LLVM  10.0.0svn

This pass performs 'jump threading', which looks at blocks that have multiple predecessors and multiple successors. More...

#include "llvm/Transforms/Scalar/JumpThreading.h"

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## Public Member Functions

bool runImpl (Function &F, TargetLibraryInfo *TLI_, LazyValueInfo *LVI_, AliasAnalysis *AA_, DomTreeUpdater *DTU_, bool HasProfileData_, std::unique_ptr< BlockFrequencyInfo > BFI_, std::unique_ptr< BranchProbabilityInfo > BPI_)

PreservedAnalyses run (Function &F, FunctionAnalysisManager &AM)

void releaseMemory ()

FindLoopHeaders - We do not want jump threading to turn proper loop structures into irreducible loops. More...

bool ProcessBlock (BasicBlock *BB)
ProcessBlock - If there are any predecessors whose control can be threaded through to a successor, transform them now. More...

bool ThreadEdge (BasicBlock *BB, const SmallVectorImpl< BasicBlock *> &PredBBs, BasicBlock *SuccBB)
ThreadEdge - We have decided that it is safe and profitable to factor the blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB across BB. More...

bool DuplicateCondBranchOnPHIIntoPred (BasicBlock *BB, const SmallVectorImpl< BasicBlock *> &PredBBs)
DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch to BB which contains an i1 PHI node and a conditional branch on that PHI. More...

bool ComputeValueKnownInPredecessorsImpl (Value *V, BasicBlock *BB, jumpthreading::PredValueInfo &Result, jumpthreading::ConstantPreference Preference, DenseSet< std::pair< Value *, BasicBlock *>> &RecursionSet, Instruction *CxtI=nullptr)
ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see if we can infer that the value is a known ConstantInt/BlockAddress or undef in any of our predecessors. More...

bool ComputeValueKnownInPredecessors (Value *V, BasicBlock *BB, jumpthreading::PredValueInfo &Result, jumpthreading::ConstantPreference Preference, Instruction *CxtI=nullptr)

bool ProcessBranchOnPHI (PHINode *PN)
ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on a PHI node in the current block. More...

bool ProcessBranchOnXOR (BinaryOperator *BO)
ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on a xor instruction in the current block. More...

bool ProcessImpliedCondition (BasicBlock *BB)

SimplifyPartiallyRedundantLoad - If LoadI is an obviously partially redundant load instruction, eliminate it by replacing it with a PHI node. More...

void UnfoldSelectInstr (BasicBlock *Pred, BasicBlock *BB, SelectInst *SI, PHINode *SIUse, unsigned Idx)

bool TryToUnfoldSelect (CmpInst *CondCmp, BasicBlock *BB)
TryToUnfoldSelect - Look for blocks of the form bb1: a = select br bb2. More...

bool TryToUnfoldSelect (SwitchInst *SI, BasicBlock *BB)

bool TryToUnfoldSelectInCurrBB (BasicBlock *BB)
TryToUnfoldSelectInCurrBB - Look for PHI/Select or PHI/CMP/Select in the same BB in the form bb: p = phi [false, bb1], [true, bb2], [false, bb3], [true, bb4], ... More...

bool ProcessGuards (BasicBlock *BB)
Try to propagate a guard from the current BB into one of its predecessors in case if another branch of execution implies that the condition of this guard is always true. More...

bool ThreadGuard (BasicBlock *BB, IntrinsicInst *Guard, BranchInst *BI)
Try to propagate the guard from BB which is the lower block of a diamond to one of its branches, in case if diamond's condition implies guard's condition. More...

Static Public Member Functions inherited from llvm::PassInfoMixin< JumpThreadingPass >
static StringRef name ()
Gets the name of the pass we are mixed into. More...

## Detailed Description

This pass performs 'jump threading', which looks at blocks that have multiple predecessors and multiple successors.

If one or more of the predecessors of the block can be proven to always jump to one of the successors, we forward the edge from the predecessor to the successor by duplicating the contents of this block.

