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

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//===- SpeculateAroundPHIs.cpp --------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/SpeculateAroundPHIs.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
#define DEBUG_TYPE "spec-phis"
STATISTIC(NumPHIsSpeculated, "Number of PHI nodes we speculated around");
STATISTIC(NumEdgesSplit,
"Number of critical edges which were split for speculation");
STATISTIC(NumSpeculatedInstructions,
"Number of instructions we speculated around the PHI nodes");
STATISTIC(NumNewRedundantInstructions,
"Number of new, redundant instructions inserted");
/// Check whether speculating the users of a PHI node around the PHI
/// will be safe.
///
/// This checks both that all of the users are safe and also that all of their
/// operands are either recursively safe or already available along an incoming
/// edge to the PHI.
///
/// This routine caches both all the safe nodes explored in `PotentialSpecSet`
/// and the chain of nodes that definitively reach any unsafe node in
/// `UnsafeSet`. By preserving these between repeated calls to this routine for
/// PHIs in the same basic block, the exploration here can be reused. However,
/// these caches must no be reused for PHIs in a different basic block as they
/// reflect what is available along incoming edges.
static bool
isSafeToSpeculatePHIUsers(PHINode &PN, DominatorTree &DT,
SmallPtrSetImpl<Instruction *> &PotentialSpecSet,
SmallPtrSetImpl<Instruction *> &UnsafeSet) {
auto *PhiBB = PN.getParent();
SmallPtrSet<Instruction *, 4> Visited;
SmallVector<std::pair<Instruction *, User::value_op_iterator>, 16> DFSStack;
// Walk each user of the PHI node.
for (Use &U : PN.uses()) {
auto *UI = cast<Instruction>(U.getUser());
// Ensure the use post-dominates the PHI node. This ensures that, in the
// absence of unwinding, the use will actually be reached.
// FIXME: We use a blunt hammer of requiring them to be in the same basic
// block. We should consider using actual post-dominance here in the
// future.
if (UI->getParent() != PhiBB) {
LLVM_DEBUG(dbgs() << " Unsafe: use in a different BB: " << *UI << "\n");
return false;
}
// FIXME: This check is much too conservative. We're not going to move these
// instructions onto new dynamic paths through the program unless there is
// a call instruction between the use and the PHI node. And memory isn't
// changing unless there is a store in that same sequence. We should
// probably change this to do at least a limited scan of the intervening
// instructions and allow handling stores in easily proven safe cases.
if (mayBeMemoryDependent(*UI)) {
LLVM_DEBUG(dbgs() << " Unsafe: can't speculate use: " << *UI << "\n");
return false;
}
// Now do a depth-first search of everything these users depend on to make
// sure they are transitively safe. This is a depth-first search, but we
// check nodes in preorder to minimize the amount of checking.
Visited.insert(UI);
DFSStack.push_back({UI, UI->value_op_begin()});
do {
User::value_op_iterator OpIt;
std::tie(UI, OpIt) = DFSStack.pop_back_val();
while (OpIt != UI->value_op_end()) {
auto *OpI = dyn_cast<Instruction>(*OpIt);
// Increment to the next operand for whenever we continue.
++OpIt;
// No need to visit non-instructions, which can't form dependencies.
if (!OpI)
continue;
// Now do the main pre-order checks that this operand is a viable
// dependency of something we want to speculate.
// First do a few checks for instructions that won't require
// speculation at all because they are trivially available on the
// incoming edge (either through dominance or through an incoming value
// to a PHI).
//
// The cases in the current block will be trivially dominated by the
// edge.
auto *ParentBB = OpI->getParent();
if (ParentBB == PhiBB) {
if (isa<PHINode>(OpI)) {
// We can trivially map through phi nodes in the same block.
continue;
}
} else if (DT.dominates(ParentBB, PhiBB)) {
// Instructions from dominating blocks are already available.
continue;
}
// Once we know that we're considering speculating the operand, check
// if we've already explored this subgraph and found it to be safe.
if (PotentialSpecSet.count(OpI))
continue;
// If we've already explored this subgraph and found it unsafe, bail.
