//===- SSAUpdaterImpl.h - SSA Updater Implementation ------------*- C++ -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file provides a template that implements the core algorithm for the
// SSAUpdater and MachineSSAUpdater.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
#define LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#define DEBUG_TYPE "ssaupdater"
namespace llvm {
template<typename T> class SSAUpdaterTraits;
template<typename UpdaterT>
class SSAUpdaterImpl {
private:
UpdaterT *Updater;
using Traits = SSAUpdaterTraits<UpdaterT>;
using BlkT = typename Traits::BlkT;
using ValT = typename Traits::ValT;
using PhiT = typename Traits::PhiT;
/// BBInfo - Per-basic block information used internally by SSAUpdaterImpl.
/// The predecessors of each block are cached here since pred_iterator is
/// slow and we need to iterate over the blocks at least a few times.
class BBInfo {
public:
// Back-pointer to the corresponding block.
BlkT *BB;
// Value to use in this block.
ValT AvailableVal;
// Block that defines the available value.
BBInfo *DefBB;
// Postorder number.
int BlkNum = 0;
// Immediate dominator.
BBInfo *IDom = nullptr;
// Number of predecessor blocks.
unsigned NumPreds = 0;
// Array[NumPreds] of predecessor blocks.
BBInfo **Preds = nullptr;
// Marker for existing PHIs that match.
PhiT *PHITag = nullptr;
BBInfo(BlkT *ThisBB, ValT V)
: BB(ThisBB), AvailableVal(V), DefBB(V ? this : nullptr) {}
};
using AvailableValsTy = DenseMap<BlkT *, ValT>;
AvailableValsTy *AvailableVals;
SmallVectorImpl<PhiT *> *InsertedPHIs;
using BlockListTy = SmallVectorImpl<BBInfo *>;
using BBMapTy = DenseMap<BlkT *, BBInfo *>;
BBMapTy BBMap;
BumpPtrAllocator Allocator;
public:
explicit SSAUpdaterImpl(UpdaterT *U, AvailableValsTy *A,
SmallVectorImpl<PhiT *> *Ins) :
Updater(U), AvailableVals(A), InsertedPHIs(Ins) {}
/// GetValue - Check to see if AvailableVals has an entry for the specified
/// BB and if so, return it. If not, construct SSA form by first
/// calculating the required placement of PHIs and then inserting new PHIs
/// where needed.
ValT GetValue(BlkT *BB) {
SmallVector<BBInfo *, 100> BlockList;
BBInfo *PseudoEntry = BuildBlockList(BB, &BlockList);
// Special case: bail out if BB is unreachable.
if (BlockList.size() == 0) {
ValT V = Traits::GetUndefVal(BB, Updater);
(*AvailableVals)[BB] = V;
return V;
}
FindDominators(&BlockList, PseudoEntry);
FindPHIPlacement(&BlockList);
FindAvailableVals(&BlockList);
return BBMap[BB]->DefBB->AvailableVal;
}
/// BuildBlockList - Starting from the specified basic block, traverse back
/// through its predecessors until reaching blocks with known values.
/// Create BBInfo structures for the blocks and append them to the block
/// list.
BBInfo *BuildBlockList(BlkT *BB, BlockListTy *BlockList) {
SmallVector<BBInfo *, 10> RootList;
SmallVector<BBInfo *, 64> WorkList;
BBInfo *Info = new (Allocator) BBInfo(BB, 0);
BBMap[BB] = Info;
WorkList.push_back(Info);
// Search backward from BB, creating BBInfos along the way and stopping
// when reaching blocks that define the value. Record those defining
// blocks on the RootList.
SmallVector<BlkT *, 10> Preds;
while (!WorkList.empty()) {
Info = WorkList.pop_back_val();
Preds.clear();
Traits::FindPredecessorBlocks(Info->BB, &Preds);
Info->NumPreds = Preds.size();
if (Info->NumPreds == 0)
Info->Preds = nullptr;
else
Info->Preds = static_cast<BBInfo **>(Allocator.Allocate(
Info->NumPreds * sizeof(BBInfo *), alignof(BBInfo *)));
for (unsigned p = 0; p != Info->NumPreds; ++p) {
BlkT *Pred = Preds[p];
// Check if BBMap already has a BBInfo for the predecessor block.
typename BBMapTy::value_type &BBMapBucket =
BBMap.FindAndConstruct(Pred);
if (BBMapBucket.second) {
Info->Preds[p] = BBMapBucket.second;
continue;
}
// Create a new BBInfo for the predecessor.
