Add -unroll-runtime for unrolling loops with run-time trip counts.
Patch by Brendon Cahoon!
This extends the existing LoopUnroll and LoopUnrollPass. Brendon
measured no regressions in the llvm test suite with -unroll-runtime
enabled. This implementation works by using the existing loop
unrolling code to unroll the loop by a power-of-two (default 8). It
generates an if-then-else sequence of code prior to the loop to
execute the extra iterations before entering the unrolled loop.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146245 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/lib/Transforms/Utils/LoopUnrollRuntime.cpp
new file mode 100644
index 0000000..2b92e59
--- /dev/null
+++ b/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -0,0 +1,375 @@
+//===-- UnrollLoopRuntime.cpp - Runtime Loop unrolling utilities ----------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements some loop unrolling utilities for loops with run-time
+// trip counts. See LoopUnroll.cpp for unrolling loops with compile-time
+// trip counts.
+//
+// The functions in this file are used to generate extra code when the
+// run-time trip count modulo the unroll factor is not 0. When this is the
+// case, we need to generate code to execute these 'left over' iterations.
+//
+// The current strategy generates an if-then-else sequence prior to the
+// unrolled loop to execute the 'left over' iterations. Other strategies
+// include generate a loop before or after the unrolled loop.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "loop-unroll"
+#include "llvm/Transforms/Utils/UnrollLoop.h"
+#include "llvm/BasicBlock.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/LoopIterator.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionExpander.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include <algorithm>
+
+using namespace llvm;
+
+STATISTIC(NumRuntimeUnrolled,
+ "Number of loops unrolled with run-time trip counts");
+
+/// Connect the unrolling prolog code to the original loop.
+/// The unrolling prolog code contains code to execute the
+/// 'extra' iterations if the run-time trip count modulo the
+/// unroll count is non-zero.
+///
+/// This function performs the following:
+/// - Create PHI nodes at prolog end block to combine values
+/// that exit the prolog code and jump around the prolog.
+/// - Add a PHI operand to a PHI node at the loop exit block
+/// for values that exit the prolog and go around the loop.
+/// - Branch around the original loop if the trip count is less
+/// than the unroll factor.
+///
+static void ConnectProlog(Loop *L, Value *TripCount, unsigned Count,
+ BasicBlock *LastPrologBB, BasicBlock *PrologEnd,
+ BasicBlock *OrigPH, BasicBlock *NewPH,
+ ValueToValueMapTy &LVMap, Pass *P) {
+ BasicBlock *Latch = L->getLoopLatch();
+ assert(Latch != 0 && "Loop must have a latch");
+
+ // Create a PHI node for each outgoing value from the original loop
+ // (which means it is an outgoing value from the prolog code too).
+ // The new PHI node is inserted in the prolog end basic block.
+ // The new PHI name is added as an operand of a PHI node in either
+ // the loop header or the loop exit block.
+ for (succ_iterator SBI = succ_begin(Latch), SBE = succ_end(Latch);
+ SBI != SBE; ++SBI) {
+ for (BasicBlock::iterator BBI = (*SBI)->begin();
+ PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
+
+ // Add a new PHI node to the prolog end block and add the
+ // appropriate incoming values.
+ PHINode *NewPN = PHINode::Create(PN->getType(), 2, PN->getName()+".unr",
+ PrologEnd->getTerminator());
+ // Adding a value to the new PHI node from the original loop preheader.
+ // This is the value that skips all the prolog code.
+ if (L->contains(PN)) {
+ NewPN->addIncoming(PN->getIncomingValueForBlock(NewPH), OrigPH);
+ } else {
+ NewPN->addIncoming(Constant::getNullValue(PN->getType()), OrigPH);
+ }
+ Value *OrigVal = PN->getIncomingValueForBlock(Latch);
+ Value *V = OrigVal;
+ if (Instruction *I = dyn_cast<Instruction>(V)) {
+ if (L->contains(I)) {
+ V = LVMap[I];
+ }
+ }
+ // Adding a value to the new PHI node from the last prolog block
+ // that was created.
+ NewPN->addIncoming(V, LastPrologBB);
+
+ // Update the existing PHI node operand with the value from the
+ // new PHI node. How this is done depends on if the existing
+ // PHI node is in the original loop block, or the exit block.