// If when we directly test whether this is safe it fails, bail.
if (UnsafeSet.count(OpI) || ParentBB != PhiBB ||
mayBeMemoryDependent(*OpI)) {
LLVM_DEBUG(dbgs() << " Unsafe: can't speculate transitive use: "
<< *OpI << "\n");
// Record the stack of instructions which reach this node as unsafe
// so we prune subsequent searches.
UnsafeSet.insert(OpI);
for (auto &StackPair : DFSStack) {
Instruction *I = StackPair.first;
UnsafeSet.insert(I);
}
return false;
}
// Skip any operands we're already recursively checking.
if (!Visited.insert(OpI).second)
continue;
// Push onto the stack and descend. We can directly continue this
// loop when ascending.
DFSStack.push_back({UI, OpIt});
UI = OpI;
OpIt = OpI->value_op_begin();
}
// This node and all its operands are safe. Go ahead and cache that for
// reuse later.
PotentialSpecSet.insert(UI);
// Continue with the next node on the stack.
} while (!DFSStack.empty());
}
#ifndef NDEBUG
// Every visited operand should have been marked as safe for speculation at
// this point. Verify this and return success.
for (auto *I : Visited)
assert(PotentialSpecSet.count(I) &&
"Failed to mark a visited instruction as safe!");
#endif
return true;
}
/// Check whether, in isolation, a given PHI node is both safe and profitable
/// to speculate users around.
///
/// This handles checking whether there are any constant operands to a PHI
/// which could represent a useful speculation candidate, whether the users of
/// the PHI are safe to speculate including all their transitive dependencies,
/// and whether after speculation there will be some cost savings (profit) to
/// folding the operands into the users of the PHI node. Returns true if both
/// safe and profitable with relevant cost savings updated in the map and with
/// an update to the `PotentialSpecSet`. Returns false if either safety or
/// profitability are absent. Some new entries may be made to the
/// `PotentialSpecSet` even when this routine returns false, but they remain
/// conservatively correct.
///
/// The profitability check here is a local one, but it checks this in an
/// interesting way. Beyond checking that the total cost of materializing the
/// constants will be less than the cost of folding them into their users, it
/// also checks that no one incoming constant will have a higher cost when
/// folded into its users rather than materialized. This higher cost could
/// result in a dynamic *path* that is more expensive even when the total cost
/// is lower. Currently, all of the interesting cases where this optimization
/// should fire are ones where it is a no-loss operation in this sense. If we
/// ever want to be more aggressive here, we would need to balance the
/// different incoming edges' cost by looking at their respective
/// probabilities.
static bool isSafeAndProfitableToSpeculateAroundPHI(
PHINode &PN, SmallDenseMap<PHINode *, int, 16> &CostSavingsMap,
SmallPtrSetImpl<Instruction *> &PotentialSpecSet,
SmallPtrSetImpl<Instruction *> &UnsafeSet, DominatorTree &DT,
TargetTransformInfo &TTI) {
// First see whether there is any cost savings to speculating around this
// PHI, and build up a map of the constant inputs to how many times they
// occur.
bool NonFreeMat = false;
struct CostsAndCount {
int MatCost = TargetTransformInfo::TCC_Free;
int FoldedCost = TargetTransformInfo::TCC_Free;
int Count = 0;
};
SmallDenseMap<ConstantInt *, CostsAndCount, 16> CostsAndCounts;
SmallPtrSet<BasicBlock *, 16> IncomingConstantBlocks;
for (int i : llvm::seq<int>(0, PN.getNumIncomingValues())) {
auto *IncomingC = dyn_cast<ConstantInt>(PN.getIncomingValue(i));
if (!IncomingC)
continue;
// Only visit each incoming edge with a constant input once.
if (!IncomingConstantBlocks.insert(PN.getIncomingBlock(i)).second)
continue;
auto InsertResult = CostsAndCounts.insert({IncomingC, {}});
// Count how many edges share a given incoming costant.
