Restore minor deletion.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70892 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Scalar/JumpThreading.cpp b/lib/Transforms/Scalar/JumpThreading.cpp
index 69d1799..c0ca2df 100644
--- a/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/lib/Transforms/Scalar/JumpThreading.cpp
@@ -15,17 +15,19 @@
#include "llvm/Transforms/Scalar.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Pass.h"
-#include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/Statistic.h"
-#include "llvm/ADT/STLExtras.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetData.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallSet.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
-#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/Support/ValueHandle.h"
using namespace llvm;
STATISTIC(NumThreads, "Number of jumps threaded");
@@ -55,6 +57,11 @@
///
class VISIBILITY_HIDDEN JumpThreading : public FunctionPass {
TargetData *TD;
+#ifdef NDEBUG
+ SmallPtrSet<BasicBlock*, 16> LoopHeaders;
+#else
+ SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
+#endif
public:
static char ID; // Pass identification
JumpThreading() : FunctionPass(&ID) {}
@@ -64,8 +71,11 @@
}
bool runOnFunction(Function &F);
+ void FindLoopHeaders(Function &F);
+
bool ProcessBlock(BasicBlock *BB);
- void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
+ bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB,
+ unsigned JumpThreadCost);
BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal);
bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
@@ -91,6 +101,8 @@
DOUT << "Jump threading on function '" << F.getNameStart() << "'\n";
TD = &getAnalysis<TargetData>();
+ FindLoopHeaders(F);
+
bool AnotherIteration = true, EverChanged = false;
while (AnotherIteration) {
AnotherIteration = false;
@@ -108,6 +120,7 @@
BB != &BB->getParent()->getEntryBlock()) {
DOUT << " JT: Deleting dead block '" << BB->getNameStart()
<< "' with terminator: " << *BB->getTerminator();
+ LoopHeaders.erase(BB);
DeleteDeadBlock(BB);
Changed = true;
}
@@ -115,9 +128,35 @@
AnotherIteration = Changed;
EverChanged |= Changed;
}
+
+ LoopHeaders.clear();
return EverChanged;
}
+/// 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.
+///
+void JumpThreading::FindLoopHeaders(Function &F) {
+ SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
+ FindFunctionBackedges(F, Edges);
+
+ for (unsigned i = 0, e = Edges.size(); i != e; ++i)
+ LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
+}
+
+
/// FactorCommonPHIPreds - If there are multiple preds with the same incoming
/// value for the PHI, factor them together so we get one block to thread for
/// the whole group.
@@ -191,6 +230,10 @@
if (BasicBlock *SinglePred = BB->getSinglePredecessor())
if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
SinglePred != BB) {
+ // If SinglePred was a loop header, BB becomes one.
+ if (LoopHeaders.erase(SinglePred))
+ LoopHeaders.insert(BB);
+
// Remember if SinglePred was the entry block of the function. If so, we
// will need to move BB back to the entry position.
bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
@@ -389,22 +432,8 @@
// Next, figure out which successor we are threading to.
BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
- // If threading to the same block as we come from, we would infinite loop.
- if (SuccBB == BB) {
- DOUT << " Not threading BB '" << BB->getNameStart()
- << "' - would thread to self!\n";
- return false;
- }
-
- // And finally, do it!
- DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '"
- << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
- << ", across block:\n "
- << *BB << "\n";
-
- ThreadEdge(BB, PredBB, SuccBB);
- ++NumThreads;
- return true;
+ // Ok, try to thread it!
+ return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost);
}
/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
@@ -675,22 +704,8 @@
SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
}
- // If threading to the same block as we come from, we would infinite loop.
- if (SuccBB == BB) {
- DOUT << " Not threading BB '" << BB->getNameStart()
- << "' - would thread to self!\n";
- return false;
- }
-
- // And finally, do it!
- DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '"
- << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
- << ", across block:\n "
- << *BB << "\n";
-
- ThreadEdge(BB, PredBB, SuccBB);
- ++NumThreads;
- return true;
+ // Ok, try to thread it!
+ return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost);
}
/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch
@@ -751,22 +766,8 @@
// 'true' block.
BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd);
- // If threading to the same block as we come from, we would infinite loop.
- if (SuccBB == BB) {
- DOUT << " Not threading BB '" << BB->getNameStart()
- << "' - would thread to self!\n";
- return false;
- }
-
- // And finally, do it!
- DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
- << "' to '" << SuccBB->getNameStart() << "' with cost: "
- << JumpThreadCost << ", across block:\n "
- << *BB << "\n";
-
- ThreadEdge(BB, PredBB, SuccBB);
- ++NumThreads;
- return true;
+ // Ok, try to thread it!
+ return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost);
}
/// ProcessBranchOnCompare - We found a branch on a comparison between a phi
@@ -829,32 +830,40 @@
// Next, get our successor.
BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection);
- // If threading to the same block as we come from, we would infinite loop.
- if (SuccBB == BB) {
- DOUT << " Not threading BB '" << BB->getNameStart()
- << "' - would thread to self!\n";
- return false;
- }
-
-
- // And finally, do it!
- DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
- << "' to '" << SuccBB->getNameStart() << "' with cost: "
- << JumpThreadCost << ", across block:\n "
- << *BB << "\n";
-
- ThreadEdge(BB, PredBB, SuccBB);
- ++NumThreads;
- return true;
+ // Ok, try to thread it!
+ return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost);
}
/// ThreadEdge - We have decided that it is safe and profitable to thread an
/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this
/// change.
-void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
- BasicBlock *SuccBB) {
+bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
+ BasicBlock *SuccBB, unsigned JumpThreadCost) {
+ // If threading to the same block as we come from, we would infinite loop.
+ if (SuccBB == BB) {
+ DOUT << " Not threading across BB '" << BB->getNameStart()
+ << "' - would thread to self!\n";
+ return false;
+ }
+
+ // If threading this would thread across a loop header, don't thread the edge.
+ // See the comments above FindLoopHeaders for justifications and caveats.
+ if (LoopHeaders.count(BB)) {
+ DOUT << " Not threading from '" << PredBB->getNameStart()
+ << "' across loop header BB '" << BB->getNameStart()
+ << "' to dest BB '" << SuccBB->getNameStart()
+ << "' - it might create an irreducible loop!\n";
+ return false;
+ }
+
+ // And finally, do it!
+ DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '"
+ << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
+ << ", across block:\n "
+ << *BB << "\n";
+
// Jump Threading can not update SSA properties correctly if the values
// defined in the duplicated block are used outside of the block itself. For
// this reason, we spill all values that are used outside of BB to the stack.
@@ -938,4 +947,8 @@
RecursivelyDeleteTriviallyDeadInstructions(Inst);
}
+
+ // Threaded an edge!
+ ++NumThreads;
+ return true;
}