LLVM_TRANSFORMS_SCALAR_SIMPLELOOPUNSWITCH_H 10 #define LLVM_TRANSFORMS_SCALAR_SIMPLELOOPUNSWITCH_H 11 12 #include "llvm/Analysis/LoopAnalysisManager.h" 13 #include "llvm/Analysis/LoopInfo.h" 14 #include "llvm/IR/PassManager.h" 15 #include "llvm/Transforms/Scalar/LoopPassManager.h" 16 17 namespace llvm { 18 19 /// This pass transforms loops that contain branches or switches on loop- 20 /// invariant conditions to have multiple loops. For example, it turns the left 21 /// into the right code: 22 /// 23 /// for (...) if (lic) 24 /// A for (...) 25 /// if (lic) A; B; C 26 /// B else 27 /// C for (...) 28 /// A; C 29 /// 30 /// This can increase the size of the code exponentially (doubling it every time 31 /// a loop is unswitched) so we only unswitch if the resultant code will be 32 /// smaller than a threshold. 33 /// 34 /// This pass expects LICM to be run before it to hoist invariant conditions out 35 /// of the loop, to make the unswitching opportunity obvious. 36 /// 37 /// There is a taxonomy of unswitching that we use to classify different forms 38 /// of this transformaiton: 39 /// 40 /// - Trival unswitching: this is when the condition can be unswitched without 41 /// cloning any code from inside the loop. A non-trivial unswitch requires 42 /// code duplication. 43 /// 44 /// - Full unswitching: this is when the branch or switch is completely moved 45 /// from inside the loop to outside the loop. Partial unswitching removes the 46 /// branch from the clone of the loop but must leave a (somewhat simplified) 47 /// branch in the original loop. While theoretically partial unswitching can 48 /// be done for switches, the requirements are extreme - we need the loop 49 /// invariant input to the switch to be sufficient to collapse to a single 50 /// successor in each clone. 51 /// 52 /// This pass always does trivial, full unswitching for both branches and 53 /// switches. For branches, it also always does trivial, partial unswitching. 54 /// 55 /// If enabled (via the constructor's `NonTrivial` parameter), this pass will 56 /// additionally do non-trivial, full unswitching for branches and switches, and 57 /// will do non-trivial, partial unswitching for branches. 58 /// 59 /// Because partial unswitching of switches is extremely unlikely to be possible 60 /// in practice and significantly complicates the implementation, this pass does 61 /// not currently implement that in any mode. 62 class SimpleLoopUnswitchPass : public PassInfoMixin<SimpleLoopUnswitchPass> { 63 bool NonTrivial; 64 65 public: 66 SimpleLoopUnswitchPass(bool NonTrivial = false) : NonTrivial(NonTrivial) {} 67 68 PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM, 69 LoopStandardAnalysisResults &AR, LPMUpdater &U); 70 }; 71 72 /// Create the legacy pass object for the simple loop unswitcher. 73 /// 74 /// See the documentaion for `SimpleLoopUnswitchPass` for details. 75 Pass *createSimpleLoopUnswitchLegacyPass(bool NonTrivial = false); 76 77 } // end namespace llvm 78 79 #endif // LLVM_TRANSFORMS_SCALAR_SIMPLELOOPUNSWITCH_H 80
Indexes created Fri Sep 25 00:27:33 UTC 2026