++InsertResult.first->second.Count;
// Only compute the cost the first time we see a particular constant.
if (!InsertResult.second)
continue;
int &MatCost = InsertResult.first->second.MatCost;
MatCost = TTI.getIntImmCost(IncomingC->getValue(), IncomingC->getType());
NonFreeMat |= MatCost != TTI.TCC_Free;
}
if (!NonFreeMat) {
LLVM_DEBUG(dbgs() << " Free: " << PN << "\n");
// No profit in free materialization.
return false;
}
// Now check that the uses of this PHI can actually be speculated,
// otherwise we'll still have to materialize the PHI value.
if (!isSafeToSpeculatePHIUsers(PN, DT, PotentialSpecSet, UnsafeSet)) {
LLVM_DEBUG(dbgs() << " Unsafe PHI: " << PN << "\n");
return false;
}
// Compute how much (if any) savings are available by speculating around this
// PHI.
for (Use &U : PN.uses()) {
auto *UserI = cast<Instruction>(U.getUser());
// Now check whether there is any savings to folding the incoming constants
// into this use.
unsigned Idx = U.getOperandNo();
// If we have a binary operator that is commutative, an actual constant
// operand would end up on the RHS, so pretend the use of the PHI is on the
// RHS.
//
// Technically, this is a bit weird if *both* operands are PHIs we're
// speculating. But if that is the case, giving an "optimistic" cost isn't
// a bad thing because after speculation it will constant fold. And
// moreover, such cases should likely have been constant folded already by
// some other pass, so we shouldn't worry about "modeling" them terribly
// accurately here. Similarly, if the other operand is a constant, it still
// seems fine to be "optimistic" in our cost modeling, because when the
// incoming operand from the PHI node is also a constant, we will end up
// constant folding.
if (UserI->isBinaryOp() && UserI->isCommutative() && Idx != 1)
// Assume we will commute the constant to the RHS to be canonical.
Idx = 1;
// Get the intrinsic ID if this user is an intrinsic.
Intrinsic::ID IID = Intrinsic::not_intrinsic;
if (auto *UserII = dyn_cast<IntrinsicInst>(UserI))
IID = UserII->getIntrinsicID();
for (auto &IncomingConstantAndCostsAndCount : CostsAndCounts) {
ConstantInt *IncomingC = IncomingConstantAndCostsAndCount.first;
int MatCost = IncomingConstantAndCostsAndCount.second.MatCost;
int &FoldedCost = IncomingConstantAndCostsAndCount.second.FoldedCost;
if (IID)
FoldedCost += TTI.getIntImmCost(IID, Idx, IncomingC->getValue(),
IncomingC->getType());
else
FoldedCost +=
TTI.getIntImmCost(UserI->getOpcode(), Idx, IncomingC->getValue(),
IncomingC->getType());
// If we accumulate more folded cost for this incoming constant than
// materialized cost, then we'll regress any edge with this constant so
// just bail. We're only interested in cases where folding the incoming
// constants is at least break-even on all paths.
if (FoldedCost > MatCost) {
LLVM_DEBUG(dbgs() << " Not profitable to fold imm: " << *IncomingC
<< "\n"
" Materializing cost: "
<< MatCost
<< "\n"
" Accumulated folded cost: "
<< FoldedCost << "\n");
return false;
}
}
}
// Compute the total cost savings afforded by this PHI node.
int TotalMatCost = TTI.TCC_Free, TotalFoldedCost = TTI.TCC_Free;
for (auto IncomingConstantAndCostsAndCount : CostsAndCounts) {
int MatCost = IncomingConstantAndCostsAndCount.second.MatCost;
int FoldedCost = IncomingConstantAndCostsAndCount.second.FoldedCost;
int Count = IncomingConstantAndCostsAndCount.second.Count;
TotalMatCost += MatCost * Count;
TotalFoldedCost += FoldedCost * Count;
}
assert(TotalFoldedCost <= TotalMatCost && "If each constant's folded cost is "
"less that its materialized cost, "
"the sum must be as well.");
LLVM_DEBUG(dbgs() << " Cost savings " << (TotalMatCost - TotalFoldedCost)
<< ": " << PN << "\n");
CostSavingsMap[&PN] = TotalMatCost - TotalFoldedCost;
return true;
}
/// Simple helper to walk all the users of a list of phis depth first, and call
/// a visit function on each one in post-order.
