f12ad95991
PR #125442 replaces the pass-based Polly architecture with a monolithic pass consisting of phases. Reasons listed in https://github.com/llvm/llvm-project/pull/125442. With this change, the SCoP-passes became redundant problematic versions of the same functionality and are removed.
146 lines
4.9 KiB
C++
146 lines
4.9 KiB
C++
//===- DeadCodeElimination.cpp - Eliminate dead iteration ----------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// The polyhedral dead code elimination pass analyses a SCoP to eliminate
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// statement instances that can be proven dead.
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// As a consequence, the code generated for this SCoP may execute a statement
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// less often. This means, a statement may be executed only in certain loop
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// iterations or it may not even be part of the generated code at all.
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//
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// This code:
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//
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// for (i = 0; i < N; i++)
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// arr[i] = 0;
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// for (i = 0; i < N; i++)
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// arr[i] = 10;
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// for (i = 0; i < N; i++)
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// arr[i] = i;
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//
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// is e.g. simplified to:
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//
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// for (i = 0; i < N; i++)
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// arr[i] = i;
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//
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// The idea and the algorithm used was first implemented by Sven Verdoolaege in
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// the 'ppcg' tool.
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//
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//===----------------------------------------------------------------------===//
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#include "polly/DeadCodeElimination.h"
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#include "polly/DependenceInfo.h"
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#include "polly/Options.h"
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#include "polly/ScopInfo.h"
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#include "llvm/Support/CommandLine.h"
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#include "isl/isl-noexceptions.h"
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using namespace llvm;
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using namespace polly;
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namespace {
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cl::opt<int> DCEPreciseSteps(
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"polly-dce-precise-steps",
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cl::desc("The number of precise steps between two approximating "
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"iterations. (A value of -1 schedules another approximation stage "
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"before the actual dead code elimination."),
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cl::init(-1), cl::cat(PollyCategory));
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/// Return the set of live iterations.
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///
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/// The set of live iterations are all iterations that write to memory and for
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/// which we can not prove that there will be a later write that _must_
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/// overwrite the same memory location and is consequently the only one that
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/// is visible after the execution of the SCoP.
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///
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/// To compute the live outs, we compute for the data-locations that are
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/// must-written to the last statement that touches these locations. On top of
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/// this we add all statements that perform may-write accesses.
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///
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/// We could be more precise by removing may-write accesses for which we know
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/// that they are overwritten by a must-write after. However, at the moment the
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/// only may-writes we introduce access the full (unbounded) array, such that
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/// bounded write accesses can not overwrite all of the data-locations. As
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/// this means may-writes are in the current situation always live, there is
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/// no point in trying to remove them from the live-out set.
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static isl::union_set getLiveOut(Scop &S) {
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isl::union_map Schedule = S.getSchedule();
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isl::union_map MustWrites = S.getMustWrites();
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isl::union_map WriteIterations = MustWrites.reverse();
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isl::union_map WriteTimes = WriteIterations.apply_range(Schedule);
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isl::union_map LastWriteTimes = WriteTimes.lexmax();
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isl::union_map LastWriteIterations =
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LastWriteTimes.apply_range(Schedule.reverse());
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isl::union_set Live = LastWriteIterations.range();
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isl::union_map MayWrites = S.getMayWrites();
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Live = Live.unite(MayWrites.domain());
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return Live.coalesce();
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}
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/// Performs polyhedral dead iteration elimination by:
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/// o Assuming that the last write to each location is live.
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/// o Following each RAW dependency from a live iteration backwards and adding
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/// that iteration to the live set.
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///
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/// To ensure the set of live iterations does not get too complex we always
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/// combine a certain number of precise steps with one approximating step that
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/// simplifies the life set with an affine hull.
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static bool runDeadCodeElimination(Scop &S, int PreciseSteps,
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const Dependences &D) {
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if (!D.hasValidDependences())
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return false;
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isl::union_set Live = getLiveOut(S);
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isl::union_map Dep =
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D.getDependences(Dependences::TYPE_RAW | Dependences::TYPE_RED);
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Dep = Dep.reverse();
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if (PreciseSteps == -1)
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Live = Live.affine_hull();
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isl::union_set OriginalDomain = S.getDomains();
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int Steps = 0;
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while (true) {
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Steps++;
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isl::union_set Extra = Live.apply(Dep);
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if (Extra.is_subset(Live))
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break;
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Live = Live.unite(Extra);
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if (Steps > PreciseSteps) {
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Steps = 0;
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Live = Live.affine_hull();
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}
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Live = Live.intersect(OriginalDomain);
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}
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Live = Live.coalesce();
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return S.restrictDomains(Live);
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}
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} // namespace
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bool polly::runDeadCodeElim(Scop &S, DependenceAnalysis::Result &DA) {
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const Dependences &Deps = DA.getDependences(Dependences::AL_Statement);
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bool Changed = runDeadCodeElimination(S, DCEPreciseSteps, Deps);
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// FIXME: We can probably avoid the recomputation of all dependences by
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// updating them explicitly.
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if (Changed)
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DA.recomputeDependences(Dependences::AL_Statement);
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return Changed;
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}
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