#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <map>
#include <set>
class SimplifyCFGOpt {
const TargetTransformInfo &TTI;
+ unsigned BonusInstThreshold;
const DataLayout *const DL;
AssumptionTracker *AT;
Value *isValueEqualityComparison(TerminatorInst *TI);
bool SimplifyCondBranch(BranchInst *BI, IRBuilder <>&Builder);
public:
- SimplifyCFGOpt(const TargetTransformInfo &TTI, const DataLayout *DL,
- AssumptionTracker *AT)
- : TTI(TTI), DL(DL), AT(AT) {}
+ SimplifyCFGOpt(const TargetTransformInfo &TTI, unsigned BonusInstThreshold,
+ const DataLayout *DL, AssumptionTracker *AT)
+ : TTI(TTI), BonusInstThreshold(BonusInstThreshold), DL(DL), AT(AT) {}
bool run(BasicBlock *BB);
};
}
/// FoldBranchToCommonDest - If this basic block is simple enough, and if a
/// predecessor branches to us and one of our successors, fold the block into
/// the predecessor and use logical operations to pick the right destination.
-bool llvm::FoldBranchToCommonDest(BranchInst *BI, const DataLayout *DL) {
+bool llvm::FoldBranchToCommonDest(BranchInst *BI, const DataLayout *DL,
+ unsigned BonusInstThreshold) {
BasicBlock *BB = BI->getParent();
Instruction *Cond = nullptr;
Cond->getParent() != BB || !Cond->hasOneUse())
return false;
- // Only allow this if the condition is a simple instruction that can be
- // executed unconditionally. It must be in the same block as the branch, and
- // must be at the front of the block.
- BasicBlock::iterator FrontIt = BB->front();
-
- // Ignore dbg intrinsics.
- while (isa<DbgInfoIntrinsic>(FrontIt)) ++FrontIt;
-
- // Allow a single instruction to be hoisted in addition to the compare
- // that feeds the branch. We later ensure that any values that _it_ uses
- // were also live in the predecessor, so that we don't unnecessarily create
- // register pressure or inhibit out-of-order execution.
- Instruction *BonusInst = nullptr;
- if (&*FrontIt != Cond &&
- FrontIt->hasOneUse() && FrontIt->user_back() == Cond &&
- isSafeToSpeculativelyExecute(FrontIt, DL)) {
- BonusInst = &*FrontIt;
- ++FrontIt;
-
- // Ignore dbg intrinsics.
- while (isa<DbgInfoIntrinsic>(FrontIt)) ++FrontIt;
- }
-
- // Only a single bonus inst is allowed.
- if (&*FrontIt != Cond)
- return false;
-
// Make sure the instruction after the condition is the cond branch.
BasicBlock::iterator CondIt = Cond; ++CondIt;
if (&*CondIt != BI)
return false;
+ // Only allow this transformation if computing the condition doesn't involve
+ // too many instructions and these involved instructions can be executed
+ // unconditionally. We denote all involved instructions except the condition
+ // as "bonus instructions", and only allow this transformation when the
+ // number of the bonus instructions does not exceed a certain threshold.
+ unsigned NumBonusInsts = 0;
+ for (auto I = BB->begin(); Cond != I; ++I) {
+ // Ignore dbg intrinsics.
+ if (isa<DbgInfoIntrinsic>(I))
+ continue;
+ if (!I->hasOneUse() || !isSafeToSpeculativelyExecute(I, DL))
+ return false;
+ // I has only one use and can be executed unconditionally.
+ Instruction *User = dyn_cast<Instruction>(I->user_back());
+ if (User == nullptr || User->getParent() != BB)
+ return false;
+ // I is used in the same BB. Since BI uses Cond and doesn't have more slots
+ // to use any other instruction, User must be an instruction between next(I)
+ // and Cond.
+ ++NumBonusInsts;
+ // Early exits once we reach the limit.
