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Canonicalizing OSSA provably minimizes the number of retains and releases within the boundaries of that lifetime. This eliminates the need for ad-hoc optimization of OSSA copies. This initial implementation only canonicalizes owned values, but canonicalizing guaranteed values is a simple extension. This was originally part of the CopyPropagation prototype years ago. Now OSSA is specified completely enough that it can be turned into a simple utility instead. CanonicalOSSALifetime uses PrunedLiveness to find the extended live range and identify the consumes on the boundary. All other consumes need their own copy. No other copies are needed. By running this after other transformations that affect OSSA lifetimes, we can avoid the need to run pattern-matching optimization to SemanticARC to recover from suboptimal patterns, which is not robust, maintainable, or efficient.
85 lines
3.1 KiB
C++
85 lines
3.1 KiB
C++
//===--------- CopyPropagation.cpp - Remove redundant SSA copies. ---------===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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///
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/// SSA Copy propagation pass to remove unnecessary copy_value and destroy_value
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/// instructions.
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///
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/// Because this algorithm rewrites copies and destroys without attempting to
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/// balance the retain count, it is only sound when SIL is in ownership-SSA
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/// form. The pass itself is mostly for testing the underlying functionality
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/// which can also be invoked as a utility for any owned value.
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///
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/// TODO: Cleanup the resulting SIL by deleting instructions that produce dead
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/// values (after removing its copies).
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///
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/// ===----------------------------------------------------------------------===
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#define DEBUG_TYPE "copy-propagation"
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#include "swift/SIL/BasicBlockUtils.h"
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#include "swift/SILOptimizer/PassManager/Passes.h"
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#include "swift/SILOptimizer/PassManager/Transforms.h"
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#include "swift/SILOptimizer/Utils/CanonicalOSSALifetime.h"
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using namespace swift;
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//===----------------------------------------------------------------------===//
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// CopyPropagation: Top-Level Function Transform.
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//===----------------------------------------------------------------------===//
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namespace {
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class CopyPropagation : public SILFunctionTransform {
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/// The entry point to this function transformation.
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void run() override;
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};
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} // end anonymous namespace
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/// Top-level pass driver.
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void CopyPropagation::run() {
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auto *f = getFunction();
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// Debug label for unit testing.
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LLVM_DEBUG(llvm::dbgs() << "*** CopyPropagation: " << f->getName() << "\n");
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// This algorithm fundamentally assumes ownership.
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if (!f->hasOwnership())
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return;
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// Driver: Find all copied defs.
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llvm::SmallSetVector<SILValue, 16> copiedDefs;
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for (auto &bb : *f) {
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for (auto &i : bb) {
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if (auto *copy = dyn_cast<CopyValueInst>(&i))
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copiedDefs.insert(
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CanonicalizeOSSALifetime::getCanonicalCopiedDef(copy));
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}
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}
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// Perform copy propgation for each copied value.
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CanonicalizeOSSALifetime canonicalizer(/*pruneDebug*/ true);
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for (auto &def : copiedDefs) {
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canonicalizer.canonicalizeValueLifetime(def);
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if (SILValue outerCopy = canonicalizer.createdOuterCopy()) {
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SILValue outerDef = canonicalizer.getCanonicalCopiedDef(outerCopy);
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canonicalizer.canonicalizeValueLifetime(outerDef);
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}
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// TODO: also canonicalize any lifetime.persistentCopies like separate owned
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// live ranges.
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}
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if (canonicalizer.hasChanged()) {
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invalidateAnalysis(SILAnalysis::InvalidationKind::Instructions);
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DeadEndBlocks deBlocks(f);
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f->verifyOwnership(&deBlocks);
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}
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}
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SILTransform *swift::createCopyPropagation() { return new CopyPropagation(); }
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