forked from OSchip/llvm-project
completely rewrite the memory promotion algorithm in LICM.
Among other things, this uses SSAUpdater instead of PromoteMemToReg. llvm-svn: 112417
This commit is contained in:
Chris Lattner
parent
d0c054886c
commit
1dc98b47b5
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@ -26,8 +26,7 @@
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// pointer. There are no calls in the loop which mod/ref the pointer.
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// If these conditions are true, we can promote the loads and stores in the
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// loop of the pointer to use a temporary alloca'd variable. We then use
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// the mem2reg functionality to construct the appropriate SSA form for the
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// variable.
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// the SSAUpdater to construct the appropriate SSA form for the value.
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//
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//===----------------------------------------------------------------------===//
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@ -44,7 +43,6 @@
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#include "llvm/Analysis/AliasSetTracker.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Transforms/Utils/PromoteMemToReg.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/CommandLine.h"
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@ -205,20 +203,7 @@ namespace {
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bool isLoopInvariantInst(Instruction &I);
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bool isNotUsedInLoop(Instruction &I);
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/// PromoteValuesInLoop - Look at the stores in the loop and promote as many
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/// to scalars as we can.
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///
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void PromoteValuesInLoop();
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/// FindPromotableValuesInLoop - Check the current loop for stores to
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/// definite pointers, which are not loaded and stored through may aliases.
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/// If these are found, create an alloca for the value, add it to the
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/// PromotedValues list, and keep track of the mapping from value to
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/// alloca...
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///
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void FindPromotableValuesInLoop(
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std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
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DenseMap<Value*, AllocaInst*> &Val2AlMap);
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void PromoteAliasSet(AliasSet &AS);
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};
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}
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@ -284,9 +269,13 @@ bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
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HoistRegion(DT->getNode(L->getHeader()));
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// Now that all loop invariants have been removed from the loop, promote any
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// memory references to scalars that we can...
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if (!DisablePromotion && Preheader && L->hasDedicatedExits())
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PromoteValuesInLoop();
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// memory references to scalars that we can.
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if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
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// Loop over all of the alias sets in the tracker object.
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for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
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I != E; ++I)
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PromoteAliasSet(*I);
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}
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// Clear out loops state information for the next iteration
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CurLoop = 0;
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@ -622,160 +611,22 @@ bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
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return true;
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}
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/// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
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/// PromoteAliasSet - Try to promote memory values to scalars by sinking
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/// stores out of the loop and moving loads to before the loop. We do this by
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/// looping over the stores in the loop, looking for stores to Must pointers
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/// which are loop invariant. We promote these memory locations to use allocas
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/// instead. These allocas can easily be raised to register values by the
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/// PromoteMem2Reg functionality.
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/// which are loop invariant.
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///
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void LICM::PromoteValuesInLoop() {
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// PromotedValues - List of values that are promoted out of the loop. Each
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// value has an alloca instruction for it, and a canonical version of the
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// pointer.
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std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
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DenseMap<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
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FindPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
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if (ValueToAllocaMap.empty()) return; // If there are values to promote.
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Changed = true;
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NumPromoted += PromotedValues.size();
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std::vector<Value*> PointerValueNumbers;
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// Emit a copy from the value into the alloca'd value in the loop preheader
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TerminatorInst *LoopPredInst = Preheader->getTerminator();
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for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
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Value *Ptr = PromotedValues[i].second;
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// If we are promoting a pointer value, update alias information for the
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// inserted load.
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Value *LoadValue = 0;
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if (cast<PointerType>(Ptr->getType())->getElementType()->isPointerTy()) {
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// Locate a load or store through the pointer, and assign the same value
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// to LI as we are loading or storing. Since we know that the value is
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// stored in this loop, this will always succeed.
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for (Value::use_iterator UI = Ptr->use_begin(), E = Ptr->use_end();
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UI != E; ++UI) {
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User *U = *UI;
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if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
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LoadValue = LI;
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break;
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} else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
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if (SI->getOperand(1) == Ptr) {
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LoadValue = SI->getOperand(0);
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break;
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}
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}
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}
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assert(LoadValue && "No store through the pointer found!");
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PointerValueNumbers.push_back(LoadValue); // Remember this for later.
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}
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// Load from the memory we are promoting.