+ if (NumBonusInsts > BonusInstThreshold)
+ return false;
+ }
+
// Cond is known to be a compare or binary operator. Check to make sure that
// neither operand is a potentially-trapping constant expression.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0)))
PBI->swapSuccessors();
}
- // If we have a bonus inst, clone it into the predecessor block.
- Instruction *NewBonus = nullptr;
- if (BonusInst) {
- NewBonus = BonusInst->clone();
+ // If we have bonus instructions, clone them into the predecessor block.
+ // Note that there may be mutliple predecessor blocks, so we cannot move
+ // bonus instructions to a predecessor block.
+ ValueToValueMapTy VMap; // maps original values to cloned values
+ // We already make sure Cond is the last instruction before BI. Therefore,
+ // every instructions before Cond other than DbgInfoIntrinsic are bonus
+ // instructions.
+ for (auto BonusInst = BB->begin(); Cond != BonusInst; ++BonusInst) {
+ if (isa<DbgInfoIntrinsic>(BonusInst))
+ continue;
+ Instruction *NewBonusInst = BonusInst->clone();
+ RemapInstruction(NewBonusInst, VMap,
+ RF_NoModuleLevelChanges | RF_IgnoreMissingEntries);
+ VMap[BonusInst] = NewBonusInst;
// If we moved a load, we cannot any longer claim any knowledge about
// its potential value. The previous information might have been valid
// only given the branch precondition.
// For an analogous reason, we must also drop all the metadata whose
// semantics we don't understand.
- NewBonus->dropUnknownMetadata(LLVMContext::MD_dbg);
+ NewBonusInst->dropUnknownMetadata(LLVMContext::MD_dbg);
- PredBlock->getInstList().insert(PBI, NewBonus);
- NewBonus->takeName(BonusInst);
- BonusInst->setName(BonusInst->getName()+".old");
+ PredBlock->getInstList().insert(PBI, NewBonusInst);
+ NewBonusInst->takeName(BonusInst);
+ BonusInst->setName(BonusInst->getName() + ".old");
}
// Clone Cond into the predecessor basic block, and or/and the
// two conditions together.
Instruction *New = Cond->clone();
- if (BonusInst) New->replaceUsesOfWith(BonusInst, NewBonus);
+ RemapInstruction(New, VMap,
+ RF_NoModuleLevelChanges | RF_IgnoreMissingEntries);
PredBlock->getInstList().insert(PBI, New);
New->takeName(Cond);
- Cond->setName(New->getName()+".old");
+ Cond->setName(New->getName() + ".old");
if (BI->isConditional()) {
Instruction *NewCond =
/// the PHI, merging the third icmp into the switch.
static bool TryToSimplifyUncondBranchWithICmpInIt(
ICmpInst *ICI, IRBuilder<> &Builder, const TargetTransformInfo &TTI,
- const DataLayout *DL, AssumptionTracker *AT) {
+ unsigned BonusInstThreshold, const DataLayout *DL, AssumptionTracker *AT) {
BasicBlock *BB = ICI->getParent();
// If the block has any PHIs in it or the icmp has multiple uses, it is too
ICI->eraseFromParent();
}
// BB is now empty, so it is likely to simplify away.
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
}
// Ok, the block is reachable from the default dest. If the constant we're
ICI->replaceAllUsesWith(V);
ICI->eraseFromParent();
// BB is now empty, so it is likely to simplify away.
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
}
// The use of the icmp has to be in the 'end' block, by the only PHI node in
// see if that predecessor totally determines the outcome of this switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred, Builder))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
Value *Cond = SI->getCondition();
if (SelectInst *Select = dyn_cast<SelectInst>(Cond))
if (SimplifySwitchOnSelect(SI, Select))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
// If the block only contains the switch, see if we can fold the block
// away into any preds.