ValT PredVal = AvailableVals->lookup(Pred);
BBInfo *PredInfo = new (Allocator) BBInfo(Pred, PredVal);
BBMapBucket.second = PredInfo;
Info->Preds[p] = PredInfo;
if (PredInfo->AvailableVal) {
RootList.push_back(PredInfo);
continue;
}
WorkList.push_back(PredInfo);
}
}
// Now that we know what blocks are backwards-reachable from the starting
// block, do a forward depth-first traversal to assign postorder numbers
// to those blocks.
BBInfo *PseudoEntry = new (Allocator) BBInfo(nullptr, 0);
unsigned BlkNum = 1;
// Initialize the worklist with the roots from the backward traversal.
while (!RootList.empty()) {
Info = RootList.pop_back_val();
Info->IDom = PseudoEntry;
Info->BlkNum = -1;
WorkList.push_back(Info);
}
while (!WorkList.empty()) {
Info = WorkList.back();
if (Info->BlkNum == -2) {
// All the successors have been handled; assign the postorder number.
Info->BlkNum = BlkNum++;
// If not a root, put it on the BlockList.
if (!Info->AvailableVal)
BlockList->push_back(Info);
WorkList.pop_back();
continue;
}
// Leave this entry on the worklist, but set its BlkNum to mark that its
// successors have been put on the worklist. When it returns to the top
// the list, after handling its successors, it will be assigned a
// number.
Info->BlkNum = -2;
// Add unvisited successors to the work list.
for (typename Traits::BlkSucc_iterator SI =
Traits::BlkSucc_begin(Info->BB),
E = Traits::BlkSucc_end(Info->BB); SI != E; ++SI) {
BBInfo *SuccInfo = BBMap[*SI];
if (!SuccInfo || SuccInfo->BlkNum)
continue;
SuccInfo->BlkNum = -1;
WorkList.push_back(SuccInfo);
}
}
PseudoEntry->BlkNum = BlkNum;
return PseudoEntry;
}
/// IntersectDominators - This is the dataflow lattice "meet" operation for
/// finding dominators. Given two basic blocks, it walks up the dominator
/// tree until it finds a common dominator of both. It uses the postorder
/// number of the blocks to determine how to do that.
BBInfo *IntersectDominators(BBInfo *Blk1, BBInfo *Blk2) {
while (Blk1 != Blk2) {
while (Blk1->BlkNum < Blk2->BlkNum) {
Blk1 = Blk1->IDom;
if (!Blk1)
return Blk2;
}
while (Blk2->BlkNum < Blk1->BlkNum) {
Blk2 = Blk2->IDom;
if (!Blk2)
return Blk1;
}
}
return Blk1;
}
/// FindDominators - Calculate the dominator tree for the subset of the CFG
/// corresponding to the basic blocks on the BlockList. This uses the
/// algorithm from: "A Simple, Fast Dominance Algorithm" by Cooper, Harvey
/// and Kennedy, published in Software--Practice and Experience, 2001,
/// 4:1-10. Because the CFG subset does not include any edges leading into
/// blocks that define the value, the results are not the usual dominator
/// tree. The CFG subset has a single pseudo-entry node with edges to a set
/// of root nodes for blocks that define the value. The dominators for this
/// subset CFG are not the standard dominators but they are adequate for
/// placing PHIs within the subset CFG.
void FindDominators(BlockListTy *BlockList, BBInfo *PseudoEntry) {
bool Changed;
do {
Changed = false;
// Iterate over the list in reverse order, i.e., forward on CFG edges.
for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
E = BlockList->rend(); I != E; ++I) {
BBInfo *Info = *I;
BBInfo *NewIDom = nullptr;
// Iterate through the block's predecessors.
for (unsigned p = 0; p != Info->NumPreds; ++p) {
BBInfo *Pred = Info->Preds[p];
// Treat an unreachable predecessor as a definition with 'undef'.