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LoadInst *LI = new LoadInst(Ptr, Ptr->getName()+".promoted", LoopPredInst);
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if (LoadValue) CurAST->copyValue(LoadValue, LI);
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// Store into the temporary alloca.
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new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
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}
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// Scan the basic blocks in the loop, replacing uses of our pointers with
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// uses of the allocas in question.
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//
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for (Loop::block_iterator I = CurLoop->block_begin(),
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E = CurLoop->block_end(); I != E; ++I) {
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BasicBlock *BB = *I;
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// Rewrite all loads and stores in the block of the pointer...
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for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
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if (LoadInst *L = dyn_cast<LoadInst>(II)) {
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DenseMap<Value*, AllocaInst*>::iterator
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I = ValueToAllocaMap.find(L->getOperand(0));
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if (I != ValueToAllocaMap.end())
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L->setOperand(0, I->second); // Rewrite load instruction...
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} else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
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DenseMap<Value*, AllocaInst*>::iterator
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I = ValueToAllocaMap.find(S->getOperand(1));
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if (I != ValueToAllocaMap.end())
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S->setOperand(1, I->second); // Rewrite store instruction...
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}
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}
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}
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// Now that the body of the loop uses the allocas instead of the original
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// memory locations, insert code to copy the alloca value back into the
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// original memory location on all exits from the loop.
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SmallVector<BasicBlock*, 8> ExitBlocks;
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CurLoop->getUniqueExitBlocks(ExitBlocks);
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
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// Copy all of the allocas into their memory locations.
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BasicBlock::iterator BI = ExitBlocks[i]->getFirstNonPHI();
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Instruction *InsertPos = BI;
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unsigned PVN = 0;
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for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
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// Load from the alloca.
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LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
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// If this is a pointer type, update alias info appropriately.
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if (LI->getType()->isPointerTy())
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CurAST->copyValue(PointerValueNumbers[PVN++], LI);
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// Store into the memory we promoted.
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new StoreInst(LI, PromotedValues[i].second, InsertPos);
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}
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}
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// Now that we have done the deed, use the mem2reg functionality to promote
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// all of the new allocas we just created into real SSA registers.
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//
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std::vector<AllocaInst*> PromotedAllocas;
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PromotedAllocas.reserve(PromotedValues.size());
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for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
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PromotedAllocas.push_back(PromotedValues[i].first);
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PromoteMemToReg(PromotedAllocas, *DT, *DF, CurAST);
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}
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/// FindPromotableValuesInLoop - Check the current loop for stores to definite
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/// pointers, which are not loaded and stored through may aliases and are safe
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/// for promotion. If these are found, create an alloca for the value, add it
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/// to the PromotedValues list, and keep track of the mapping from value to
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/// alloca.
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void LICM::FindPromotableValuesInLoop(
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std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
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DenseMap<Value*, AllocaInst*> &ValueToAllocaMap) {
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Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
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// Loop over all of the alias sets in the tracker object.
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for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
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I != E; ++I) {
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AliasSet &AS = *I;
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void LICM::PromoteAliasSet(AliasSet &AS) {
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// We can promote this alias set if it has a store, if it is a "Must" alias
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// set, if the pointer is loop invariant, and if we are not eliminating any
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// volatile loads or stores.
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if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
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AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
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continue;
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return;
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assert(!AS.empty() &&
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"Must alias set should have at least one pointer element in it!");
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Value *V = AS.begin()->getValue();
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// Check that all of the pointers in the alias set have the same type. We
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// cannot (yet) promote a memory location that is loaded and stored in
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// different sizes.
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{
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bool PointerOk = true;
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for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
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if (V->getType() != I->getValue()->getType()) {
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PointerOk = false;
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break;
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}
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if (!PointerOk)
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continue;
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}
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Value *SomePtr = AS.begin()->getValue();
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// It isn't safe to promote a load/store from the loop if the load/store is
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// conditional. For example, turning:
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@ -791,43 +642,207 @@ void LICM::FindPromotableValuesInLoop(
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// It is safe to promote P if all uses are direct load/stores and if at
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// least one is guaranteed to be executed.