///
/// All of the PHIs should be in the same basic block, and this is primarily
/// used to make a single depth-first walk across their collective users
/// without revisiting any subgraphs. Callers should provide a fast, idempotent
/// callable to test whether a node has been visited and the more important
/// callable to actually visit a particular node.
///
/// Depth-first and postorder here refer to the *operand* graph -- we start
/// from a collection of users of PHI nodes and walk "up" the operands
/// depth-first.
template <typename IsVisitedT, typename VisitT>
static void visitPHIUsersAndDepsInPostOrder(ArrayRef<PHINode *> PNs,
IsVisitedT IsVisited,
VisitT Visit) {
SmallVector<std::pair<Instruction *, User::value_op_iterator>, 16> DFSStack;
for (auto *PN : PNs)
for (Use &U : PN->uses()) {
auto *UI = cast<Instruction>(U.getUser());
if (IsVisited(UI))
// Already visited this user, continue across the roots.
continue;
// Otherwise, walk the operand graph depth-first and visit each
// dependency in postorder.
DFSStack.push_back({UI, UI->value_op_begin()});
do {
User::value_op_iterator OpIt;
std::tie(UI, OpIt) = DFSStack.pop_back_val();
while (OpIt != UI->value_op_end()) {
auto *OpI = dyn_cast<Instruction>(*OpIt);
// Increment to the next operand for whenever we continue.
++OpIt;
// No need to visit non-instructions, which can't form dependencies,
// or instructions outside of our potential dependency set that we
// were given. Finally, if we've already visited the node, continue
// to the next.
if (!OpI || IsVisited(OpI))
continue;
// Push onto the stack and descend. We can directly continue this
// loop when ascending.
DFSStack.push_back({UI, OpIt});
UI = OpI;
OpIt = OpI->value_op_begin();
}
// Finished visiting children, visit this node.
assert(!IsVisited(UI) && "Should not have already visited a node!");
Visit(UI);
} while (!DFSStack.empty());
}
}
/// Find profitable PHIs to speculate.
///
/// For a PHI node to be profitable, we need the cost of speculating its users
/// (and their dependencies) to not exceed the savings of folding the PHI's
/// constant operands into the speculated users.
///
/// Computing this is surprisingly challenging. Because users of two different
/// PHI nodes can depend on each other or on common other instructions, it may
/// be profitable to speculate two PHI nodes together even though neither one
/// in isolation is profitable. The straightforward way to find all the
/// profitable PHIs would be to check each combination of PHIs' cost, but this
/// is exponential in complexity.
///
/// Even if we assume that we only care about cases where we can consider each
/// PHI node in isolation (rather than considering cases where none are
/// profitable in isolation but some subset are profitable as a set), we still
/// have a challenge. The obvious way to find all individually profitable PHIs
/// is to iterate until reaching a fixed point, but this will be quadratic in
/// complexity. =/
///
/// This code currently uses a linear-to-compute order for a greedy approach.
/// It won't find cases where a set of PHIs must be considered together, but it
/// handles most cases of order dependence without quadratic iteration. The
/// specific order used is the post-order across the operand DAG. When the last
/// user of a PHI is visited in this postorder walk, we check it for
/// profitability.
///
/// There is an orthogonal extra complexity to all of this: computing the cost
/// itself can easily become a linear computation making everything again (at
/// best) quadratic. Using a postorder over the operand graph makes it
/// particularly easy to avoid this through dynamic programming. As we do the
/// postorder walk, we build the transitive cost of that subgraph. It is also
/// straightforward to then update these costs when we mark a PHI for
/// speculation so that subsequent PHIs don't re-pay the cost of already
/// speculated instructions.
static SmallVector<PHINode *, 16>
findProfitablePHIs(ArrayRef<PHINode *> PNs,
const SmallDenseMap<PHINode *, int, 16> &CostSavingsMap,
const SmallPtrSetImpl<Instruction *> &PotentialSpecSet,
int NumPreds, DominatorTree &DT, TargetTransformInfo &TTI) {
SmallVector<PHINode *, 16> SpecPNs;
// First, establish a reverse mapping from immediate users of the PHI nodes
// to the nodes themselves, and count how many users each PHI node has in
// a way we can update while processing them.
SmallDenseMap<Instruction *, TinyPtrVector<PHINode *>, 16> UserToPNMap;
SmallDenseMap<PHINode *, int, 16> PNUserCountMap;
SmallPtrSet<Instruction *, 16> UserSet;
for (auto *PN : PNs) {
assert(UserSet.empty() && "Must start with an empty user set!");
for (Use &U : PN->uses())
UserSet.insert(cast<Instruction>(U.getUser()));
PNUserCountMap[PN] = UserSet.size();
for (auto *UI : UserSet)
UserToPNMap.insert({UI, {}}).first->second.push_back(PN);
UserSet.clear();
}
// Now do a DFS across the operand graph of the users, computing cost as we
// go and when all costs for a given PHI are known, checking that PHI for
// profitability.
SmallDenseMap<Instruction *, int, 16> SpecCostMap;
visitPHIUsersAndDepsInPostOrder(
PNs,
/*IsVisited*/
[&](Instruction *I) {
// We consider anything that isn't potentially speculated to be
// "visited" as it is already handled. Similarly, anything that *is*
// potentially speculated but for which we have an entry in our cost
// map, we're done.
return !PotentialSpecSet.count(I) || SpecCostMap.count(I);
},
/*Visit*/
[&](Instruction *I) {
// We've fully visited the operands, so sum their cost with this node
// and update the cost map.
int Cost = TTI.TCC_Free;
for (Value *OpV : I->operand_values())
if (auto *OpI = dyn_cast<Instruction>(OpV)) {
auto CostMapIt = SpecCostMap.find(OpI);
if (CostMapIt != SpecCostMap.end())
Cost += CostMapIt->second;
}
Cost += TTI.getUserCost(I);
bool Inserted = SpecCostMap.insert({I, Cost}).second;
(void)Inserted;
assert(Inserted && "Must not re-insert a cost during the DFS!");
// Now check if this node had a corresponding PHI node using it. If so,
// we need to decrement the outstanding user count for it.
auto UserPNsIt = UserToPNMap.find(I);
if (UserPNsIt == UserToPNMap.end())
return;
auto &UserPNs = UserPNsIt->second;
auto UserPNsSplitIt = std::stable_partition(
UserPNs.begin(), UserPNs.end(), [&](PHINode *UserPN) {
int &PNUserCount = PNUserCountMap.find(UserPN)->second;
assert(
PNUserCount > 0 &&
"Should never re-visit a PN after its user count hits zero!");
--PNUserCount;
return PNUserCount != 0;
});
// FIXME: Rather than one at a time, we should sum the savings as the
// cost will be completely shared.
SmallVector<Instruction *, 16> SpecWorklist;
for (auto *PN : llvm::make_range(UserPNsSplitIt, UserPNs.end())) {
int SpecCost = TTI.TCC_Free;
for (Use &U : PN->uses())
SpecCost +=
SpecCostMap.find(cast<Instruction>(U.getUser()))->second;
SpecCost *= (NumPreds - 1);
// When the user count of a PHI node hits zero, we should check its
// profitability. If profitable, we should mark it for speculation
// and zero out the cost of everything it depends on.
int CostSavings = CostSavingsMap.find(PN)->second;
if (SpecCost > CostSavings) {
LLVM_DEBUG(dbgs() << " Not profitable, speculation cost: " << *PN
<< "\n"
" Cost savings: "
<< CostSavings
<< "\n"
" Speculation cost: "
<< SpecCost << "\n");
continue;
}
// We're going to speculate this user-associated PHI. Copy it out and
// add its users to the worklist to update their cost.