+ if (L->contains(PN)) {
+ PN->setIncomingValue(PN->getBasicBlockIndex(NewPH), NewPN);
+ } else {
+ PN->addIncoming(NewPN, PrologEnd);
+ }
+ }
+ }
+
+ // Create a branch around the orignal loop, which is taken if the
+ // trip count is less than the unroll factor.
+ Instruction *InsertPt = PrologEnd->getTerminator();
+ Instruction *BrLoopExit =
+ new ICmpInst(InsertPt, ICmpInst::ICMP_ULT, TripCount,
+ ConstantInt::get(TripCount->getType(), Count));
+ BasicBlock *Exit = L->getUniqueExitBlock();
+ assert(Exit != 0 && "Loop must have a single exit block only");
+ // Split the exit to maintain loop canonicalization guarantees
+ SmallVector<BasicBlock*, 4> Preds(pred_begin(Exit), pred_end(Exit));
+ if (!Exit->isLandingPad()) {
+ SplitBlockPredecessors(Exit, Preds.data(), Preds.size(),
+ ".unr-lcssa", P);
+ } else {
+ SmallVector<BasicBlock*, 2> NewBBs;
+ SplitLandingPadPredecessors(Exit, Preds, ".unr1-lcssa", ".unr2-lcssa",
+ P, NewBBs);
+ }
+ // Add the branch to the exit block (around the unrolled loop)
+ BranchInst::Create(Exit, NewPH, BrLoopExit, InsertPt);
+ InsertPt->eraseFromParent();
+}
+
+/// Create a clone of the blocks in a loop and connect them together.
+/// This function doesn't create a clone of the loop structure.
+///
+/// There are two value maps that are defined and used. VMap is
+/// for the values in the current loop instance. LVMap contains
+/// the values from the last loop instance. We need the LVMap values
+/// to update the inital values for the current loop instance.
+///
+static void CloneLoopBlocks(Loop *L,
+ bool FirstCopy,
+ BasicBlock *InsertTop,
+ BasicBlock *InsertBot,
+ std::vector<BasicBlock *> &NewBlocks,
+ LoopBlocksDFS &LoopBlocks,
+ ValueToValueMapTy &VMap,
+ ValueToValueMapTy &LVMap,
+ LoopInfo *LI) {
+
+ BasicBlock *Preheader = L->getLoopPreheader();
+ BasicBlock *Header = L->getHeader();
+ BasicBlock *Latch = L->getLoopLatch();
+ Function *F = Header->getParent();
+ LoopBlocksDFS::RPOIterator BlockBegin = LoopBlocks.beginRPO();
+ LoopBlocksDFS::RPOIterator BlockEnd = LoopBlocks.endRPO();
+ // For each block in the original loop, create a new copy,
+ // and update the value map with the newly created values.
+ for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
+ BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, ".unr", F);
+ NewBlocks.push_back(NewBB);
+
+ if (Loop *ParentLoop = L->getParentLoop())
+ ParentLoop->addBasicBlockToLoop(NewBB, LI->getBase());
+
+ VMap[*BB] = NewBB;
+ if (Header == *BB) {
+ // For the first block, add a CFG connection to this newly
+ // created block
+ InsertTop->getTerminator()->setSuccessor(0, NewBB);
+
+ // Change the incoming values to the ones defined in the
+ // previously cloned loop.
+ for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
+ PHINode *NewPHI = cast<PHINode>(VMap[I]);
+ if (FirstCopy) {
+ // We replace the first phi node with the value from the preheader
+ VMap[I] = NewPHI->getIncomingValueForBlock(Preheader);
+ NewBB->getInstList().erase(NewPHI);
+ } else {
+ // Update VMap with values from the previous block
+ unsigned idx = NewPHI->getBasicBlockIndex(Latch);
+ Value *InVal = NewPHI->getIncomingValue(idx);
+ if (Instruction *I = dyn_cast<Instruction>(InVal))
+ if (L->contains(I))
+ InVal = LVMap[InVal];
+ NewPHI->setIncomingValue(idx, InVal);
+ NewPHI->setIncomingBlock(idx, InsertTop);
+ }
+ }
+ }
+
+ if (Latch == *BB) {
+ VMap.erase((*BB)->getTerminator());
+ NewBB->getTerminator()->eraseFromParent();
+ BranchInst::Create(InsertBot, NewBB);
+ }
+ }
+ // LastValueMap is updated with the values for the current loop
+ // which are used the next time this function is called.
