//===- DeadCodeElimination.cpp - Eliminate dead iteration ----------------===// // // 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 // //===----------------------------------------------------------------------===// // // The polyhedral dead code elimination pass analyses a SCoP to eliminate // statement instances that can be proven dead. // As a consequence, the code generated for this SCoP may execute a statement // less often. This means, a statement may be executed only in certain loop // iterations or it may not even be part of the generated code at all. // // This code: // // for (i = 0; i < N; i++) // arr[i] = 0; // for (i = 0; i < N; i++) // arr[i] = 10; // for (i = 0; i < N; i++) // arr[i] = i; // // is e.g. simplified to: // // for (i = 0; i < N; i++) // arr[i] = i; // // The idea and the algorithm used was first implemented by Sven Verdoolaege in // the 'ppcg' tool. // //===----------------------------------------------------------------------===// #include "polly/DeadCodeElimination.h" #include "polly/DependenceInfo.h" #include "polly/LinkAllPasses.h" #include "polly/Options.h" #include "polly/ScopInfo.h" #include "llvm/Support/CommandLine.h" #include "isl/isl-noexceptions.h" usingnamespacellvm; usingnamespacepolly; namespace { cl::opt<int> DCEPreciseSteps( "polly-dce-precise-steps", cl::desc("The number of precise steps between two approximating " "iterations. (A value of -1 schedules another approximation stage " "before the actual dead code elimination."), cl::init(-1), cl::cat(PollyCategory)); class DeadCodeElimWrapperPass final : public ScopPass { … }; char DeadCodeElimWrapperPass::ID = …; /// Return the set of live iterations. /// /// The set of live iterations are all iterations that write to memory and for /// which we can not prove that there will be a later write that _must_ /// overwrite the same memory location and is consequently the only one that /// is visible after the execution of the SCoP. /// /// To compute the live outs, we compute for the data-locations that are /// must-written to the last statement that touches these locations. On top of /// this we add all statements that perform may-write accesses. /// /// We could be more precise by removing may-write accesses for which we know /// that they are overwritten by a must-write after. However, at the moment the /// only may-writes we introduce access the full (unbounded) array, such that /// bounded write accesses can not overwrite all of the data-locations. As /// this means may-writes are in the current situation always live, there is /// no point in trying to remove them from the live-out set. static isl::union_set getLiveOut(Scop &S) { … } /// Performs polyhedral dead iteration elimination by: /// o Assuming that the last write to each location is live. /// o Following each RAW dependency from a live iteration backwards and adding /// that iteration to the live set. /// /// To ensure the set of live iterations does not get too complex we always /// combine a certain number of precise steps with one approximating step that /// simplifies the life set with an affine hull. static bool runDeadCodeElimination(Scop &S, int PreciseSteps, const Dependences &D) { … } bool DeadCodeElimWrapperPass::runOnScop(Scop &S) { … } void DeadCodeElimWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const { … } } // namespace Pass *polly::createDeadCodeElimWrapperPass() { … } llvm::PreservedAnalyses DeadCodeElimPass::run(Scop &S, ScopAnalysisManager &SAM, ScopStandardAnalysisResults &SAR, SPMUpdater &U) { … } INITIALIZE_PASS_BEGIN(DeadCodeElimWrapperPass, "polly-dce", "Polly - Remove dead iterations", false, false) INITIALIZE_PASS_DEPENDENCY(DependenceInfo) INITIALIZE_PASS_DEPENDENCY(ScopInfoRegionPass) INITIALIZE_PASS_END(DeadCodeElimWrapperPass, "polly-dce", "Polly - Remove dead iterations", false, false)