An example of when this can occur is code like this:

if () { ... X = 4; } if (X < 3) {

In this case, the unconditional branch at the end of the first if can be revectored to the false side of the second if.

Definition at line 77 of file JumpThreading.h.

## Constructor & Destructor Documentation

 JumpThreadingPass::JumpThreadingPass ( int T = -1 )

Definition at line 171 of file JumpThreading.cpp.

References BBDuplicateThreshold.

## ◆ ComputeValueKnownInPredecessors()

 bool llvm::JumpThreadingPass::ComputeValueKnownInPredecessors ( Value * V, BasicBlock * BB, jumpthreading::PredValueInfo & Result, jumpthreading::ConstantPreference Preference, Instruction * CxtI = nullptr )
inline

Definition at line 123 of file JumpThreading.h.

## ◆ ComputeValueKnownInPredecessorsImpl()

 bool JumpThreadingPass::ComputeValueKnownInPredecessorsImpl ( Value * V, BasicBlock * BB, jumpthreading::PredValueInfo & Result, jumpthreading::ConstantPreference Preference, DenseSet< std::pair< Value *, BasicBlock *>> & RecursionSet, Instruction * CxtI = nullptr )

ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see if we can infer that the value is a known ConstantInt/BlockAddress or undef in any of our predecessors.

If so, return the known list of value and pred BB in the result vector.

This returns true if there were any known values.

TODO: Per PR2563, we could infer value range information about a predecessor based on its terminator.

If I is a PHI node, then we know the incoming values for any constants.

Definition at line 593 of file JumpThreading.cpp.

## ◆ DuplicateCondBranchOnPHIIntoPred()

 bool JumpThreadingPass::DuplicateCondBranchOnPHIIntoPred ( BasicBlock * BB, const SmallVectorImpl< BasicBlock *> & PredBBs )

DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch to BB which contains an i1 PHI node and a conditional branch on that PHI.

If we can duplicate the contents of BB up into PredBB do so now, this improves the odds that the branch will be on an analyzable instruction like a compare.

Definition at line 2262 of file JumpThreading.cpp.

FindLoopHeaders - We do not want jump threading to turn proper loop structures into irreducible loops.

Doing this breaks up the loop nesting hierarchy and pessimizes later transformations. To prevent this from happening, we first have to find the loop headers. Here we approximate this by finding targets of backedges in the CFG.

Note that there definitely are cases when we want to allow threading of edges across a loop header. For example, threading a jump from outside the loop (the preheader) to an exit block of the loop is definitely profitable. It is also almost always profitable to thread backedges from within the loop to exit blocks, and is often profitable to thread backedges to other blocks within the loop (forming a nested loop). This simple analysis is not rich enough to track all of these properties and keep it up-to-date as the CFG mutates, so we don't allow any of these transformations.

Definition at line 560 of file JumpThreading.cpp.

References llvm::FindFunctionBackedges().

## ◆ ProcessBranchOnPHI()

 bool JumpThreadingPass::ProcessBranchOnPHI ( PHINode * PN )

ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on a PHI node in the current block.

See if there are any simplifications we can do based on inputs to the phi node.

Definition at line 1767 of file JumpThreading.cpp.

## ◆ ProcessBranchOnXOR()

 bool JumpThreadingPass::ProcessBranchOnXOR ( BinaryOperator * BO )

ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on a xor instruction in the current block.

See if there are any simplifications we can do based on inputs to the xor.

Definition at line 1796 of file JumpThreading.cpp.

## ◆ ProcessGuards()

 bool JumpThreadingPass::ProcessGuards ( BasicBlock * BB )

Try to propagate a guard from the current BB into one of its predecessors in case if another branch of execution implies that the condition of this guard is always true.

Currently we only process the simplest case that looks like:

Start: cond = ... br i1 cond, label T1, label F1 T1: br label Merge F1: br label Merge Merge: condGuard = ... call void(i1, ...) .experimental.guard( i1 condGuard )[ "deopt"() ]

And cond either implies condGuard or !condGuard. In this case all the instructions before the guard can be duplicated in both branches, and the guard is then threaded to one of them.