+ for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
+ VI != VE; ++VI) {
+ LVMap[VI->first] = VI->second;
+ }
+}
+
+/// Insert code in the prolog code when unrolling a loop with a
+/// run-time trip-count.
+///
+/// This method assumes that the loop unroll factor is total number
+/// of loop bodes in the loop after unrolling. (Some folks refer
+/// to the unroll factor as the number of *extra* copies added).
+/// We assume also that the loop unroll factor is a power-of-two. So, after
+/// unrolling the loop, the number of loop bodies executed is 2,
+/// 4, 8, etc. Note - LLVM converts the if-then-sequence to a switch
+/// instruction in SimplifyCFG.cpp. Then, the backend decides how code for
+/// the switch instruction is generated.
+///
+/// extraiters = tripcount % loopfactor
+/// if (extraiters == 0) jump Loop:
+/// if (extraiters == loopfactor) jump L1
+/// if (extraiters == loopfactor-1) jump L2
+/// ...
+/// L1: LoopBody;
+/// L2: LoopBody;
+/// ...
+/// if tripcount < loopfactor jump End
+/// Loop:
+/// ...
+/// End:
+///
+bool llvm::UnrollRuntimeLoopProlog(Loop *L, unsigned Count, LoopInfo *LI,
+ LPPassManager *LPM) {
+ // for now, only unroll loops that contain a single exit
+ SmallVector<BasicBlock*, 4> ExitingBlocks;
+ L->getExitingBlocks(ExitingBlocks);
+ if (ExitingBlocks.size() > 1)
+ return false;
+
+ // Make sure the loop is in canonical form, and there is a single
+ // exit block only.
+ if (!L->isLoopSimplifyForm() || L->getUniqueExitBlock() == 0)
+ return false;
+
+ // Use Scalar Evolution to compute the trip count. This allows more
+ // loops to be unrolled than relying on induction var simplification
+ ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
+ if (SE == 0)
+ return false;
+
+ // Only unroll loops with a computable trip count and the trip count needs
+ // to be an int value (allowing a pointer type is a TODO item)
+ const SCEV *BECount = SE->getBackedgeTakenCount(L);
+ if (isa<SCEVCouldNotCompute>(BECount) || !BECount->getType()->isIntegerTy())
+ return false;
+
+ // Add 1 since the backedge count doesn't include the first loop iteration
+ const SCEV *TripCountSC =
+ SE->getAddExpr(BECount, SE->getConstant(BECount->getType(), 1));
+ if (isa<SCEVCouldNotCompute>(TripCountSC))
+ return false;
+
+ // We only handle cases when the unroll factor is a power of 2.
+ // Count is the loop unroll factor, the number of extra copies added + 1.
+ if ((Count & (Count-1)) != 0)
+ return false;
+
+ // If this loop is nested, then the loop unroller changes the code in
+ // parent loop, so the Scalar Evolution pass needs to be run again
+ if (Loop *ParentLoop = L->getParentLoop())
+ SE->forgetLoop(ParentLoop);
+
+ BasicBlock *PH = L->getLoopPreheader();
+ BasicBlock *Header = L->getHeader();
+ BasicBlock *Latch = L->getLoopLatch();
+ // It helps to splits the original preheader twice, one for the end of the
+ // prolog code and one for a new loop preheader
+ BasicBlock *PEnd = SplitEdge(PH, Header, LPM->getAsPass());
+ BasicBlock *NewPH = SplitBlock(PEnd, PEnd->getTerminator(), LPM->getAsPass());
+ BranchInst *PreHeaderBR = cast<BranchInst>(PH->getTerminator());
+
+ // Compute the number of extra iterations required, which is:
+ // extra iterations = run-time trip count % (loop unroll factor + 1)
+ SCEVExpander Expander(*SE, "loop-unroll");
+ Value *TripCount = Expander.expandCodeFor(TripCountSC, TripCountSC->getType(),
+ PreHeaderBR);
+ Type *CountTy = TripCount->getType();
+ BinaryOperator *ModVal =
+ BinaryOperator::CreateURem(TripCount,
+ ConstantInt::get(CountTy, Count),
+ "xtraiter");
+ ModVal->insertBefore(PreHeaderBR);
+
+ // Check if for no extra iterations, then jump to unrolled loop
+ Value *BranchVal = new ICmpInst(PreHeaderBR,
+ ICmpInst::ICMP_NE, ModVal,
+ ConstantInt::get(CountTy, 0), "lcmp");
+ // Branch to either the extra iterations or the unrolled loop
+ // We will fix up the true branch label when adding loop body copies
+ BranchInst::Create(PEnd, PEnd, BranchVal, PreHeaderBR);
+ assert(PreHeaderBR->isUnconditional() &&
+ PreHeaderBR->getSuccessor(0) == PEnd &&
+ "CFG edges in Preheader are not correct");
+ PreHeaderBR->eraseFromParent();
+
+ ValueToValueMapTy LVMap;
+ Function *F = Header->getParent();
+ // These variables are used to update the CFG links in each iteration
+ BasicBlock *CompareBB = 0;
+ BasicBlock *LastLoopBB = PH;
+ // Get an ordered list of blocks in the loop to help with the ordering of the
+ // cloned blocks in the prolog code
+ LoopBlocksDFS LoopBlocks(L);
+ LoopBlocks.perform(LI);
+
+ //
+ // For each extra loop iteration, create a copy of the loop's basic blocks
+ // and generate a condition that branches to the copy depending on the
+ // number of 'left over' iterations.
+ //
+ for (unsigned leftOverIters = Count-1; leftOverIters > 0; --leftOverIters) {
+ std::vector<BasicBlock*> NewBlocks;
+ ValueToValueMapTy VMap;
+
+ // Clone all the basic blocks in the loop, but we don't clone the loop
+ // This function adds the appropriate CFG connections.
+ CloneLoopBlocks(L, (leftOverIters == Count-1), LastLoopBB, PEnd, NewBlocks,
+ LoopBlocks, VMap, LVMap, LI);
+ LastLoopBB = cast<BasicBlock>(VMap[Latch]);
+
+ // Insert the cloned blocks into function just before the original loop
+ F->getBasicBlockList().splice(PEnd, F->getBasicBlockList(),
+ NewBlocks[0], F->end());
+
+ // Generate the code for the comparison which determines if the loop
+ // prolog code needs to be executed.
+ if (leftOverIters == Count-1) {
+ // There is no compare block for the fall-thru case when for the last
+ // left over iteration
+ CompareBB = NewBlocks[0];
+ } else {
+ // Create a new block for the comparison
+ BasicBlock *NewBB = BasicBlock::Create(CompareBB->getContext(), "unr.cmp",
+ F, CompareBB);
+ if (Loop *ParentLoop = L->getParentLoop()) {
+ // Add the new block to the parent loop, if needed
+ ParentLoop->addBasicBlockToLoop(NewBB, LI->getBase());
+ }
+
+ // The comparison w/ the extra iteration value and branch
+ Value *BranchVal = new ICmpInst(*NewBB, ICmpInst::ICMP_EQ, ModVal,
+ ConstantInt::get(CountTy, leftOverIters),
+ "un.tmp");
+ // Branch to either the extra iterations or the unrolled loop
+ BranchInst::Create(NewBlocks[0], CompareBB,
+ BranchVal, NewBB);
+ CompareBB = NewBB;
+ PH->getTerminator()->setSuccessor(0, NewBB);
+ VMap[NewPH] = CompareBB;
+ }
+
+ // Rewrite the cloned instruction operands to use the values
+ // created when the clone is created.
+ for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) {
+ for (BasicBlock::iterator I = NewBlocks[i]->begin(),
+ E = NewBlocks[i]->end(); I != E; ++I) {
+ RemapInstruction(I, VMap,
+ RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
+ }
+ }
+ }
+
+ // Connect the prolog code to the original loop and update the
+ // PHI functions.
+ ConnectProlog(L, TripCount, Count, LastLoopBB, PEnd, PH, NewPH, LVMap,
+ LPM->getAsPass());
+ NumRuntimeUnrolled++;
+ return true;
+}