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4d1e1851a4
This change makes ScalarEvolution a stand-alone object and just produces one from a pass as needed. Making this work well requires making the object movable, using references instead of overwritten pointers in a number of places, and other refactorings. I've also wired it up to the new pass manager and added a RUN line to a test to exercise it under the new pass manager. This includes basic printing support much like with other analyses. But there is a big and somewhat scary change here. Prior to this patch ScalarEvolution was never *actually* invalidated!!! Re-running the pass just re-wired up the various other analyses and didn't remove any of the existing entries in the SCEV caches or clear out anything at all. This might seem OK as everything in SCEV that can uses ValueHandles to track updates to the values that serve as SCEV keys. However, this still means that as we ran SCEV over each function in the module, we kept accumulating more and more SCEVs into the cache. At the end, we would have a SCEV cache with every value that we ever needed a SCEV for in the entire module!!! Yowzers. The releaseMemory routine would dump all of this, but that isn't realy called during normal runs of the pipeline as far as I can see. To make matters worse, there *is* actually a key that we don't update with value handles -- there is a map keyed off of Loop*s. Because LoopInfo *does* release its memory from run to run, it is entirely possible to run SCEV over one function, then over another function, and then lookup a Loop* from the second function but find an entry inserted for the first function! Ouch. To make matters still worse, there are plenty of updates that *don't* trip a value handle. It seems incredibly unlikely that today GVN or another pass that invalidates SCEV can update values in *just* such a way that a subsequent run of SCEV will incorrectly find lookups in a cache, but it is theoretically possible and would be a nightmare to debug. With this refactoring, I've fixed all this by actually destroying and recreating the ScalarEvolution object from run to run. Technically, this could increase the amount of malloc traffic we see, but then again it is also technically correct. ;] I don't actually think we're suffering from tons of malloc traffic from SCEV because if we were, the fact that we never clear the memory would seem more likely to have come up as an actual problem before now. So, I've made the simple fix here. If in fact there are serious issues with too much allocation and deallocation, I can work on a clever fix that preserves the allocations (while clearing the data) between each run, but I'd prefer to do that kind of optimization with a test case / benchmark that shows why we need such cleverness (and that can test that we actually make it faster). It's possible that this will make some things faster by making the SCEV caches have higher locality (due to being significantly smaller) so until there is a clear benchmark, I think the simple change is best. Differential Revision: http://reviews.llvm.org/D12063 llvm-svn: 245193
233 lines
8.0 KiB
C++
233 lines
8.0 KiB
C++
//===-------- PPCLoopDataPrefetch.cpp - Loop Data Prefetching Pass --------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a Loop Data Prefetching Pass.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "ppc-loop-data-prefetch"
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#include "PPC.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AssumptionCache.h"
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#include "llvm/Analysis/CodeMetrics.h"
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#include "llvm/Analysis/InstructionSimplify.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/ScalarEvolutionExpander.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/ValueMapper.h"
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using namespace llvm;
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// By default, we limit this to creating 16 PHIs (which is a little over half
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// of the allocatable register set).
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static cl::opt<bool>
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PrefetchWrites("ppc-loop-prefetch-writes", cl::Hidden, cl::init(false),
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cl::desc("Prefetch write addresses"));
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// This seems like a reasonable default for the BG/Q (this pass is enabled, by
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// default, only on the BG/Q).
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static cl::opt<unsigned>
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PrefDist("ppc-loop-prefetch-distance", cl::Hidden, cl::init(300),
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cl::desc("The loop prefetch distance"));
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static cl::opt<unsigned>
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CacheLineSize("ppc-loop-prefetch-cache-line", cl::Hidden, cl::init(64),
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cl::desc("The loop prefetch cache line size"));
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namespace llvm {
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void initializePPCLoopDataPrefetchPass(PassRegistry&);
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}
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namespace {
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class PPCLoopDataPrefetch : public FunctionPass {
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public:
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static char ID; // Pass ID, replacement for typeid
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PPCLoopDataPrefetch() : FunctionPass(ID) {
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initializePPCLoopDataPrefetchPass(*PassRegistry::getPassRegistry());
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<AssumptionCacheTracker>();
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addPreserved<LoopInfoWrapperPass>();
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AU.addRequired<ScalarEvolutionWrapperPass>();
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// FIXME: For some reason, preserving SE here breaks LSR (even if
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// this pass changes nothing).