Definition at line 2652 of file JumpThreading.cpp.

## ◆ ProcessImpliedCondition()

 bool JumpThreadingPass::ProcessImpliedCondition ( BasicBlock * BB )

Definition at line 1242 of file JumpThreading.cpp.

References llvm::dyn_cast(), and llvm::BasicBlock::getTerminator().

 bool JumpThreadingPass::ProcessThreadableEdges ( Value * Cond, BasicBlock * BB, jumpthreading::ConstantPreference Preference, Instruction * CxtI = nullptr )

Definition at line 1592 of file JumpThreading.cpp.

## ◆ releaseMemory()

inline

Definition at line 105 of file JumpThreading.h.

References ProcessBlock().

## ◆ run()

 PreservedAnalyses JumpThreadingPass::run ( Function & F, FunctionAnalysisManager & AM )

## ◆ runImpl()

 bool JumpThreadingPass::runImpl ( Function & F, TargetLibraryInfo * TLI_, LazyValueInfo * LVI_, AliasAnalysis * AA_, DomTreeUpdater * DTU_, bool HasProfileData_, std::unique_ptr< BlockFrequencyInfo > BFI_, std::unique_ptr< BranchProbabilityInfo > BPI_ )

SimplifyPartiallyRedundantLoad - If LoadI is an obviously partially redundant load instruction, eliminate it by replacing it with a PHI node.

This is an important optimization that encourages jump threading, and needs to be run interlaced with other jump threading tasks.

Definition at line 1293 of file JumpThreading.cpp.

 bool JumpThreadingPass::ThreadEdge ( BasicBlock * BB, const SmallVectorImpl< BasicBlock *> & PredBBs, BasicBlock * SuccBB )

 bool JumpThreadingPass::ThreadGuard ( BasicBlock * BB, IntrinsicInst * Guard, BranchInst * BI )

Try to propagate the guard from BB which is the lower block of a diamond to one of its branches, in case if diamond's condition implies guard's condition.

Definition at line 2686 of file JumpThreading.cpp.

## ◆ TryToUnfoldSelect() [1/2]

 bool JumpThreadingPass::TryToUnfoldSelect ( CmpInst * CondCmp, BasicBlock * BB )

TryToUnfoldSelect - Look for blocks of the form bb1: a = select br bb2.

bb2: p = phi [a, bb1] ... c = icmp p br i1 c

And expand the select into a branch structure if one of its arms allows c to be folded. This later enables threading from bb1 over bb2.

Definition at line 2497 of file JumpThreading.cpp.

## ◆ TryToUnfoldSelect() [2/2]

 bool JumpThreadingPass::TryToUnfoldSelect ( SwitchInst * SI, BasicBlock * BB )

## ◆ TryToUnfoldSelectInCurrBB()

 bool JumpThreadingPass::TryToUnfoldSelectInCurrBB ( BasicBlock * BB )

TryToUnfoldSelectInCurrBB - Look for PHI/Select or PHI/CMP/Select in the same BB in the form bb: p = phi [false, bb1], [true, bb2], [false, bb3], [true, bb4], ...

s = select p, trueval, falseval

or

bb: p = phi [0, bb1], [1, bb2], [0, bb3], [1, bb4], ... c = cmp p, 0 s = select c, trueval, falseval

And expand the select into a branch structure. This later enables jump-threading over bb in this pass.

Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold select if the associated PHI has at least one constant. If the unfolded select is not jump-threaded, it will be folded again in the later optimizations.

Definition at line 2562 of file JumpThreading.cpp.

## ◆ UnfoldSelectInstr()

 void JumpThreadingPass::UnfoldSelectInstr ( BasicBlock * Pred, BasicBlock * BB, SelectInst * SI, PHINode * SIUse, unsigned Idx )

The documentation for this class was generated from the following files: