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Replace SplitKit SSA update with an iterative algorithm very similar to the one

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in SSAUpdaterImpl.h

Verifying live intervals revealed that the old method was completely wrong, and
we need an iterative approach to calculating PHI placemant. Fortunately, we have
MachineDominators available, so we don't have to compute that over and over
like SSAUpdaterImpl.h must.

Live-out values are cached between calls to mapValue() and computed in a greedy
way, so most calls will be working with very small block sets.

Thanks to Bob for explaining how this should work.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117599 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jakob Stoklund Olesen 2010-10-28 20:34:52 +00:00
parent d68f458244
commit e1dde7b05a
2 changed files with 181 additions and 88 deletions
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245
lib/CodeGen/SplitKit.cpp
View File

@ -336,6 +336,7 @@ makeVV(const VNInfo *a, VNInfo *b) {
void LiveIntervalMap::reset(LiveInterval *li) {
li_ = li;
valueMap_.clear();
liveOutCache_.clear();
}
bool LiveIntervalMap::isComplexMapped(const VNInfo *ParentVNI) const {
@ -408,105 +409,175 @@ VNInfo *LiveIntervalMap::mapValue(const VNInfo *ParentVNI, SlotIndex Idx,
// Now for the fun part. We know that ParentVNI potentially has multiple defs,
// and we may need to create even more phi-defs to preserve VNInfo SSA form.
// Perform a depth-first search for predecessor blocks where we know the
// dominating VNInfo. Insert phi-def VNInfos along the path back to IdxMBB.
// Perform a search for all predecessor blocks where we know the dominating
// VNInfo. Insert phi-def VNInfos along the path back to IdxMBB.
DEBUG(dbgs() << "\n Reaching defs for BB#" << IdxMBB->getNumber()
<< " at " << Idx << " in " << *li_ << '\n');
// Track MBBs where we have created or learned the dominating value.
// This may change during the DFS as we create new phi-defs.
typedef DenseMap<MachineBasicBlock*, VNInfo*> MBBValueMap;
MBBValueMap DomValue;
typedef SplitAnalysis::BlockPtrSet BlockPtrSet;
BlockPtrSet Visited;
// Blocks where li_ should be live-in.
SmallVector<MachineDomTreeNode*, 16> LiveIn;
LiveIn.push_back(mdt_[IdxMBB]);
// Iterate over IdxMBB predecessors in a depth-first order.
// Skip begin() since that is always IdxMBB.
for (idf_ext_iterator<MachineBasicBlock*, BlockPtrSet>
IDFI = llvm::next(idf_ext_begin(IdxMBB, Visited)),
IDFE = idf_ext_end(IdxMBB, Visited); IDFI != IDFE;) {
MachineBasicBlock *MBB = *IDFI;
SlotIndex End = lis_.getMBBEndIdx(MBB).getPrevSlot();
// Using liveOutCache_ as a visited set, perform a BFS for all reaching defs.
for (unsigned i = 0; i != LiveIn.size(); ++i) {
MachineBasicBlock *MBB = LiveIn[i]->getBlock();
for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
PE = MBB->pred_end(); PI != PE; ++PI) {
MachineBasicBlock *Pred = *PI;
// Is this a known live-out block?
std::pair<LiveOutMap::iterator,bool> LOIP =
liveOutCache_.insert(std::make_pair(Pred, LiveOutPair()));
// Yes, we have been here before.
if (!LOIP.second) {
if (VNInfo *VNI = LOIP.first->second.first) {
DEBUG(dbgs() << " known valno #" << VNI->id
<< " at BB#" << Pred->getNumber() << '\n');
}
continue;
}
// We are operating on the restricted CFG where ParentVNI is live.
if (parentli_.getVNInfoAt(End) != ParentVNI) {
IDFI.skipChildren();
continue;
// Does Pred provide a live-out value?
SlotIndex Last = lis_.getMBBEndIdx(Pred).getPrevSlot();
if (VNInfo *VNI = extendTo(Pred, Last)) {
MachineBasicBlock *DefMBB = lis_.getMBBFromIndex(VNI->def);
DEBUG(dbgs() << " found valno #" << VNI->id
<< " at BB#" << DefMBB->getNumber() << '\n');
LiveOutPair &LOP = LOIP.first->second;
LOP.first = VNI;
LOP.second = mdt_[lis_.getMBBFromIndex(VNI->def)];
continue;
}
// No, we need a live-in value for Pred as well
if (Pred != IdxMBB)
LiveIn.push_back(mdt_[Pred]);
}
}
// Do we have a dominating value in this block?
VNInfo *VNI = extendTo(MBB, End);
if (!VNI) {
++IDFI;
continue;
}
// We may need to add phi-def values to preserve the SSA form.
// This is essentially the same iterative algorithm that SSAUpdater uses,
// except we already have a dominator tree, so we don't have to recompute it.
VNInfo *IdxVNI = 0;
unsigned Changes;
do {
Changes = 0;
DEBUG(dbgs() << " Iterating over " << LiveIn.size() << " blocks.\n");
// Propagate live-out values down the dominator tree, inserting phi-defs when
// necessary. Since LiveIn was created by a BFS, going backwards makes it more
// likely for us to visit immediate dominators before their children.
for (unsigned i = LiveIn.size(); i; --i) {
MachineDomTreeNode *Node = LiveIn[i-1];
MachineBasicBlock *MBB = Node->getBlock();
MachineDomTreeNode *IDom = Node->getIDom();
LiveOutPair IDomValue;
// We need a live-in value to a block with no immediate dominator?
// This is probably an unreachable block that has survived somehow.
bool needPHI = !IDom;
// Yes, VNI dominates MBB. Make sure we visit MBB again from other paths.
Visited.erase(MBB);
// Get the IDom live-out value.
if (!needPHI) {
LiveOutMap::iterator I = liveOutCache_.find(IDom->getBlock());
if (I != liveOutCache_.end())
IDomValue = I->second;
else
// If IDom is outside our set of live-out blocks, there must be new
// defs, and we need a phi-def here.
needPHI = true;
}
// Track the path back to IdxMBB, creating phi-defs
// as needed along the way.
for (unsigned PI = IDFI.getPathLength()-1; PI != 0; --PI) {