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// AU.addPreserved<ScalarEvolutionWrapperPass>();
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AU.addRequired<TargetTransformInfoWrapperPass>();
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}
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bool runOnFunction(Function &F) override;
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bool runOnLoop(Loop *L);
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private:
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AssumptionCache *AC;
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LoopInfo *LI;
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ScalarEvolution *SE;
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const TargetTransformInfo *TTI;
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const DataLayout *DL;
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};
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}
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char PPCLoopDataPrefetch::ID = 0;
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INITIALIZE_PASS_BEGIN(PPCLoopDataPrefetch, "ppc-loop-data-prefetch",
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"PPC Loop Data Prefetch", false, false)
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INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
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INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
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INITIALIZE_PASS_END(PPCLoopDataPrefetch, "ppc-loop-data-prefetch",
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"PPC Loop Data Prefetch", false, false)
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FunctionPass *llvm::createPPCLoopDataPrefetchPass() { return new PPCLoopDataPrefetch(); }
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bool PPCLoopDataPrefetch::runOnFunction(Function &F) {
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LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
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DL = &F.getParent()->getDataLayout();
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AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
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TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
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bool MadeChange = false;
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for (auto I = LI->begin(), IE = LI->end(); I != IE; ++I)
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for (auto L = df_begin(*I), LE = df_end(*I); L != LE; ++L)
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MadeChange |= runOnLoop(*L);
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return MadeChange;
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}
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bool PPCLoopDataPrefetch::runOnLoop(Loop *L) {
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bool MadeChange = false;
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// Only prefetch in the inner-most loop
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if (!L->empty())
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return MadeChange;
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SmallPtrSet<const Value *, 32> EphValues;
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CodeMetrics::collectEphemeralValues(L, AC, EphValues);
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// Calculate the number of iterations ahead to prefetch
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CodeMetrics Metrics;
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for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
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I != IE; ++I) {
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// If the loop already has prefetches, then assume that the user knows
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// what he or she is doing and don't add any more.
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for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
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J != JE; ++J)
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if (CallInst *CI = dyn_cast<CallInst>(J))
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if (Function *F = CI->getCalledFunction())
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if (F->getIntrinsicID() == Intrinsic::prefetch)
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return MadeChange;
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Metrics.analyzeBasicBlock(*I, *TTI, EphValues);
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}
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unsigned LoopSize = Metrics.NumInsts;
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if (!LoopSize)
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LoopSize = 1;
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unsigned ItersAhead = PrefDist/LoopSize;
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if (!ItersAhead)
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ItersAhead = 1;
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SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>, 16> PrefLoads;
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for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
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I != IE; ++I) {
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for (BasicBlock::iterator J = (*I)->begin(), JE = (*I)->end();
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J != JE; ++J) {
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Value *PtrValue;
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Instruction *MemI;
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if (LoadInst *LMemI = dyn_cast<LoadInst>(J)) {
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MemI = LMemI;
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PtrValue = LMemI->getPointerOperand();
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} else if (StoreInst *SMemI = dyn_cast<StoreInst>(J)) {
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if (!PrefetchWrites) continue;
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MemI = SMemI;
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PtrValue = SMemI->getPointerOperand();
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} else continue;
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unsigned PtrAddrSpace = PtrValue->getType()->getPointerAddressSpace();
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if (PtrAddrSpace)
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continue;
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if (L->isLoopInvariant(PtrValue))
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continue;
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const SCEV *LSCEV = SE->getSCEV(PtrValue);
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const SCEVAddRecExpr *LSCEVAddRec = dyn_cast<SCEVAddRecExpr>(LSCEV);
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if (!LSCEVAddRec)
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continue;
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// We don't want to double prefetch individual cache lines. If this load
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// is known to be within one cache line of some other load that has
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// already been prefetched, then don't prefetch this one as well.
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bool DupPref = false;
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for (SmallVector<std::pair<Instruction *, const SCEVAddRecExpr *>,
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16>::iterator K = PrefLoads.begin(), KE = PrefLoads.end();
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K != KE; ++K) {
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const SCEV *PtrDiff = SE->getMinusSCEV(LSCEVAddRec, K->second);
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if (const SCEVConstant *ConstPtrDiff =
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dyn_cast<SCEVConstant>(PtrDiff)) {
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int64_t PD = std::abs(ConstPtrDiff->getValue()->getSExtValue());
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if (PD < (int64_t) CacheLineSize) {
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DupPref = true;
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break;
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}
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}
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}
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if (DupPref)
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continue;
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const SCEV *NextLSCEV = SE->getAddExpr(LSCEVAddRec, SE->getMulExpr(
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SE->getConstant(LSCEVAddRec->getType(), ItersAhead),
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LSCEVAddRec->getStepRecurrence(*SE)));
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if (!isSafeToExpand(NextLSCEV, *SE))
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continue;
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PrefLoads.push_back(std::make_pair(MemI, LSCEVAddRec));
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Type *I8Ptr = Type::getInt8PtrTy((*I)->getContext(), PtrAddrSpace);
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SCEVExpander SCEVE(*SE, J->getModule()->getDataLayout(), "prefaddr");
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Value *PrefPtrValue = SCEVE.expandCodeFor(NextLSCEV, I8Ptr, MemI);
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IRBuilder<> Builder(MemI);
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Module *M = (*I)->getParent()->getParent();
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Type *I32 = Type::getInt32Ty((*I)->getContext());
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Value *PrefetchFunc = Intrinsic::getDeclaration(M, Intrinsic::prefetch);
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Builder.CreateCall(
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PrefetchFunc,
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{PrefPtrValue,
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ConstantInt::get(I32, MemI->mayReadFromMemory() ? 0 : 1),
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ConstantInt::get(I32, 3), ConstantInt::get(I32, 1)});
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MadeChange = true;
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}
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}
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return MadeChange;
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}
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