SpecPNs.push_back(PN);
for (Use &U : PN->uses()) {
auto *UI = cast<Instruction>(U.getUser());
auto CostMapIt = SpecCostMap.find(UI);
if (CostMapIt->second == 0)
continue;
// Zero out this cost entry to avoid duplicates.
CostMapIt->second = 0;
SpecWorklist.push_back(UI);
}
}
// Now walk all the operands of the users in the worklist transitively
// to zero out all the memoized costs.
while (!SpecWorklist.empty()) {
Instruction *SpecI = SpecWorklist.pop_back_val();
assert(SpecCostMap.find(SpecI)->second == 0 &&
"Didn't zero out a cost!");
// Walk the operands recursively to zero out their cost as well.
for (auto *OpV : SpecI->operand_values()) {
auto *OpI = dyn_cast<Instruction>(OpV);
if (!OpI)
continue;
auto CostMapIt = SpecCostMap.find(OpI);
if (CostMapIt == SpecCostMap.end() || CostMapIt->second == 0)
continue;
CostMapIt->second = 0;
SpecWorklist.push_back(OpI);
}
}
});
return SpecPNs;
}
/// Speculate users around a set of PHI nodes.
///
/// This routine does the actual speculation around a set of PHI nodes where we
/// have determined this to be both safe and profitable.
///
/// This routine handles any spliting of critical edges necessary to create
/// a safe block to speculate into as well as cloning the instructions and
/// rewriting all uses.
static void speculatePHIs(ArrayRef<PHINode *> SpecPNs,
SmallPtrSetImpl<Instruction *> &PotentialSpecSet,
SmallSetVector<BasicBlock *, 16> &PredSet,
DominatorTree &DT) {
LLVM_DEBUG(dbgs() << " Speculating around " << SpecPNs.size() << " PHIs!\n");
NumPHIsSpeculated += SpecPNs.size();
// Split any critical edges so that we have a block to hoist into.
auto *ParentBB = SpecPNs[0]->getParent();
SmallVector<BasicBlock *, 16> SpecPreds;
SpecPreds.reserve(PredSet.size());
for (auto *PredBB : PredSet) {
auto *NewPredBB = SplitCriticalEdge(
PredBB, ParentBB,
CriticalEdgeSplittingOptions(&DT).setMergeIdenticalEdges());
if (NewPredBB) {
++NumEdgesSplit;
LLVM_DEBUG(dbgs() << " Split critical edge from: " << PredBB->getName()
<< "\n");
SpecPreds.push_back(NewPredBB);
} else {
assert(PredBB->getSingleSuccessor() == ParentBB &&
"We need a non-critical predecessor to speculate into.");
assert(!isa<InvokeInst>(PredBB->getTerminator()) &&
"Cannot have a non-critical invoke!");
// Already non-critical, use existing pred.
SpecPreds.push_back(PredBB);
}
}
SmallPtrSet<Instruction *, 16> SpecSet;
SmallVector<Instruction *, 16> SpecList;
visitPHIUsersAndDepsInPostOrder(SpecPNs,
/*IsVisited*/
[&](Instruction *I) {
// This is visited if we don't need to
// speculate it or we already have
// speculated it.
return !PotentialSpecSet.count(I) ||
SpecSet.count(I);
},
/*Visit*/
[&](Instruction *I) {
// All operands scheduled, schedule this
// node.
SpecSet.insert(I);
SpecList.push_back(I);
});
int NumSpecInsts = SpecList.size() * SpecPreds.size();
int NumRedundantInsts = NumSpecInsts - SpecList.size();
LLVM_DEBUG(dbgs() << " Inserting " << NumSpecInsts
<< " speculated instructions, " << NumRedundantInsts
<< " redundancies\n");
NumSpeculatedInstructions += NumSpecInsts;
NumNewRedundantInstructions += NumRedundantInsts;
// Each predecessor is numbered by its index in `SpecPreds`, so for each
// instruction we speculate, the speculated instruction is stored in that
// index of the vector associated with the original instruction. We also
// store the incoming values for each predecessor from any PHIs used.