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bool GuaranteedToExecute = false;
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bool InvalidInst = false;
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for (Value::use_iterator UI = V->use_begin(), UE = V->use_end();
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SmallVector<Instruction*, 64> LoopUses;
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SmallPtrSet<Value*, 4> PointerMustAliases;
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// Check that all of the pointers in the alias set have the same type. We
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// cannot (yet) promote a memory location that is loaded and stored in
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// different sizes.
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for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
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Value *ASIV = ASI->getValue();
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PointerMustAliases.insert(ASIV);
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// Check that all of the pointers in the alias set have the same type. We
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// cannot (yet) promote a memory location that is loaded and stored in
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// different sizes.
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if (SomePtr->getType() != ASIV->getType())
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return;
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for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
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UI != UE; ++UI) {
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// Ignore instructions not in this loop.
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// Ignore instructions that are outside the loop.
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Instruction *Use = dyn_cast<Instruction>(*UI);
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if (!Use || !CurLoop->contains(Use))
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continue;
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if (!isa<LoadInst>(Use) && !isa<StoreInst>(Use)) {
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InvalidInst = true;
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break;
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}
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// If there is an non-load/store instruction in the loop, we can't promote
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// it.
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if (isa<LoadInst>(Use))
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assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken");
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else if (isa<StoreInst>(Use))
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assert(!cast<StoreInst>(Use)->isVolatile() &&
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Use->getOperand(0) != ASIV && "AST broken");
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else
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return; // Not a load or store.
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if (!GuaranteedToExecute)
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GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
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LoopUses.push_back(Use);
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}
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}
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// If there is an non-load/store instruction in the loop, we can't promote
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// it. If there isn't a guaranteed-to-execute instruction, we can't
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// promote.
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if (InvalidInst || !GuaranteedToExecute)
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// If there isn't a guaranteed-to-execute instruction, we can't promote.
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if (!GuaranteedToExecute)
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return;
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// Otherwise, this is safe to promote, lets do it!
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DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
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Changed = true;
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++NumPromoted;
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// We use the SSAUpdater interface to insert phi nodes as required.
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SmallVector<PHINode*, 16> NewPHIs;
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SSAUpdater SSA(&NewPHIs);
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// It wants to know some value of the same type as what we'll be inserting.
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Value *SomeValue;
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if (isa<LoadInst>(LoopUses[0]))
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SomeValue = LoopUses[0];
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else
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SomeValue = cast<StoreInst>(LoopUses[0])->getOperand(0);
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SSA.Initialize(SomeValue);
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// First step: bucket up uses of the pointers by the block they occur in.
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// This is important because we have to handle multiple defs/uses in a block
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// ourselves: SSAUpdater is purely for cross-block references.
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// FIXME: Want a TinyVector<Instruction*> since there is usually 0/1 element.
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DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock;
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for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
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Instruction *User = LoopUses[i];
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UsesByBlock[User->getParent()].push_back(User);
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}
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// Okay, now we can iterate over all the blocks in the loop with uses,
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// processing them. Keep track of which loads are loading a live-in value.
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SmallVector<LoadInst*, 32> LiveInLoads;
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for (unsigned LoopUse = 0, e = LoopUses.size(); LoopUse != e; ++LoopUse) {
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Instruction *User = LoopUses[LoopUse];
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std::vector<Instruction*> &BlockUses = UsesByBlock[User->getParent()];
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// If this block has already been processed, ignore this repeat use.
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if (BlockUses.empty()) continue;
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// Okay, this is the first use in the block. If this block just has a
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// single user in it, we can rewrite it trivially.
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if (BlockUses.size() == 1) {
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// If it is a store, it is a trivial def of the value in the block.
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if (isa<StoreInst>(User)) {
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SSA.AddAvailableValue(User->getParent(),
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cast<StoreInst>(User)->getOperand(0));
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} else {
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// Otherwise it is a load, queue it to rewrite as a live-in load.
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LiveInLoads.push_back(cast<LoadInst>(User));
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}
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BlockUses.clear();
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continue;
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}
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const Type *Ty = cast<PointerType>(V->getType())->getElementType();
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AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
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PromotedValues.push_back(std::make_pair(AI, V));
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// Update the AST and alias analysis.
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CurAST->copyValue(V, AI);
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for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
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ValueToAllocaMap[I->getValue()] = AI;
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DEBUG(dbgs() << "LICM: Promoting value: " << *V << "\n");
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// Otherwise, check to see if this block is all loads. If so, we can queue
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// them all as live in loads.
