#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
}
bool runOnLoop(Loop *L, LPPassManager &LPM);
+ void simplifyLoopLatch(Loop *L);
bool rotateLoop(Loop *L);
private:
bool LoopRotate::runOnLoop(Loop *L, LPPassManager &LPM) {
LI = &getAnalysis<LoopInfo>();
+ // Simplify the loop latch before attempting to rotate the header
+ // upward. Rotation may not be needed if the loop tail can be folded into the
+ // loop exit.
+ simplifyLoopLatch(L);
+
// One loop can be rotated multiple times.
bool MadeChange = false;
while (rotateLoop(L))
}
}
+/// Determine whether the instructions in this range my be safely and cheaply
+/// speculated. This is not an important enough situation to develop complex
+/// heuristics. We handle a single arithmetic instruction along with any type
+/// conversions.
+static bool shouldSpeculateInstrs(BasicBlock::iterator Begin,
+ BasicBlock::iterator End) {
+ bool seenIncrement = false;
+ for (BasicBlock::iterator I = Begin; I != End; ++I) {
+
+ if (!isSafeToSpeculativelyExecute(I))
+ return false;
+
+ if (isa<DbgInfoIntrinsic>(I))
+ continue;
+
+ switch (I->getOpcode()) {
+ default:
+ return false;
+ case Instruction::GetElementPtr:
+ // GEPs are cheap if all indices are constant.
+ if (!cast<GEPOperator>(I)->hasAllConstantIndices())
+ return false;
+ // fall-thru to increment case
+ case Instruction::Add:
+ case Instruction::Sub:
+ case Instruction::And:
+ case Instruction::Or:
+ case Instruction::Xor:
+ case Instruction::Shl:
+ case Instruction::LShr:
+ case Instruction::AShr:
+ if (seenIncrement)
+ return false;
+ seenIncrement = true;
+ break;
+ case Instruction::Trunc:
+ case Instruction::ZExt:
+ case Instruction::SExt:
+ // ignore type conversions
+ break;
+ }
+ }
+ return true;
+}
+
+/// Fold the loop tail into the loop exit by speculating the loop tail
+/// instructions. Typically, this is a single post-increment. In the case of a
+/// simple 2-block loop, hoisting the increment can be much better than
+/// duplicating the entire loop header. In the cast of loops with early exits,
+/// rotation will not work anyway, but simplifyLoopLatch will put the loop in
+/// canonical form so downstream passes can handle it.
+///
+/// I don't believe this invalidates SCEV.
+void LoopRotate::simplifyLoopLatch(Loop *L) {
+ BasicBlock *Latch = L->getLoopLatch();
+ if (!Latch || Latch->hasAddressTaken())
+ return;
+
+ BranchInst *Jmp = dyn_cast<BranchInst>(Latch->getTerminator());
+ if (!Jmp || !Jmp->isUnconditional())
+ return;
+
+ BasicBlock *LastExit = Latch->getSinglePredecessor();
+ if (!LastExit || !L->isLoopExiting(LastExit))
+ return;
+
+ BranchInst *BI = dyn_cast<BranchInst>(LastExit->getTerminator());
+ if (!BI)
+ return;
+
+ if (!shouldSpeculateInstrs(Latch->begin(), Jmp))
+ return;
+
+ DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into "
+ << LastExit->getName() << "\n");
+
+ // Hoist the instructions from Latch into LastExit.
+ LastExit->getInstList().splice(BI, Latch->getInstList(), Latch->begin(), Jmp);
+
+ unsigned FallThruPath = BI->getSuccessor(0) == Latch ? 0 : 1;
+ BasicBlock *Header = Jmp->getSuccessor(0);
+ assert(Header == L->getHeader() && "expected a backward branch");
+
+ // Remove Latch from the CFG so that LastExit becomes the new Latch.
+ BI->setSuccessor(FallThruPath, Header);
+ Latch->replaceSuccessorsPhiUsesWith(LastExit);
+ Jmp->eraseFromParent();
+
+ // Nuke the Latch block.
+ assert(Latch->empty() && "unable to evacuate Latch");
+ LI->removeBlock(Latch);
+ if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>())
+ DT->eraseNode(Latch);
+ Latch->eraseFromParent();
+}
+
/// Rotate loop LP. Return true if the loop is rotated.
bool LoopRotate::rotateLoop(Loop *L) {
// If the loop has only one block then there is not much to rotate.
// memory (without proving that the loop doesn't write).
if (L->hasLoopInvariantOperands(Inst) &&
!Inst->mayReadFromMemory() && !Inst->mayWriteToMemory() &&
- !isa<TerminatorInst>(Inst) && !isa<DbgInfoIntrinsic>(Inst)) {
+ !isa<TerminatorInst>(Inst) && !isa<DbgInfoIntrinsic>(Inst) &&
+ !isa<AllocaInst>(Inst)) {
Inst->moveBefore(LoopEntryBranch);
continue;
}
}
// Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and
- // thus is not a preheader anymore. Split the edge to form a real preheader.
+ // thus is not a preheader anymore.
+ // Split the edge to form a real preheader.
BasicBlock *NewPH = SplitCriticalEdge(OrigPreheader, NewHeader, this);
NewPH->setName(NewHeader->getName() + ".lr.ph");
- // Preserve canonical loop form, which means that 'Exit' should have only one
- // predecessor.
+ // Preserve canonical loop form, which means that 'Exit' should have only
+ // one predecessor.
BasicBlock *ExitSplit = SplitCriticalEdge(L->getLoopLatch(), Exit, this);
ExitSplit->moveBefore(Exit);
} else {