[llvm-commits] [llvm] r60470 - in /llvm/trunk: lib/Transforms/Scalar/JumpThreading.cpp test/Transforms/JumpThreading/basic.ll
Chris Lattner
sabre at nondot.org
Tue Dec 2 23:48:08 PST 2008
Author: lattner
Date: Wed Dec 3 01:48:08 2008
New Revision: 60470
URL: http://llvm.org/viewvc/llvm-project?rev=60470&view=rev
Log:
Teach jump threading some more simple tricks:
1) have it fold "br undef", which does occur with
surprising frequency as jump threading iterates.
2) teach j-t to delete dead blocks. This removes the successor
edges, reducing the in-edges of other blocks, allowing
recursive simplification.
3) Fold things like:
br COND, BBX, BBY
BBX:
br COND, BBZ, BBW
which also happens because jump threading iterates.
Modified:
llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
llvm/trunk/test/Transforms/JumpThreading/basic.ll
Modified: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp?rev=60470&r1=60469&r2=60470&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Wed Dec 3 01:48:08 2008
@@ -67,6 +67,7 @@
bool ProcessBlock(BasicBlock *BB);
void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal);
+ bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
bool ProcessJumpOnPHI(PHINode *PN);
bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd);
@@ -93,9 +94,23 @@
while (AnotherIteration) {
AnotherIteration = false;
bool Changed = false;
- for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
- while (ProcessBlock(I))
+ for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
+ BasicBlock *BB = I;
+ while (ProcessBlock(BB))
Changed = true;
+
+ ++I;
+
+ // If the block is trivially dead, zap it. This eliminates the successor
+ // edges which simplifies the CFG.
+ if (pred_begin(BB) == pred_end(BB) &&
+ BB != &BB->getParent()->getEntryBlock()) {
+ DOUT << " JT: Deleting dead block '" << BB->getNameStart()
+ << "' with terminator: " << *BB->getTerminator();
+ DeleteDeadBlock(BB);
+ Changed = true;
+ }
+ }
AnotherIteration = Changed;
EverChanged |= Changed;
}
@@ -209,29 +224,80 @@
return true;
}
+ // If the terminator is branching on an undef, we can pick any of the
+ // successors to branch to. Since this is arbitrary, we pick the successor
+ // with the fewest predecessors. This should reduce the in-degree of the
+ // others.
+ if (isa<UndefValue>(Condition)) {
+ TerminatorInst *BBTerm = BB->getTerminator();
+ unsigned MinSucc = 0;
+ BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
+ // Compute the successor with the minimum number of predecessors.
+ unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
+ for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
+ TestBB = BBTerm->getSuccessor(i);
+ unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
+ if (NumPreds < MinNumPreds)
+ MinSucc = i;
+ }
+
+ // Fold the branch/switch.
+ for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
+ if (i == MinSucc) continue;
+ BBTerm->getSuccessor(i)->removePredecessor(BB);
+ }
+
+ DOUT << " In block '" << BB->getNameStart()
+ << "' folding undef terminator: " << *BBTerm;
+ BranchInst::Create(BBTerm->getSuccessor(MinSucc), BBTerm);
+ BBTerm->eraseFromParent();
+ return true;
+ }
+
+ Instruction *CondInst = dyn_cast<Instruction>(Condition);
+
+ // If the condition is an instruction defined in another block, see if a
+ // predecessor has the same condition:
+ // br COND, BBX, BBY
+ // BBX:
+ // br COND, BBZ, BBW
+ if (!Condition->hasOneUse() && // Multiple uses.
+ (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
+ pred_iterator PI = pred_begin(BB), E = pred_end(BB);
+ if (isa<BranchInst>(BB->getTerminator())) {
+ for (; PI != E; ++PI)
+ if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
+ if (PBI->isConditional() && PBI->getCondition() == Condition &&
+ ProcessBranchOnDuplicateCond(*PI, BB))
+ return true;
+ }
+ }
+
// If there is only a single predecessor of this block, nothing to fold.
if (BB->getSinglePredecessor())
return false;
-
+
+ // All the rest of our checks depend on the condition being an instruction.
+ if (CondInst == 0)
+ return false;
+
// See if this is a phi node in the current block.
- PHINode *PN = dyn_cast<PHINode>(Condition);
- if (PN && PN->getParent() == BB)
- return ProcessJumpOnPHI(PN);
+ if (PHINode *PN = dyn_cast<PHINode>(CondInst))
+ if (PN->getParent() == BB)
+ return ProcessJumpOnPHI(PN);
// If this is a conditional branch whose condition is and/or of a phi, try to
// simplify it.
