llvm-project/llvm/lib/Transforms/Scalar/LICM.cpp

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//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
// This pass is a simple loop invariant code motion pass.
//
// Note that this pass does NOT require pre-headers to exist on loops in the
// CFG, but if there is not distinct preheader for a loop, the hoisted code will
// be *DUPLICATED* in every basic block, outside of the loop, that preceeds the
// loop header. Additionally, any use of one of these hoisted expressions
// cannot be loop invariant itself, because the expression hoisted gets a PHI
// node that is loop variant.
//
// For these reasons, and many more, it makes sense to run a pass before this
// that ensures that there are preheaders on all loops. That said, we don't
// REQUIRE it. :)
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/iOperators.h"
#include "llvm/iPHINode.h"
#include "llvm/iMemory.h"
#include "llvm/Support/InstVisitor.h"
#include "llvm/Support/CFG.h"
#include "Support/STLExtras.h"
#include "Support/StatisticReporter.h"
#include <algorithm>
using std::string;
static Statistic<> NumHoistedNPH("licm\t\t- Number of insts hoisted to multiple"
" loop preds (bad, no loop pre-header)");
static Statistic<> NumHoistedPH("licm\t\t- Number of insts hoisted to a loop "
"pre-header");
static Statistic<> NumHoistedLoads("licm\t\t- Number of load insts hoisted");
namespace {
struct LICM : public FunctionPass, public InstVisitor<LICM> {
virtual bool runOnFunction(Function &F);
// This transformation requires natural loop information...
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.preservesCFG();
AU.addRequired<LoopInfo>();
AU.addRequired<AliasAnalysis>();
}
private:
// List of predecessor blocks for the current loop - These blocks are where
// we hoist loop invariants to for the current loop.
//
std::vector<BasicBlock*> LoopPreds, LoopBackEdges;
Loop *CurLoop; // The current loop we are working on...
bool Changed; // Set to true when we change anything.
AliasAnalysis *AA; // Currently AliasAnalysis information
// visitLoop - Hoist expressions out of the specified loop...
void visitLoop(Loop *L);
// notInCurrentLoop - Little predicate that returns true if the specified
// basic block is in a subloop of the current one, not the current one
// itself.
//
bool notInCurrentLoop(BasicBlock *BB) {
for (unsigned i = 0, e = CurLoop->getSubLoops().size(); i != e; ++i)
if (CurLoop->getSubLoops()[i]->contains(BB))
return true; // A subloop actually contains this block!
return false;
}
// hoist - When an instruction is found to only use loop invariant operands
// that is safe to hoist, this instruction is called to do the dirty work.
//
void hoist(Instruction &I);
// pointerInvalidatedByLoop - Return true if the body of this loop may store
// into the memory location pointed to by V.
//
bool pointerInvalidatedByLoop(Value *V);
// isLoopInvariant - Return true if the specified value is loop invariant
inline bool isLoopInvariant(Value *V) {
if (Instruction *I = dyn_cast<Instruction>(V))
return !CurLoop->contains(I->getParent());
return true; // All non-instructions are loop invariant
}
// visitBasicBlock - Run LICM on a particular block.
void visitBasicBlock(BasicBlock *BB);
// Instruction visitation handlers... these basically control whether or not
// the specified instruction types are hoisted.
//
friend class InstVisitor<LICM>;
void visitBinaryOperator(Instruction &I) {
if (isLoopInvariant(I.getOperand(0)) && isLoopInvariant(I.getOperand(1)))
hoist(I);
}
void visitCastInst(CastInst &CI) {
Instruction &I = (Instruction&)CI;
if (isLoopInvariant(I.getOperand(0))) hoist(I);
}
void visitShiftInst(ShiftInst &I) { visitBinaryOperator((Instruction&)I); }
void visitLoadInst(LoadInst &LI) {
if (isLoopInvariant(LI.getOperand(0)) &&
!pointerInvalidatedByLoop(LI.getOperand(0)))
hoist(LI);
}
void visitGetElementPtrInst(GetElementPtrInst &GEPI) {
Instruction &I = (Instruction&)GEPI;
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
if (!isLoopInvariant(I.getOperand(i))) return;
hoist(I);
}
};
RegisterOpt<LICM> X("licm", "Loop Invariant Code Motion");
}
Pass *createLICMPass() { return new LICM(); }
bool LICM::runOnFunction(Function &) {
// get our loop information...
const std::vector<Loop*> &TopLevelLoops =
getAnalysis<LoopInfo>().getTopLevelLoops();
// Get our alias analysis information...
AA = &getAnalysis<AliasAnalysis>();
// Traverse loops in postorder, hoisting expressions out of the deepest loops
// first.
