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- CodeGenPrepare does not split loop back edges but it only knows about back edges of single block loops. It now does a DFS walk to find loop back edges.

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- Use SplitBlockPredecessors to factor out common predecessors of the critical edge destination. This is disabled for now due to some regressions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61248 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng
2008-12-19 18:03:11 +00:00
parent a33649e98c
commit ab63152871
3 changed files with 185 additions and 45 deletions
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178
lib/Transforms/Scalar/CodeGenPrepare.cpp
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@@ -30,6 +30,7 @@
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
@@ -37,11 +38,18 @@
using namespace llvm;
using namespace llvm::PatternMatch;
static cl::opt<bool> FactorCommonPreds("split-critical-paths-tweak",
cl::init(false), cl::Hidden);
namespace {
class VISIBILITY_HIDDEN CodeGenPrepare : public FunctionPass {
/// TLI - Keep a pointer of a TargetLowering to consult for determining
/// transformation profitability.
const TargetLowering *TLI;
/// BackEdges - Keep a set of all the loop back edges.
///
SmallSet<std::pair<BasicBlock*,BasicBlock*>, 8> BackEdges;
public:
static char ID; // Pass identification, replacement for typeid
explicit CodeGenPrepare(const TargetLowering *tli = 0)
@@ -58,6 +66,7 @@ namespace {
bool OptimizeInlineAsmInst(Instruction *I, CallSite CS,
DenseMap<Value*,Value*> &SunkAddrs);
bool OptimizeExtUses(Instruction *I);
void findLoopBackEdges(Function &F);
};
}
@@ -69,10 +78,55 @@ FunctionPass *llvm::createCodeGenPreparePass(const TargetLowering *TLI) {
return new CodeGenPrepare(TLI);
}
/// findLoopBackEdges - Do a DFS walk to find loop back edges.
///
void CodeGenPrepare::findLoopBackEdges(Function &F) {
SmallPtrSet<BasicBlock*, 8> Visited;
SmallVector<std::pair<BasicBlock*, succ_iterator>, 8> VisitStack;
SmallPtrSet<BasicBlock*, 8> InStack;
BasicBlock *BB = &F.getEntryBlock();
if (succ_begin(BB) == succ_end(BB))
return;
Visited.insert(BB);
VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
InStack.insert(BB);
do {
std::pair<BasicBlock*, succ_iterator> &Top = VisitStack.back();
BasicBlock *ParentBB = Top.first;
succ_iterator &I = Top.second;
bool FoundNew = false;
while (I != succ_end(ParentBB)) {
BB = *I++;
if (Visited.insert(BB)) {
FoundNew = true;
break;
}
// Successor is in VisitStack, it's a back edge.
if (InStack.count(BB))
BackEdges.insert(std::make_pair(ParentBB, BB));
}
if (FoundNew) {
// Go down one level if there is a unvisited successor.
InStack.insert(BB);
VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
} else {
// Go up one level.
std::pair<BasicBlock*, succ_iterator> &Pop = VisitStack.back();
InStack.erase(Pop.first);
VisitStack.pop_back();
}
} while (!VisitStack.empty());
}
bool CodeGenPrepare::runOnFunction(Function &F) {
bool EverMadeChange = false;
findLoopBackEdges(F);
// First pass, eliminate blocks that contain only PHI nodes and an
// unconditional branch.
EverMadeChange |= EliminateMostlyEmptyBlocks(F);
@@ -262,7 +316,9 @@ void CodeGenPrepare::EliminateMostlyEmptyBlock(BasicBlock *BB) {
/// phi nodes (otherwise critical edges are ok). If there is already another
/// predecessor of the succ that is empty (and thus has no phi nodes), use it
/// instead of introducing a new block.
static void SplitEdgeNicely(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
static void SplitEdgeNicely(TerminatorInst *TI, unsigned SuccNum,
SmallSet<std::pair<BasicBlock*,BasicBlock*>, 8> &BackEdges,
Pass *P) {
BasicBlock *TIBB = TI->getParent();
BasicBlock *Dest = TI->getSuccessor(SuccNum);
assert(isa<PHINode>(Dest->begin()) &&
@@ -271,55 +327,90 @@ static void SplitEdgeNicely(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
// As a hack, never split backedges of loops. Even though the copy for any
// PHIs inserted on the backedge would be dead for exits from the loop, we
// assume that the cost of *splitting* the backedge would be too high.
if (Dest == TIBB)
if (BackEdges.count(std::make_pair(TIBB, Dest)))
return;
/// TIPHIValues - This array is lazily computed to determine the values of
/// PHIs in Dest that TI would provide.
SmallVector<Value*, 32> TIPHIValues;
if (!FactorCommonPreds) {
/// TIPHIValues - This array is lazily computed to determine the values of
/// PHIs in Dest that TI would provide.
SmallVector<Value*, 32> TIPHIValues;
// Check to see if Dest has any blocks that can be used as a split edge for
// this terminator.
for (pred_iterator PI = pred_begin(Dest), E = pred_end(Dest); PI != E; ++PI) {
BasicBlock *Pred = *PI;
// To be usable, the pred has to end with an uncond branch to the dest.
BranchInst *PredBr = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredBr || !PredBr->isUnconditional() ||
// Must be empty other than the branch.
&Pred->front() != PredBr ||
// Cannot be the entry block; its label does not get emitted.
Pred == &(Dest->getParent()->getEntryBlock()))
continue;
// Check to see if Dest has any blocks that can be used as a split edge for
// this terminator.
for (pred_iterator PI = pred_begin(Dest), E = pred_end(Dest); PI != E; ++PI) {
BasicBlock *Pred = *PI;
// To be usable, the pred has to end with an uncond branch to the dest.
BranchInst *PredBr = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredBr || !PredBr->isUnconditional() ||
// Must be empty other than the branch.
&Pred->front() != PredBr ||
// Cannot be the entry block; its label does not get emitted.
Pred == &(Dest->getParent()->getEntryBlock()))
continue;
// Finally, since we know that Dest has phi nodes in it, we have to make
// sure that jumping to Pred will have the same affect as going to Dest in
// terms of PHI values.
PHINode *PN;
unsigned PHINo = 0;
bool FoundMatch = true;
for (BasicBlock::iterator I = Dest->begin();
(PN = dyn_cast<PHINode>(I)); ++I, ++PHINo) {
if (PHINo == TIPHIValues.size())
TIPHIValues.push_back(PN->getIncomingValueForBlock(TIBB));
// Finally, since we know that Dest has phi nodes in it, we have to make
// sure that jumping to Pred will have the same affect as going to Dest in
// terms of PHI values.
PHINode *PN;
unsigned PHINo = 0;
bool FoundMatch = true;
for (BasicBlock::iterator I = Dest->begin();
(PN = dyn_cast<PHINode>(I)); ++I, ++PHINo) {
if (PHINo == TIPHIValues.size())
TIPHIValues.push_back(PN->getIncomingValueForBlock(TIBB));
// If the PHI entry doesn't work, we can't use this pred.
if (TIPHIValues[PHINo] != PN->getIncomingValueForBlock(Pred)) {
FoundMatch = false;
break;
// If the PHI entry doesn't work, we can't use this pred.
if (TIPHIValues[PHINo] != PN->getIncomingValueForBlock(Pred)) {
FoundMatch = false;
break;
}
}
// If we found a workable predecessor, change TI to branch to Succ.
if (FoundMatch) {
Dest->removePredecessor(TIBB);
TI->setSuccessor(SuccNum, Pred);
return;
}
}
// If we found a workable predecessor, change TI to branch to Succ.
if (FoundMatch) {
Dest->removePredecessor(TIBB);
TI->setSuccessor(SuccNum, Pred);
return;
SplitCriticalEdge(TI, SuccNum, P, true);
return;
}
PHINode *PN;
SmallVector<Value*, 8> TIPHIValues;
for (BasicBlock::iterator I = Dest->begin();
(PN = dyn_cast<PHINode>(I)); ++I)
TIPHIValues.push_back(PN->getIncomingValueForBlock(TIBB));
SmallVector<BasicBlock*, 8> IdenticalPreds;
for (pred_iterator PI = pred_begin(Dest), E = pred_end(Dest); PI != E; ++PI) {
BasicBlock *Pred = *PI;
if (BackEdges.count(std::make_pair(Pred, Dest)))
continue;
if (PI == TIBB)
IdenticalPreds.push_back(Pred);
else {
bool Identical = true;
unsigned PHINo = 0;
for (BasicBlock::iterator I = Dest->begin();
(PN = dyn_cast<PHINode>(I)); ++I, ++PHINo)
if (TIPHIValues[PHINo] != PN->getIncomingValueForBlock(Pred)) {
Identical = false;
break;
}
if (Identical)
IdenticalPreds.push_back(Pred);
}
}
SplitCriticalEdge(TI, SuccNum, P, true);
assert(!IdenticalPreds.empty());
SplitBlockPredecessors(Dest, &IdenticalPreds[0], IdenticalPreds.size(),
".critedge", P);
}
/// OptimizeNoopCopyExpression - If the specified cast instruction is a noop
/// copy (e.g. it's casting from one pointer type to another, int->uint, or
/// int->sbyte on PPC), sink it into user blocks to reduce the number of virtual
@@ -1350,17 +1441,16 @@ bool CodeGenPrepare::OptimizeExtUses(Instruction *I) {
bool CodeGenPrepare::OptimizeBlock(BasicBlock &BB) {
bool MadeChange = false;
// Split all critical edges where the dest block has a PHI and where the phi
// has shared immediate operands.
// Split all critical edges where the dest block has a PHI.
TerminatorInst *BBTI = BB.getTerminator();
if (BBTI->getNumSuccessors() > 1) {
for (unsigned i = 0, e = BBTI->getNumSuccessors(); i != e; ++i)
if (isa<PHINode>(BBTI->getSuccessor(i)->begin()) &&
isCriticalEdge(BBTI, i, true))
SplitEdgeNicely(BBTI, i, this);
for (unsigned i = 0, e = BBTI->getNumSuccessors(); i != e; ++i) {
BasicBlock *SuccBB = BBTI->getSuccessor(i);
if (isa<PHINode>(SuccBB->begin()) && isCriticalEdge(BBTI, i, true))
SplitEdgeNicely(BBTI, i, BackEdges, this);
}
}
// Keep track of non-local addresses that have been sunk into this block.
// This allows us to avoid inserting duplicate code for blocks with multiple
// load/stores of the same address.