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bool HasStore = false;
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for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
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if (isa<StoreInst>(BlockUses[i])) {
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HasStore = true;
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break;
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}
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}
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if (!HasStore) {
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for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
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LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
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BlockUses.clear();
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continue;
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}
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// Otherwise, we have mixed loads and stores (or just a bunch of stores).
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// Since SSAUpdater is purely for cross-block values, we need to determine
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// the order of these instructions in the block. If the first use in the
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// block is a load, then it uses the live in value. The last store defines
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// the live out value. We handle this by doing a linear scan of the block.
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BasicBlock *BB = User->getParent();
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Value *StoredValue = 0;
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for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
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if (LoadInst *L = dyn_cast<LoadInst>(II)) {
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// If this is a load to an unrelated pointer, ignore it.
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if (!PointerMustAliases.count(L->getOperand(0))) continue;
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// If we haven't seen a store yet, this is a live in use, otherwise
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// use the stored value.
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if (StoredValue)
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L->replaceAllUsesWith(StoredValue);
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else
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LiveInLoads.push_back(L);
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continue;
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}
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if (StoreInst *S = dyn_cast<StoreInst>(II)) {
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// If this is a load to an unrelated pointer, ignore it.
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if (!PointerMustAliases.count(S->getOperand(1))) continue;
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// Remember that this is the active value in the block.
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StoredValue = S->getOperand(0);
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}
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}
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// The last stored value that happened is the live-out for the block.
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assert(StoredValue && "Already checked that there is a store in block");
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SSA.AddAvailableValue(BB, StoredValue);
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BlockUses.clear();
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}
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// Now that all the intra-loop values are classified, set up the preheader.
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// It gets a load of the pointer we're promoting, and it is the live-out value
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// from the preheader.
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LoadInst *PreheaderLoad = new LoadInst(SomePtr,SomePtr->getName()+".promoted",
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Preheader->getTerminator());
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SSA.AddAvailableValue(Preheader, PreheaderLoad);
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// Now that the preheader is good to go, set up the exit blocks. Each exit
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// block gets a store of the live-out values that feed them. Since we've
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// already told the SSA updater about the defs in the loop and the preheader
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// definition, it is all set and we can start using it.
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SmallVector<BasicBlock*, 8> ExitBlocks;
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CurLoop->getUniqueExitBlocks(ExitBlocks);
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
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BasicBlock *ExitBlock = ExitBlocks[i];
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Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
|
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Instruction *InsertPos = ExitBlock->getFirstNonPHI();
|
||||
new StoreInst(LiveInValue, SomePtr, InsertPos);
|
||||
}
|
||||
|
||||
// Okay, now we rewrite all loads that use live-in values in the loop,
|
||||
// inserting PHI nodes as necessary.
|
||||
for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
|
||||
LoadInst *ALoad = LiveInLoads[i];
|
||||
ALoad->replaceAllUsesWith(SSA.GetValueInMiddleOfBlock(ALoad->getParent()));
|
||||
}
|
||||
|
||||
// Now that everything is rewritten, delete the old instructions from the body
|
||||
// of the loop. They should all be dead now.
|
||||
for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
|
||||
Instruction *User = LoopUses[i];
|
||||
CurAST->deleteValue(User);
|
||||
User->eraseFromParent();
|
||||
}
|
||||
|
||||
// If the preheader load is itself a pointer, we need to tell alias analysis
|
||||
// about the new pointer we created in the preheader block and about any PHI
|
||||
// nodes that just got inserted.
|
||||
if (PreheaderLoad->getType()->isPointerTy()) {
|
||||
// Copy any value stored to or loaded from a must-alias of the pointer.
|
||||
CurAST->copyValue(SomeValue, PreheaderLoad);
|
||||
|
||||
for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
|
||||
CurAST->copyValue(SomeValue, NewPHIs[i]);
|
||||
}
|
||||
|
||||
// fwew, we're done!
|
||||
}
|
||||
|
||||
|
||||
/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
|
||||
void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
|
||||
AliasSetTracker *AST = LoopToAliasMap[L];
|
||||
|
|
|
|||
Loading…
Reference in New Issue