SmallDenseMap<Instruction *, SmallVector<Value *, 2>, 16> SpeculatedValueMap;
// Inject the synthetic mappings to rewrite PHIs to the appropriate incoming
// value. This handles both the PHIs we are speculating around and any other
// PHIs that happen to be used.
for (auto *OrigI : SpecList)
for (auto *OpV : OrigI->operand_values()) {
auto *OpPN = dyn_cast<PHINode>(OpV);
if (!OpPN || OpPN->getParent() != ParentBB)
continue;
auto InsertResult = SpeculatedValueMap.insert({OpPN, {}});
if (!InsertResult.second)
continue;
auto &SpeculatedVals = InsertResult.first->second;
// Populating our structure for mapping is particularly annoying because
// finding an incoming value for a particular predecessor block in a PHI
// node is a linear time operation! To avoid quadratic behavior, we build
// a map for this PHI node's incoming values and then translate it into
// the more compact representation used below.
SmallDenseMap<BasicBlock *, Value *, 16> IncomingValueMap;
for (int i : llvm::seq<int>(0, OpPN->getNumIncomingValues()))
IncomingValueMap[OpPN->getIncomingBlock(i)] = OpPN->getIncomingValue(i);
for (auto *PredBB : SpecPreds)
SpeculatedVals.push_back(IncomingValueMap.find(PredBB)->second);
}
// Speculate into each predecessor.
for (int PredIdx : llvm::seq<int>(0, SpecPreds.size())) {
auto *PredBB = SpecPreds[PredIdx];
assert(PredBB->getSingleSuccessor() == ParentBB &&
"We need a non-critical predecessor to speculate into.");
for (auto *OrigI : SpecList) {
auto *NewI = OrigI->clone();
NewI->setName(Twine(OrigI->getName()) + "." + Twine(PredIdx));
NewI->insertBefore(PredBB->getTerminator());
// Rewrite all the operands to the previously speculated instructions.
// Because we're walking in-order, the defs must precede the uses and we
// should already have these mappings.
for (Use &U : NewI->operands()) {
auto *OpI = dyn_cast<Instruction>(U.get());
if (!OpI)
continue;
auto MapIt = SpeculatedValueMap.find(OpI);
if (MapIt == SpeculatedValueMap.end())
continue;
const auto &SpeculatedVals = MapIt->second;
assert(SpeculatedVals[PredIdx] &&
"Must have a speculated value for this predecessor!");
assert(SpeculatedVals[PredIdx]->getType() == OpI->getType() &&
"Speculated value has the wrong type!");
// Rewrite the use to this predecessor's speculated instruction.
U.set(SpeculatedVals[PredIdx]);
}
// Commute instructions which now have a constant in the LHS but not the
// RHS.
if (NewI->isBinaryOp() && NewI->isCommutative() &&
isa<Constant>(NewI->getOperand(0)) &&
!isa<Constant>(NewI->getOperand(1)))
NewI->getOperandUse(0).swap(NewI->getOperandUse(1));
SpeculatedValueMap[OrigI].push_back(NewI);
assert(SpeculatedValueMap[OrigI][PredIdx] == NewI &&
"Mismatched speculated instruction index!");
}
}
// Walk the speculated instruction list and if they have uses, insert a PHI
// for them from the speculated versions, and replace the uses with the PHI.
// Then erase the instructions as they have been fully speculated. The walk
// needs to be in reverse so that we don't think there are users when we'll
// actually eventually remove them later.
IRBuilder<> IRB(SpecPNs[0]);
for (auto *OrigI : llvm::reverse(SpecList)) {
// Check if we need a PHI for any remaining users and if so, insert it.
if (!OrigI->use_empty()) {
auto *SpecIPN = IRB.CreatePHI(OrigI->getType(), SpecPreds.size(),
Twine(OrigI->getName()) + ".phi");
// Add the incoming values we speculated.
auto &SpeculatedVals = SpeculatedValueMap.find(OrigI)->second;
for (int PredIdx : llvm::seq<int>(0, SpecPreds.size()))
SpecIPN->addIncoming(SpeculatedVals[PredIdx], SpecPreds[PredIdx]);
// And replace the uses with the PHI node.