++BBI;
if (SI == &*BBI)
if (FoldValueComparisonIntoPredecessors(SI, Builder))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
}
// Try to transform the switch into an icmp and a branch.
if (TurnSwitchRangeIntoICmp(SI, Builder))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
// Remove unreachable cases.
if (EliminateDeadSwitchCases(SI, DL, AT))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
if (ForwardSwitchConditionToPHI(SI))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
if (SwitchToLookupTable(SI, Builder, TTI, DL))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
return false;
}
if (SelectInst *SI = dyn_cast<SelectInst>(IBI->getAddress())) {
if (SimplifyIndirectBrOnSelect(IBI, SI))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
}
return Changed;
}
for (++I; isa<DbgInfoIntrinsic>(I); ++I)
;
if (I->isTerminator() &&
- TryToSimplifyUncondBranchWithICmpInIt(ICI, Builder, TTI, DL, AT))
+ TryToSimplifyUncondBranchWithICmpInIt(ICI, Builder, TTI,
+ BonusInstThreshold, DL, AT))
return true;
}
// branches to us and our successor, fold the comparison into the
// predecessor and use logical operations to update the incoming value
// for PHI nodes in common successor.
- if (FoldBranchToCommonDest(BI, DL))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ if (FoldBranchToCommonDest(BI, DL, BonusInstThreshold))
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
return false;
}
// switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred, Builder))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
// This block must be empty, except for the setcond inst, if it exists.
// Ignore dbg intrinsics.
++I;
if (&*I == BI) {
if (FoldValueComparisonIntoPredecessors(BI, Builder))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
} else if (&*I == cast<Instruction>(BI->getCondition())){
++I;
// Ignore dbg intrinsics.
while (isa<DbgInfoIntrinsic>(I))
++I;
if (&*I == BI && FoldValueComparisonIntoPredecessors(BI, Builder))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
}
}
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and one of our successors, fold the comparison into the
// predecessor and use logical operations to pick the right destination.
- if (FoldBranchToCommonDest(BI, DL))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ if (FoldBranchToCommonDest(BI, DL, BonusInstThreshold))
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
// We have a conditional branch to two blocks that are only reachable
// from BI. We know that the condbr dominates the two blocks, so see if
if (BI->getSuccessor(0)->getSinglePredecessor()) {
if (BI->getSuccessor(1)->getSinglePredecessor()) {
if (HoistThenElseCodeToIf(BI, DL))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
} else {
// If Successor #1 has multiple preds, we may be able to conditionally
// execute Successor #0 if it branches to Successor #1.
if (Succ0TI->getNumSuccessors() == 1 &&
Succ0TI->getSuccessor(0) == BI->getSuccessor(1))
if (SpeculativelyExecuteBB(BI, BI->getSuccessor(0), DL))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
}
} else if (BI->getSuccessor(1)->getSinglePredecessor()) {
// If Successor #0 has multiple preds, we may be able to conditionally
if (Succ1TI->getNumSuccessors() == 1 &&
Succ1TI->getSuccessor(0) == BI->getSuccessor(0))
if (SpeculativelyExecuteBB(BI, BI->getSuccessor(1), DL))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
}
// If this is a branch on a phi node in the current block, thread control
if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
if (PN->getParent() == BI->getParent())
if (FoldCondBranchOnPHI(BI, DL))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
// Scan predecessor blocks for conditional branches.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
if (PBI != BI && PBI->isConditional())
if (SimplifyCondBranchToCondBranch(PBI, BI))
- return SimplifyCFG(BB, TTI, DL, AT) | true;
+ return SimplifyCFG(BB, TTI, BonusInstThreshold, DL, AT) | true;
return false;
}
/// of the CFG. It returns true if a modification was made.
///
bool llvm::SimplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI,
+ unsigned BonusInstThreshold,
const DataLayout *DL, AssumptionTracker *AT) {
- return SimplifyCFGOpt(TTI, DL, AT).run(BB);
+ return SimplifyCFGOpt(TTI, BonusInstThreshold, DL, AT).run(BB);
}
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