if (Pred->BlkNum == 0) {
Pred->AvailableVal = Traits::GetUndefVal(Pred->BB, Updater);
(*AvailableVals)[Pred->BB] = Pred->AvailableVal;
Pred->DefBB = Pred;
Pred->BlkNum = PseudoEntry->BlkNum;
PseudoEntry->BlkNum++;
}
if (!NewIDom)
NewIDom = Pred;
else
NewIDom = IntersectDominators(NewIDom, Pred);
}
// Check if the IDom value has changed.
if (NewIDom && NewIDom != Info->IDom) {
Info->IDom = NewIDom;
Changed = true;
}
}
} while (Changed);
}
/// IsDefInDomFrontier - Search up the dominator tree from Pred to IDom for
/// any blocks containing definitions of the value. If one is found, then
/// the successor of Pred is in the dominance frontier for the definition,
/// and this function returns true.
bool IsDefInDomFrontier(const BBInfo *Pred, const BBInfo *IDom) {
for (; Pred != IDom; Pred = Pred->IDom) {
if (Pred->DefBB == Pred)
return true;
}
return false;
}
/// FindPHIPlacement - PHIs are needed in the iterated dominance frontiers
/// of the known definitions. Iteratively add PHIs in the dom frontiers
/// until nothing changes. Along the way, keep track of the nearest
/// dominating definitions for non-PHI blocks.
void FindPHIPlacement(BlockListTy *BlockList) {
bool Changed;
do {
Changed = false;
// Iterate over the list in reverse order, i.e., forward on CFG edges.
for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
E = BlockList->rend(); I != E; ++I) {
BBInfo *Info = *I;
// If this block already needs a PHI, there is nothing to do here.
if (Info->DefBB == Info)
continue;
// Default to use the same def as the immediate dominator.
BBInfo *NewDefBB = Info->IDom->DefBB;
for (unsigned p = 0; p != Info->NumPreds; ++p) {
if (IsDefInDomFrontier(Info->Preds[p], Info->IDom)) {
// Need a PHI here.
NewDefBB = Info;
break;
}
}
// Check if anything changed.
if (NewDefBB != Info->DefBB) {
Info->DefBB = NewDefBB;
Changed = true;
}
}
} while (Changed);
}
/// Check all predecessors and if all of them have the same AvailableVal use
/// it as value for block represented by Info. Return true if singluar value
/// is found.
bool FindSingularVal(BBInfo *Info) {
if (!Info->NumPreds)
return false;
ValT Singular = Info->Preds[0]->DefBB->AvailableVal;
if (!Singular)
return false;
for (unsigned Idx = 1; Idx < Info->NumPreds; ++Idx) {
ValT PredVal = Info->Preds[Idx]->DefBB->AvailableVal;
if (!PredVal || Singular != PredVal)
return false;
}
// Record Singular value.
(*AvailableVals)[Info->BB] = Singular;
assert(BBMap[Info->BB] == Info && "Info missed in BBMap?");
Info->AvailableVal = Singular;
Info->DefBB = Info->Preds[0]->DefBB;
return true;
}
/// FindAvailableVal - If this block requires a PHI, first check if an
/// existing PHI matches the PHI placement and reaching definitions computed
/// earlier, and if not, create a new PHI. Visit all the block's
/// predecessors to calculate the available value for each one and fill in
/// the incoming values for a new PHI.
void FindAvailableVals(BlockListTy *BlockList) {
// Go through the worklist in forward order (i.e., backward through the CFG)
// and check if existing PHIs can be used. If not, create empty PHIs where
// they are needed.
for (typename BlockListTy::iterator I = BlockList->begin(),
E = BlockList->end(); I != E; ++I) {
BBInfo *Info = *I;
// Check if there needs to be a PHI in BB.
if (Info->DefBB != Info)
continue;
// Look for singular value.
if (FindSingularVal(Info))
continue;
// Look for an existing PHI.
