05da2fe521
This file lists every pass in LLVM, and is included by Pass.h, which is very popular. Every time we add, remove, or rename a pass in LLVM, it caused lots of recompilation. I found this fact by looking at this table, which is sorted by the number of times a file was changed over the last 100,000 git commits multiplied by the number of object files that depend on it in the current checkout: recompiles touches affected_files header 342380 95 3604 llvm/include/llvm/ADT/STLExtras.h 314730 234 1345 llvm/include/llvm/InitializePasses.h 307036 118 2602 llvm/include/llvm/ADT/APInt.h 213049 59 3611 llvm/include/llvm/Support/MathExtras.h 170422 47 3626 llvm/include/llvm/Support/Compiler.h 162225 45 3605 llvm/include/llvm/ADT/Optional.h 158319 63 2513 llvm/include/llvm/ADT/Triple.h 140322 39 3598 llvm/include/llvm/ADT/StringRef.h 137647 59 2333 llvm/include/llvm/Support/Error.h 131619 73 1803 llvm/include/llvm/Support/FileSystem.h Before this change, touching InitializePasses.h would cause 1345 files to recompile. After this change, touching it only causes 550 compiles in an incremental rebuild. Reviewers: bkramer, asbirlea, bollu, jdoerfert Differential Revision: https://reviews.llvm.org/D70211
428 lines
15 KiB
C++
428 lines
15 KiB
C++
//===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===//
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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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//! \file
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//! This pass performs merges of loads and stores on both sides of a
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// diamond (hammock). It hoists the loads and sinks the stores.
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//
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// The algorithm iteratively hoists two loads to the same address out of a
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// diamond (hammock) and merges them into a single load in the header. Similar
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// it sinks and merges two stores to the tail block (footer). The algorithm
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// iterates over the instructions of one side of the diamond and attempts to
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// find a matching load/store on the other side. New tail/footer block may be
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// insterted if the tail/footer block has more predecessors (not only the two
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// predecessors that are forming the diamond). It hoists / sinks when it thinks
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// it safe to do so. This optimization helps with eg. hiding load latencies,
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// triggering if-conversion, and reducing static code size.
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//
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// NOTE: This code no longer performs load hoisting, it is subsumed by GVNHoist.
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//
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//===----------------------------------------------------------------------===//
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//
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//
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// Example:
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// Diamond shaped code before merge:
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//
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// header:
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// br %cond, label %if.then, label %if.else
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// + +
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// + +
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// + +
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// if.then: if.else:
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// %lt = load %addr_l %le = load %addr_l
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// <use %lt> <use %le>
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// <...> <...>
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// store %st, %addr_s store %se, %addr_s
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// br label %if.end br label %if.end
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// + +
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// + +
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// + +
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// if.end ("footer"):
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// <...>
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//
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// Diamond shaped code after merge:
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//
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// header:
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// %l = load %addr_l
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// br %cond, label %if.then, label %if.else
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// + +
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// + +
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// + +
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// if.then: if.else:
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// <use %l> <use %l>
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// <...> <...>
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// br label %if.end br label %if.end
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// + +
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// + +
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// + +
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// if.end ("footer"):
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// %s.sink = phi [%st, if.then], [%se, if.else]
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// <...>
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// store %s.sink, %addr_s
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// <...>
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//
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//
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//===----------------------- TODO -----------------------------------------===//
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//
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// 1) Generalize to regions other than diamonds
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// 2) Be more aggressive merging memory operations
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// Note that both changes require register pressure control
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/MergedLoadStoreMotion.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/CFG.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/Loads.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/Metadata.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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using namespace llvm;
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#define DEBUG_TYPE "mldst-motion"
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namespace {
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//===----------------------------------------------------------------------===//
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// MergedLoadStoreMotion Pass
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//===----------------------------------------------------------------------===//
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class MergedLoadStoreMotion {
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AliasAnalysis *AA = nullptr;
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// The mergeLoad/Store algorithms could have Size0 * Size1 complexity,
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// where Size0 and Size1 are the #instructions on the two sides of
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// the diamond. The constant chosen here is arbitrary. Compiler Time
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// Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.
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const int MagicCompileTimeControl = 250;
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const bool SplitFooterBB;
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public:
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MergedLoadStoreMotion(bool SplitFooterBB) : SplitFooterBB(SplitFooterBB) {}
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bool run(Function &F, AliasAnalysis &AA);
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private:
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BasicBlock *getDiamondTail(BasicBlock *BB);
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bool isDiamondHead(BasicBlock *BB);
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// Routines for sinking stores
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StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);
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PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);
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bool isStoreSinkBarrierInRange(const Instruction &Start,
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const Instruction &End, MemoryLocation Loc);
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bool canSinkStoresAndGEPs(StoreInst *S0, StoreInst *S1) const;
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void sinkStoresAndGEPs(BasicBlock *BB, StoreInst *SinkCand,
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StoreInst *ElseInst);
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bool mergeStores(BasicBlock *BB);
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};
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} // end anonymous namespace
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///
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/// Return tail block of a diamond.