- if (BinaryOperator *CondI = dyn_cast<BinaryOperator>(Condition)) {
- if ((CondI->getOpcode() == Instruction::And ||
- CondI->getOpcode() == Instruction::Or) &&
- isa<BranchInst>(BB->getTerminator()) &&
- ProcessBranchOnLogical(CondI, BB,
- CondI->getOpcode() == Instruction::And))
- return true;
- }
+ if ((CondInst->getOpcode() == Instruction::And ||
+ CondInst->getOpcode() == Instruction::Or) &&
+ isa<BranchInst>(BB->getTerminator()) &&
+ ProcessBranchOnLogical(CondInst, BB,
+ CondInst->getOpcode() == Instruction::And))
+ return true;
// If we have "br (phi != 42)" and the phi node has any constant values as
// operands, we can thread through this block.
- if (CmpInst *CondCmp = dyn_cast<CmpInst>(Condition))
+ if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst))
if (isa<PHINode>(CondCmp->getOperand(0)) &&
isa<Constant>(CondCmp->getOperand(1)) &&
ProcessBranchOnCompare(CondCmp, BB))
@@ -244,7 +310,7 @@
//
// This is particularly important because reg2mem inserts loads and stores all
// over the place, and this blocks jump threading if we don't zap them.
- Value *SimplifyValue = Condition;
+ Value *SimplifyValue = CondInst;
if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
if (isa<Constant>(CondCmp->getOperand(1)))
SimplifyValue = CondCmp->getOperand(0);
@@ -259,6 +325,80 @@
return false;
}
+/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
+/// block that jump on exactly the same condition. This means that we almost
+/// always know the direction of the edge in the DESTBB:
+/// PREDBB:
+/// br COND, DESTBB, BBY
+/// DESTBB:
+/// br COND, BBZ, BBW
+///
+/// If DESTBB has multiple predecessors, we can't just constant fold the branch
+/// in DESTBB, we have to thread over it.
+bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
+ BasicBlock *BB) {
+ BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
+
+ // If both successors of PredBB go to DESTBB, we don't know anything. We can
+ // fold the branch to an unconditional one, which allows other recursive
+ // simplifications.
+ bool BranchDir;
+ if (PredBI->getSuccessor(1) != BB)
+ BranchDir = true;
+ else if (PredBI->getSuccessor(0) != BB)
+ BranchDir = false;
+ else {
+ DOUT << " In block '" << PredBB->getNameStart()
+ << "' folding terminator: " << *PredBB->getTerminator();
+ ++NumFolds;
+ ConstantFoldTerminator(PredBB);
+ return true;
+ }
+
+ BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
+
+ // If the dest block has one predecessor, just fix the branch condition to a
+ // constant and fold it.
+ if (BB->getSinglePredecessor()) {
+ DOUT << " In block '" << BB->getNameStart()
+ << "' folding condition to '" << BranchDir << "': "
+ << *BB->getTerminator();
+ ++NumFolds;
+ DestBI->setCondition(ConstantInt::get(Type::Int1Ty, BranchDir));
+ ConstantFoldTerminator(BB);
+ return true;
+ }
+
+ // Otherwise we need to thread from PredBB to DestBB's successor which
+ // involves code duplication. Check to see if it is worth it.
+ unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
+ if (JumpThreadCost > Threshold) {
+ DOUT << " Not threading BB '" << BB->getNameStart()
+ << "' - Cost is too high: " << JumpThreadCost << "\n";
+ return false;
+ }
+
+ // 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;
+}
+
/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
/// load instruction, eliminate it by replacing it with a PHI node. This is an
/// important optimization that encourages jump threading, and needs to be run
Modified: llvm/trunk/test/Transforms/JumpThreading/basic.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/JumpThreading/basic.ll?rev=60470&r1=60469&r2=60470&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/JumpThreading/basic.ll (original)
+++ llvm/trunk/test/Transforms/JumpThreading/basic.ll Wed Dec 3 01:48:08 2008
@@ -29,3 +29,25 @@
F2:
ret i32 %B
}
+
+
+;; cond is known false on Entry -> F1 edge!
+define i32 @test2(i1 %cond) {
+Entry:
+ br i1 %cond, label %T1, label %F1
+
+T1:
+ %v1 = call i32 @f1()
+ br label %Merge
+
+F1:
+ br i1 %cond, label %Merge, label %F2
+
+Merge:
+ %B = phi i32 [47, %T1], [192, %F1]
+ ret i32 %B
+
+F2:
+ call void @f3()
+ ret i32 12
+}
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