// Start from MBB's immediate successor. End at IdxMBB.
MachineBasicBlock *Succ = IDFI.getPath(PI-1);
std::pair<MBBValueMap::iterator, bool> InsP =
DomValue.insert(MBBValueMap::value_type(Succ, VNI));
// IDom dominates all of our predecessors, but it may not be the immediate
// dominator. Check if any of them have live-out values that are properly
// dominated by IDom. If so, we need a phi-def here.
if (!needPHI) {
for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
PE = MBB->pred_end(); PI != PE; ++PI) {
LiveOutPair Value = liveOutCache_[*PI];
if (!Value.first || Value.first == IDomValue.first)
continue;
// This predecessor is carrying something other than IDomValue.
// It could be because IDomValue hasn't propagated yet, or it could be
// because MBB is in the dominance frontier of that value.
if (mdt_.dominates(IDom, Value.second)) {
needPHI = true;
break;
}
}
}
// This is the first time we backtrack to Succ.
if (InsP.second)
continue;
// We reached Succ again with the same VNI. Nothing is going to change.
VNInfo *OVNI = InsP.first->second;
if (OVNI == VNI)
break;
// Succ already has a phi-def. No need to continue.
SlotIndex Start = lis_.getMBBStartIdx(Succ);
if (OVNI->def == Start)
break;
// We have a collision between the old and new VNI at Succ. That means
// neither dominates and we need a new phi-def.
VNI = li_->getNextValue(Start, 0, lis_.getVNInfoAllocator());
VNI->setIsPHIDef(true);
InsP.first->second = VNI;
// Replace OVNI with VNI in the remaining path.
for (; PI > 1 ; --PI) {
MBBValueMap::iterator I = DomValue.find(IDFI.getPath(PI-2));
if (I == DomValue.end() || I->second != OVNI)
break;
I->second = VNI;
// Create a phi-def if required.
if (needPHI) {
++Changes;
SlotIndex Start = lis_.getMBBStartIdx(MBB);
VNInfo *VNI = li_->getNextValue(Start, 0, lis_.getVNInfoAllocator());
VNI->setIsPHIDef(true);
DEBUG(dbgs() << " - BB#" << MBB->getNumber()
<< " phi-def #" << VNI->id << " at " << Start << '\n');
// We no longer need li_ to be live-in.
LiveIn.erase(LiveIn.begin()+(i-1));
// Blocks in LiveIn are either IdxMBB, or have a value live-through.
if (MBB == IdxMBB)
IdxVNI = VNI;
// Check if we need to update live-out info.
LiveOutMap::iterator I = liveOutCache_.find(MBB);
if (I == liveOutCache_.end() || I->second.second == Node) {
// We already have a live-out defined in MBB, so this must be IdxMBB.
assert(MBB == IdxMBB && "Adding phi-def to known live-out");
li_->addRange(LiveRange(Start, Idx.getNextSlot(), VNI));
} else {
// This phi-def is also live-out, so color the whole block.
li_->addRange(LiveRange(Start, lis_.getMBBEndIdx(MBB), VNI));
I->second = LiveOutPair(VNI, Node);
}
} else if (IDomValue.first) {
// No phi-def here. Propagate IDomValue if needed.
if (MBB == IdxMBB)
IdxVNI = IDomValue.first;
LiveOutMap::iterator I = liveOutCache_.find(MBB);
if (I != liveOutCache_.end() && I->second.first != IDomValue.first) {
++Changes;
I->second = IDomValue;
DEBUG(dbgs() << " - BB#" << MBB->getNumber()
<< " idom valno #" << IDomValue.first->id
<< " from BB#" << IDom->getBlock()->getNumber() << '\n');
}
}
}
// No need to search the children, we found a dominating value.
IDFI.skipChildren();
}
// The search should at least find a dominating value for IdxMBB.
assert(!DomValue.empty() && "Couldn't find a reaching definition");
// Since we went through the trouble of a full DFS visiting all reaching defs,
// the values in DomValue are now accurate. No more phi-defs are needed for
// these blocks, so we can color the live ranges.
// This makes the next mapValue call much faster.
VNInfo *IdxVNI = 0;
for (MBBValueMap::iterator I = DomValue.begin(), E = DomValue.end(); I != E;
++I) {
MachineBasicBlock *MBB = I->first;
VNInfo *VNI = I->second;
SlotIndex Start = lis_.getMBBStartIdx(MBB);
if (MBB == IdxMBB) {
// Don't add full liveness to IdxMBB, stop at Idx.
if (Start != Idx)
li_->addRange(LiveRange(Start, Idx.getNextSlot(), VNI));
// The caller had better add some liveness to IdxVNI, or it leaks.
IdxVNI = VNI;
} else
li_->addRange(LiveRange(Start, lis_.getMBBEndIdx(MBB), VNI));
}
DEBUG(dbgs() << " - made " << Changes << " changes.\n");
} while (Changes);
assert(IdxVNI && "Didn't find value for Idx");
#ifndef NDEBUG
// Check the liveOutCache_ invariants.
for (LiveOutMap::iterator I = liveOutCache_.begin(), E = liveOutCache_.end();
I != E; ++I) {
assert(I->first && "Null MBB entry in cache");
assert(I->second.first && "Null VNInfo in cache");
assert(I->second.second && "Null DomTreeNode in cache");
if (I->second.second->getBlock() == I->first)
continue;
for (MachineBasicBlock::pred_iterator PI = I->first->pred_begin(),
PE = I->first->pred_end(); PI != PE; ++PI)
assert(liveOutCache_.lookup(*PI) == I->second && "Bad invariant");
}
#endif
// Since we went through the trouble of a full BFS visiting all reaching defs,
// the values in LiveIn are now accurate. No more phi-defs are needed
// for these blocks, so we can color the live ranges.
// This makes the next mapValue call much faster.
for (unsigned i = 0, e = LiveIn.size(); i != e; ++i) {
MachineBasicBlock *MBB = LiveIn[i]->getBlock();
SlotIndex Start = lis_.getMBBStartIdx(MBB);
if (MBB == IdxMBB) {
li_->addRange(LiveRange(Start, Idx.getNextSlot(), IdxVNI));
continue;
}
// Anything in LiveIn other than IdxMBB is live-through.
VNInfo *VNI = liveOutCache_.lookup(MBB).first;
assert(VNI && "Missing block value");
li_->addRange(LiveRange(Start, lis_.getMBBEndIdx(MBB), VNI));
}
return IdxVNI;
}