FindExistingPHI(Info->BB, BlockList);
if (Info->AvailableVal)
continue;
ValT PHI = Traits::CreateEmptyPHI(Info->BB, Info->NumPreds, Updater);
Info->AvailableVal = PHI;
(*AvailableVals)[Info->BB] = PHI;
}
// Now go back through the worklist in reverse order to fill in the
// arguments for any new PHIs added in the forward traversal.
for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
E = BlockList->rend(); I != E; ++I) {
BBInfo *Info = *I;
if (Info->DefBB != Info) {
// Record the available value to speed up subsequent uses of this
// SSAUpdater for the same value.
(*AvailableVals)[Info->BB] = Info->DefBB->AvailableVal;
continue;
}
// Check if this block contains a newly added PHI.
PhiT *PHI = Traits::ValueIsNewPHI(Info->AvailableVal, Updater);
if (!PHI)
continue;
// Iterate through the block's predecessors.
for (unsigned p = 0; p != Info->NumPreds; ++p) {
BBInfo *PredInfo = Info->Preds[p];
BlkT *Pred = PredInfo->BB;
// Skip to the nearest preceding definition.
if (PredInfo->DefBB != PredInfo)
PredInfo = PredInfo->DefBB;
Traits::AddPHIOperand(PHI, PredInfo->AvailableVal, Pred);
}
LLVM_DEBUG(dbgs() << " Inserted PHI: " << *PHI << "\n");
// If the client wants to know about all new instructions, tell it.
if (InsertedPHIs) InsertedPHIs->push_back(PHI);
}
}
/// FindExistingPHI - Look through the PHI nodes in a block to see if any of
/// them match what is needed.
void FindExistingPHI(BlkT *BB, BlockListTy *BlockList) {
for (auto &SomePHI : BB->phis()) {
if (CheckIfPHIMatches(&SomePHI)) {
RecordMatchingPHIs(BlockList);
break;
}
// Match failed: clear all the PHITag values.
for (typename BlockListTy::iterator I = BlockList->begin(),
E = BlockList->end(); I != E; ++I)
(*I)->PHITag = nullptr;
}
}
/// CheckIfPHIMatches - Check if a PHI node matches the placement and values
/// in the BBMap.
bool CheckIfPHIMatches(PhiT *PHI) {
SmallVector<PhiT *, 20> WorkList;
WorkList.push_back(PHI);
// Mark that the block containing this PHI has been visited.
BBMap[PHI->getParent()]->PHITag = PHI;
while (!WorkList.empty()) {
PHI = WorkList.pop_back_val();
// Iterate through the PHI's incoming values.
for (typename Traits::PHI_iterator I = Traits::PHI_begin(PHI),
E = Traits::PHI_end(PHI); I != E; ++I) {
ValT IncomingVal = I.getIncomingValue();
BBInfo *PredInfo = BBMap[I.getIncomingBlock()];
// Skip to the nearest preceding definition.
if (PredInfo->DefBB != PredInfo)
PredInfo = PredInfo->DefBB;
// Check if it matches the expected value.
if (PredInfo->AvailableVal) {
if (IncomingVal == PredInfo->AvailableVal)
continue;
return false;
}
// Check if the value is a PHI in the correct block.
PhiT *IncomingPHIVal = Traits::ValueIsPHI(IncomingVal, Updater);
if (!IncomingPHIVal || IncomingPHIVal->getParent() != PredInfo->BB)
return false;
// If this block has already been visited, check if this PHI matches.
if (PredInfo->PHITag) {
if (IncomingPHIVal == PredInfo->PHITag)
continue;
return false;
}
PredInfo->PHITag = IncomingPHIVal;
WorkList.push_back(IncomingPHIVal);
}
}
return true;
}
/// RecordMatchingPHIs - For each PHI node that matches, record it in both
/// the BBMap and the AvailableVals mapping.
void RecordMatchingPHIs(BlockListTy *BlockList) {
for (typename BlockListTy::iterator I = BlockList->begin(),
E = BlockList->end(); I != E; ++I)
if (PhiT *PHI = (*I)->PHITag) {
BlkT *BB = PHI->getParent();
ValT PHIVal = Traits::GetPHIValue(PHI);
(*AvailableVals)[BB] = PHIVal;
BBMap[BB]->AvailableVal = PHIVal;
}
}
};
} // end namespace llvm
#undef DEBUG_TYPE // "ssaupdater"
#endif // LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H