//
Changed = false;
std::for_each(TopLevelLoops.begin(), TopLevelLoops.end(),
bind_obj(this, &LICM::visitLoop));
return Changed;
}
void LICM::visitLoop(Loop *L) {
// Recurse through all subloops before we process this loop...
std::for_each(L->getSubLoops().begin(), L->getSubLoops().end(),
bind_obj(this, &LICM::visitLoop));
CurLoop = L;
// Calculate the set of predecessors for this loop. The predecessors for this
// loop are equal to the predecessors for the header node of the loop that are
// not themselves in the loop.
//
BasicBlock *Header = L->getHeader();
// Calculate the sets of predecessors and backedges of the loop...
LoopBackEdges.insert(LoopBackEdges.end(),pred_begin(Header),pred_end(Header));
std::vector<BasicBlock*>::iterator LPI =
std::partition(LoopBackEdges.begin(), LoopBackEdges.end(),
bind_obj(CurLoop, &Loop::contains));
// Move all predecessors to the LoopPreds vector...
LoopPreds.insert(LoopPreds.end(), LPI, LoopBackEdges.end());
// Remove predecessors from backedges list...
LoopBackEdges.erase(LPI, LoopBackEdges.end());
// The only way that there could be no predecessors to a loop is if the loop
// is not reachable. Since we don't care about optimizing dead loops,
// summarily ignore them.
//
if (LoopPreds.empty()) return;
// We want to visit all of the instructions in this loop... that are not parts
// of our subloops (they have already had their invariants hoisted out of
// their loop, into this loop, so there is no need to process the BODIES of
// the subloops).
//
std::vector<BasicBlock*> BBs(L->getBlocks().begin(), L->getBlocks().end());
// Remove blocks that are actually in subloops...
BBs.erase(std::remove_if(BBs.begin(), BBs.end(),
bind_obj(this, &LICM::notInCurrentLoop)), BBs.end());
// Visit all of the basic blocks we have chosen, hoisting out the instructions
// as neccesary. This leaves dead copies of the instruction in the loop
// unfortunately...
//
for_each(BBs.begin(), BBs.end(), bind_obj(this, &LICM::visitBasicBlock));
// Clear out loops state information for the next iteration
CurLoop = 0;
LoopPreds.clear();
LoopBackEdges.clear();
}
void LICM::visitBasicBlock(BasicBlock *BB) {
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
visit(*I);
if (dceInstruction(I))
Changed = true;
else
++I;
}
}
void LICM::hoist(Instruction &Inst) {
if (Inst.use_empty()) return; // Don't (re) hoist dead instructions!
//cerr << "Hoisting " << Inst;
BasicBlock *Header = CurLoop->getHeader();
// Old instruction will be removed, so take it's name...
string InstName = Inst.getName();
Inst.setName("");
if (isa<LoadInst>(Inst))
++NumHoistedLoads;
// The common case is that we have a pre-header. Generate special case code
// that is faster if that is the case.
//
if (LoopPreds.size() == 1) {
BasicBlock *Pred = LoopPreds[0];
// Create a new copy of the instruction, for insertion into Pred.
Instruction *New = Inst.clone();
New->setName(InstName);
// Insert the new node in Pred, before the terminator.
Pred->getInstList().insert(--Pred->end(), New);
// Kill the old instruction...
Inst.replaceAllUsesWith(New);
++NumHoistedPH;
} else {
// No loop pre-header, insert a PHI node into header to capture all of the
// incoming versions of the value.
//
PHINode *LoopVal = new PHINode(Inst.getType(), InstName+".phi");
// Insert the new PHI node into the loop header...
Header->getInstList().push_front(LoopVal);
// Insert cloned versions of the instruction into all of the loop preds.
for (unsigned i = 0, e = LoopPreds.size(); i != e; ++i) {
BasicBlock *Pred = LoopPreds[i];
// Create a new copy of the instruction, for insertion into Pred.
Instruction *New = Inst.clone();
New->setName(InstName);
// Insert the new node in Pred, before the terminator.
Pred->getInstList().insert(--Pred->end(), New);
// Add the incoming value to the PHI node.
LoopVal->addIncoming(New, Pred);
}
// Add incoming values to the PHI node for all backedges in the loop...
for (unsigned i = 0, e = LoopBackEdges.size(); i != e; ++i)
LoopVal->addIncoming(LoopVal, LoopBackEdges[i]);
// Replace all uses of the old version of the instruction in the loop with
// the new version that is out of the loop. We know that this is ok,
// because the new definition is in the loop header, which dominates the
// entire loop body. The old definition was defined _inside_ of the loop,
// so the scope cannot extend outside of the loop, so we're ok.
//
Inst.replaceAllUsesWith(LoopVal);
++NumHoistedNPH;
}
Changed = true;
}
// pointerInvalidatedByLoop - Return true if the body of this loop may store
// into the memory location pointed to by V.
//
bool LICM::pointerInvalidatedByLoop(Value *V) {
// Check to see if any of the basic blocks in CurLoop invalidate V.
for (unsigned i = 0, e = CurLoop->getBlocks().size(); i != e; ++i)
if (AA->canBasicBlockModify(*CurLoop->getBlocks()[i], V))
return true;
return false;
}