OrigI->replaceAllUsesWith(SpecIPN);
}
// It is important to immediately erase this so that it stops using other
// instructions. This avoids inserting needless PHIs of them.
OrigI->eraseFromParent();
}
// All of the uses of the speculated phi nodes should be removed at this
// point, so erase them.
for (auto *SpecPN : SpecPNs) {
assert(SpecPN->use_empty() && "All users should have been speculated!");
SpecPN->eraseFromParent();
}
}
/// Try to speculate around a series of PHIs from a single basic block.
///
/// This routine checks whether any of these PHIs are profitable to speculate
/// users around. If safe and profitable, it does the speculation. It returns
/// true when at least some speculation occurs.
static bool tryToSpeculatePHIs(SmallVectorImpl<PHINode *> &PNs,
DominatorTree &DT, TargetTransformInfo &TTI) {
LLVM_DEBUG(dbgs() << "Evaluating phi nodes for speculation:\n");
// Savings in cost from speculating around a PHI node.
SmallDenseMap<PHINode *, int, 16> CostSavingsMap;
// Remember the set of instructions that are candidates for speculation so
// that we can quickly walk things within that space. This prunes out
// instructions already available along edges, etc.
SmallPtrSet<Instruction *, 16> PotentialSpecSet;
// Remember the set of instructions that are (transitively) unsafe to
// speculate into the incoming edges of this basic block. This avoids
// recomputing them for each PHI node we check. This set is specific to this
// block though as things are pruned out of it based on what is available
// along incoming edges.
SmallPtrSet<Instruction *, 16> UnsafeSet;
// For each PHI node in this block, check whether there are immediate folding
// opportunities from speculation, and whether that speculation will be
// valid. This determise the set of safe PHIs to speculate.
PNs.erase(llvm::remove_if(PNs,
[&](PHINode *PN) {
return !isSafeAndProfitableToSpeculateAroundPHI(
*PN, CostSavingsMap, PotentialSpecSet,
UnsafeSet, DT, TTI);
}),
PNs.end());
// If no PHIs were profitable, skip.
if (PNs.empty()) {
LLVM_DEBUG(dbgs() << " No safe and profitable PHIs found!\n");
return false;
}
// We need to know how much speculation will cost which is determined by how
// many incoming edges will need a copy of each speculated instruction.
SmallSetVector<BasicBlock *, 16> PredSet;
for (auto *PredBB : PNs[0]->blocks()) {
if (!PredSet.insert(PredBB))
continue;
// We cannot speculate when a predecessor is an indirect branch.
// FIXME: We also can't reliably create a non-critical edge block for
// speculation if the predecessor is an invoke. This doesn't seem
// fundamental and we should probably be splitting critical edges
// differently.
if (isa<IndirectBrInst>(PredBB->getTerminator()) ||
isa<InvokeInst>(PredBB->getTerminator())) {
LLVM_DEBUG(dbgs() << " Invalid: predecessor terminator: "
<< PredBB->getName() << "\n");
return false;
}
}
if (PredSet.size() < 2) {
LLVM_DEBUG(dbgs() << " Unimportant: phi with only one predecessor\n");
return false;
}
SmallVector<PHINode *, 16> SpecPNs = findProfitablePHIs(
PNs, CostSavingsMap, PotentialSpecSet, PredSet.size(), DT, TTI);
if (SpecPNs.empty())
// Nothing to do.
return false;
speculatePHIs(SpecPNs, PotentialSpecSet, PredSet, DT);
return true;
}
PreservedAnalyses SpeculateAroundPHIsPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &TTI = AM.getResult<TargetIRAnalysis>(F);
bool Changed = false;
for (auto *BB : ReversePostOrderTraversal<Function *>(&F)) {
SmallVector<PHINode *, 16> PNs;
auto BBI = BB->begin();
while (auto *PN = dyn_cast<PHINode>(&*BBI)) {
PNs.push_back(PN);
++BBI;
}
if (PNs.empty())
continue;
Changed |= tryToSpeculatePHIs(PNs, DT, TTI);
}
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
}
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