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///
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BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
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assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
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return BB->getTerminator()->getSuccessor(0)->getSingleSuccessor();
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}
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///
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/// True when BB is the head of a diamond (hammock)
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///
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bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
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if (!BB)
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return false;
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auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (!BI || !BI->isConditional())
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return false;
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BasicBlock *Succ0 = BI->getSuccessor(0);
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BasicBlock *Succ1 = BI->getSuccessor(1);
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if (!Succ0->getSinglePredecessor())
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return false;
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if (!Succ1->getSinglePredecessor())
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return false;
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BasicBlock *Succ0Succ = Succ0->getSingleSuccessor();
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BasicBlock *Succ1Succ = Succ1->getSingleSuccessor();
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// Ignore triangles.
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if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ)
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return false;
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return true;
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}
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///
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/// True when instruction is a sink barrier for a store
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/// located in Loc
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///
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/// Whenever an instruction could possibly read or modify the
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/// value being stored or protect against the store from
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/// happening it is considered a sink barrier.
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///
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bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,
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const Instruction &End,
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MemoryLocation Loc) {
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for (const Instruction &Inst :
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make_range(Start.getIterator(), End.getIterator()))
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if (Inst.mayThrow())
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return true;
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return AA->canInstructionRangeModRef(Start, End, Loc, ModRefInfo::ModRef);
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}
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///
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/// Check if \p BB contains a store to the same address as \p SI
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///
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/// \return The store in \p when it is safe to sink. Otherwise return Null.
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///
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StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1,
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StoreInst *Store0) {
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LLVM_DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n");
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BasicBlock *BB0 = Store0->getParent();
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for (Instruction &Inst : reverse(*BB1)) {
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auto *Store1 = dyn_cast<StoreInst>(&Inst);
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if (!Store1)
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continue;
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MemoryLocation Loc0 = MemoryLocation::get(Store0);
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MemoryLocation Loc1 = MemoryLocation::get(Store1);
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if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) &&
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!isStoreSinkBarrierInRange(*Store1->getNextNode(), BB1->back(), Loc1) &&
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!isStoreSinkBarrierInRange(*Store0->getNextNode(), BB0->back(), Loc0)) {
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return Store1;
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}
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}
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return nullptr;
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}
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///
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/// Create a PHI node in BB for the operands of S0 and S1
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///
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PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
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StoreInst *S1) {
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// Create a phi if the values mismatch.
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Value *Opd1 = S0->getValueOperand();
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Value *Opd2 = S1->getValueOperand();
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if (Opd1 == Opd2)
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return nullptr;
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auto *NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink",
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&BB->front());
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NewPN->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc());
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NewPN->addIncoming(Opd1, S0->getParent());
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NewPN->addIncoming(Opd2, S1->getParent());
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return NewPN;
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}
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///
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/// Check if 2 stores can be sunk together with corresponding GEPs
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///
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bool MergedLoadStoreMotion::canSinkStoresAndGEPs(StoreInst *S0,
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StoreInst *S1) const {
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auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
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auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
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return A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() &&
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(A0->getParent() == S0->getParent()) && A1->hasOneUse() &&
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(A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0);
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}
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///
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/// Merge two stores to same address and sink into \p BB
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///
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/// Also sinks GEP instruction computing the store address
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///
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void MergedLoadStoreMotion::sinkStoresAndGEPs(BasicBlock *BB, StoreInst *S0,
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StoreInst *S1) {
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// Only one definition?
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auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
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auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
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LLVM_DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();
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dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";
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dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");
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// Hoist the instruction.
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BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();
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// Intersect optional metadata.
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S0->andIRFlags(S1);
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S0->dropUnknownNonDebugMetadata();
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// Create the new store to be inserted at the join point.
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StoreInst *SNew = cast<StoreInst>(S0->clone());
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Instruction *ANew = A0->clone();
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SNew->insertBefore(&*InsertPt);
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ANew->insertBefore(SNew);
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assert(S0->getParent() == A0->getParent());
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assert(S1->getParent() == A1->getParent());
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// New PHI operand? Use it.
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if (PHINode *NewPN = getPHIOperand(BB, S0, S1))
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SNew->setOperand(0, NewPN);
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S0->eraseFromParent();
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S1->eraseFromParent();
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A0->replaceAllUsesWith(ANew);
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A0->eraseFromParent();
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A1->replaceAllUsesWith(ANew);
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A1->eraseFromParent();
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}
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///
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/// True when two stores are equivalent and can sink into the footer
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///
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/// Starting from a diamond head block, iterate over the instructions in one
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/// successor block and try to match a store in the second successor.
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///
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bool MergedLoadStoreMotion::mergeStores(BasicBlock *HeadBB) {
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bool MergedStores = false;
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BasicBlock *TailBB = getDiamondTail(HeadBB);
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BasicBlock *SinkBB = TailBB;
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assert(SinkBB && "Footer of a diamond cannot be empty");
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succ_iterator SI = succ_begin(HeadBB);
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assert(SI != succ_end(HeadBB) && "Diamond head cannot have zero successors");
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BasicBlock *Pred0 = *SI;
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++SI;
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assert(SI != succ_end(HeadBB) && "Diamond head cannot have single successor");
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BasicBlock *Pred1 = *SI;
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// tail block of a diamond/hammock?
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if (Pred0 == Pred1)
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return false; // No.
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// bail out early if we can not merge into the footer BB
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if (!SplitFooterBB && TailBB->hasNPredecessorsOrMore(3))
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return false;
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// #Instructions in Pred1 for Compile Time Control
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auto InstsNoDbg = Pred1->instructionsWithoutDebug();
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int Size1 = std::distance(InstsNoDbg.begin(), InstsNoDbg.end());
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int NStores = 0;
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for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
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RBI != RBE;) {
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Instruction *I = &*RBI;
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++RBI;
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// Don't sink non-simple (atomic, volatile) stores.
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auto *S0 = dyn_cast<StoreInst>(I);
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if (!S0 || !S0->isSimple())
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continue;
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++NStores;
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if (NStores * Size1 >= MagicCompileTimeControl)
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break;
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if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {
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if (!canSinkStoresAndGEPs(S0, S1))
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// Don't attempt to sink below stores that had to stick around
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// But after removal of a store and some of its feeding
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// instruction search again from the beginning since the iterator
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// is likely stale at this point.
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break;
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if (SinkBB == TailBB && TailBB->hasNPredecessorsOrMore(3)) {
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// We have more than 2 predecessors. Insert a new block
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// postdominating 2 predecessors we're going to sink from.
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SinkBB = SplitBlockPredecessors(TailBB, {Pred0, Pred1}, ".sink.split");
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if (!SinkBB)
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break;
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}
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MergedStores = true;
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sinkStoresAndGEPs(SinkBB, S0, S1);
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RBI = Pred0->rbegin();
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RBE = Pred0->rend();
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LLVM_DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
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}
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}
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return MergedStores;
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}
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bool MergedLoadStoreMotion::run(Function &F, AliasAnalysis &AA) {
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this->AA = &AA;
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bool Changed = false;
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LLVM_DEBUG(dbgs() << "Instruction Merger\n");
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// Merge unconditional branches, allowing PRE to catch more
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// optimization opportunities.
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// This loop doesn't care about newly inserted/split blocks
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// since they never will be diamond heads.
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for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) {
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BasicBlock *BB = &*FI++;
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// Hoist equivalent loads and sink stores
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// outside diamonds when possible
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if (isDiamondHead(BB)) {
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Changed |= mergeStores(BB);
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}
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}
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return Changed;
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}
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namespace {
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class MergedLoadStoreMotionLegacyPass : public FunctionPass {
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const bool SplitFooterBB;
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public:
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static char ID; // Pass identification, replacement for typeid
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MergedLoadStoreMotionLegacyPass(bool SplitFooterBB = false)
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: FunctionPass(ID), SplitFooterBB(SplitFooterBB) {
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initializeMergedLoadStoreMotionLegacyPassPass(
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*PassRegistry::getPassRegistry());
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}
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///
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/// Run the transformation for each function
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///
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bool runOnFunction(Function &F) override {
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if (skipFunction(F))
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return false;
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MergedLoadStoreMotion Impl(SplitFooterBB);
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return Impl.run(F, getAnalysis<AAResultsWrapperPass>().getAAResults());
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}
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private:
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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if (!SplitFooterBB)
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AU.setPreservesCFG();
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AU.addRequired<AAResultsWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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}
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};
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char MergedLoadStoreMotionLegacyPass::ID = 0;
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} // anonymous namespace
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///
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/// createMergedLoadStoreMotionPass - The public interface to this file.
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///
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FunctionPass *llvm::createMergedLoadStoreMotionPass(bool SplitFooterBB) {
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return new MergedLoadStoreMotionLegacyPass(SplitFooterBB);
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}
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INITIALIZE_PASS_BEGIN(MergedLoadStoreMotionLegacyPass, "mldst-motion",
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"MergedLoadStoreMotion", false, false)
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INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
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INITIALIZE_PASS_END(MergedLoadStoreMotionLegacyPass, "mldst-motion",
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"MergedLoadStoreMotion", false, false)
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PreservedAnalyses
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MergedLoadStoreMotionPass::run(Function &F, FunctionAnalysisManager &AM) {
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MergedLoadStoreMotion Impl(Options.SplitFooterBB);
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auto &AA = AM.getResult<AAManager>(F);
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if (!Impl.run(F, AA))
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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if (!Options.SplitFooterBB)
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PA.preserveSet<CFGAnalyses>();
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PA.preserve<GlobalsAA>();
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return PA;
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}
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