24
lib/CodeGen/SplitKit.h
View File

@ -22,7 +22,6 @@ class LiveInterval;
class LiveIntervals;
class LiveRangeEdit;
class MachineInstr;
class MachineDominatorTree;
class MachineLoop;
class MachineLoopInfo;
class MachineRegisterInfo;
@ -31,6 +30,11 @@ class VirtRegMap;
class VNInfo;
class raw_ostream;
/// At some point we should just include MachineDominators.h:
class MachineDominatorTree;
template <class NodeT> class DomTreeNodeBase;
typedef DomTreeNodeBase<MachineBasicBlock> MachineDomTreeNode;
/// SplitAnalysis - Analyze a LiveInterval, looking for live range splitting
/// opportunities.
class SplitAnalysis {
@ -171,6 +175,24 @@ class LiveIntervalMap {
// values not present (unknown/unmapped).
ValueMap valueMap_;
typedef std::pair<VNInfo*, MachineDomTreeNode*> LiveOutPair;
typedef DenseMap<MachineBasicBlock*,LiveOutPair> LiveOutMap;
// liveOutCache_ - Map each basic block where li_ is live out to the live-out
// value and its defining block. One of these conditions shall be true:
//
// 1. !liveOutCache_.count(MBB)
// 2. liveOutCache_[MBB].second.getNode() == MBB
// 3. forall P in preds(MBB): liveOutCache_[P] == liveOutCache_[MBB]
//
// This is only a cache, the values can be computed as:
//
// VNI = li_->getVNInfoAt(lis_.getMBBEndIdx(MBB))
// Node = mbt_[lis_.getMBBFromIndex(VNI->def)]
//
// The cache is also used as a visiteed set by mapValue().
LiveOutMap liveOutCache_;
public:
LiveIntervalMap(LiveIntervals &lis,
MachineDominatorTree &mdt,