[llvm-commits] [llvm] r93221 - in /llvm/trunk: lib/Transforms/Scalar/JumpThreading.cpp test/Transforms/JumpThreading/basic.ll
Chris Lattner
sabre at nondot.org
Mon Jan 11 18:07:17 PST 2010
Author: lattner
Date: Mon Jan 11 20:07:17 2010
New Revision: 93221
URL: http://llvm.org/viewvc/llvm-project?rev=93221&view=rev
Log:
Teach jump threading to duplicate small blocks when the branch
condition is a xor with a phi node. This eliminates nonsense
like this from 176.gcc in several places:
LBB166_84:
testl %eax, %eax
- setne %al
- xorb %cl, %al
- notb %al
- testb $1, %al
- je LBB166_85
+ je LBB166_69
+ jmp LBB166_85
This is rdar://7391699
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=93221&r1=93220&r2=93221&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Mon Jan 11 20:07:17 2010
@@ -89,7 +89,7 @@
bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
BasicBlock *SuccBB);
bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
- BasicBlock *PredBB);
+ const SmallVectorImpl<BasicBlock *> &PredBBs);
typedef SmallVectorImpl<std::pair<ConstantInt*,
BasicBlock*> > PredValueInfo;
@@ -103,6 +103,7 @@
bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
bool ProcessBranchOnPHI(PHINode *PN);
+ bool ProcessBranchOnXOR(BinaryOperator *BO);
bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
};
@@ -603,6 +604,13 @@
if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
return ProcessBranchOnPHI(PN);
+
+ // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
+ if (CondInst->getOpcode() == Instruction::Xor &&
+ CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
+ return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
+
+
// TODO: If we have: "br (X > 0)" and we have a predecessor where we know
// "(X == 4)", thread through this block.
@@ -1073,6 +1081,11 @@
bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
BasicBlock *BB = PN->getParent();
+ // TODO: We could make use of this to do it once for blocks with common PHI
+ // values.
+ SmallVector<BasicBlock*, 1> PredBBs;
+ PredBBs.resize(1);
+
// If any of the predecessor blocks end in an unconditional branch, we can
// *duplicate* the conditional branch into that block in order to further
// encourage jump threading and to eliminate cases where we have branch on a
@@ -1080,15 +1093,96 @@
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = PN->getIncomingBlock(i);
if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
- if (PredBr->isUnconditional() &&
- // Try to duplicate BB into PredBB.
- DuplicateCondBranchOnPHIIntoPred(BB, PredBB))
- return true;
+ if (PredBr->isUnconditional()) {
+ PredBBs[0] = PredBB;
+ // Try to duplicate BB into PredBB.
+ if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
+ return true;
+ }
}
return false;
}
+/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
+/// a xor instruction in the current block. See if there are any
+/// simplifications we can do based on inputs to the xor.
+///
+bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
+ BasicBlock *BB = BO->getParent();
+
+ // If either the LHS or RHS of the xor is a constant, don't do this
+ // optimization.
+ if (isa<ConstantInt>(BO->getOperand(0)) ||
+ isa<ConstantInt>(BO->getOperand(1)))
+ return false;
+
+ // If we have a xor as the branch input to this block, and we know that the
+ // LHS or RHS of the xor in any predecessor is true/false, then we can clone
+ // the condition into the predecessor and fix that value to true, saving some
+ // logical ops on that path and encouraging other paths to simplify.
+ //
+ // This copies something like this:
+ //
+ // BB:
+ // %X = phi i1 [1], [%X']
+ // %Y = icmp eq i32 %A, %B
+ // %Z = xor i1 %X, %Y
+ // br i1 %Z, ...
+ //
+ // Into:
+ // BB':
+ // %Y = icmp ne i32 %A, %B
+ // br i1 %Z, ...
+
+ SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues;
+ bool isLHS = true;
+ if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) {
+ assert(XorOpValues.empty());
+ if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues))
+ return false;
+ isLHS = false;
+ }
+
+ assert(!XorOpValues.empty() &&
+ "ComputeValueKnownInPredecessors returned true with no values");
+
+ // Scan the information to see which is most popular: true or false. The
+ // predecessors can be of the set true, false, or undef.
+ unsigned NumTrue = 0, NumFalse = 0;
+ for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
+ if (!XorOpValues[i].first) continue; // Ignore undefs for the count.
+ if (XorOpValues[i].first->isZero())
+ ++NumFalse;
+ else
+ ++NumTrue;
+ }
+
+ // Determine which value to split on, true, false, or undef if neither.
+ ConstantInt *SplitVal = 0;
+ if (NumTrue > NumFalse)
+ SplitVal = ConstantInt::getTrue(BB->getContext());
+ else if (NumTrue != 0 || NumFalse != 0)
+ SplitVal = ConstantInt::getFalse(BB->getContext());
+
+ // Collect all of the blocks that this can be folded into so that we can
+ // factor this once and clone it once.
+ SmallVector<BasicBlock*, 8> BlocksToFoldInto;
+ for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
+ if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue;
+
+ BlocksToFoldInto.push_back(XorOpValues[i].second);
+ }
+
+ // Try to duplicate BB into PredBB.
+ if (DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto)) {
+// errs() << "CLONE XOR COND: " << *BB << "Into PRED: " << *BlocksToFoldInto[0];
+ return true;
+ }
+ return false;
+}
+
+
/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
/// predecessor to the PHIBB block. If it has PHI nodes, add entries for
/// NewPred using the entries from OldPred (suitably mapped).
@@ -1277,13 +1371,15 @@
/// improves the odds that the branch will be on an analyzable instruction like
/// a compare.
bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
- BasicBlock *PredBB) {
+ const SmallVectorImpl<BasicBlock *> &PredBBs) {
+ assert(!PredBBs.empty() && "Can't handle an empty set");
+
// If BB is a loop header, then duplicating this block outside the loop would
// cause us to transform this into an irreducible loop, don't do this.
// See the comments above FindLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB)) {
DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
- << "' into predecessor block '" << PredBB->getName()
+ << "' into predecessor block '" << PredBBs[0]->getName()
<< "' - it might create an irreducible loop!\n");
return false;
}
@@ -1295,12 +1391,32 @@
return false;
}
+ // And finally, do it! Start by factoring the predecessors is needed.
+ BasicBlock *PredBB;
+ if (PredBBs.size() == 1)
+ PredBB = PredBBs[0];
+ else {
+ DEBUG(dbgs() << " Factoring out " << PredBBs.size()
+ << " common predecessors.\n");
+ PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
+ ".thr_comm", this);
+ }
+
// Okay, we decided to do this! Clone all the instructions in BB onto the end
// of PredBB.
DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
<< PredBB->getName() << "' to eliminate branch on phi. Cost: "
<< DuplicationCost << " block is:" << *BB << "\n");
+ // Unless PredBB ends with an unconditional branch, split the edge so that we
+ // can just clone the bits from BB into the end of the new PredBB.
+ BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
+
+ if (!OldPredBranch->isUnconditional()) {
+ PredBB = SplitEdge(PredBB, BB, this);
+ OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
+ }
+
// We are going to have to map operands from the original BB block into the
// PredBB block. Evaluate PHI nodes in BB.
DenseMap<Instruction*, Value*> ValueMapping;
@@ -1309,8 +1425,6 @@
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
- BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
-
// Clone the non-phi instructions of BB into PredBB, keeping track of the
// mapping and using it to remap operands in the cloned instructions.
for (; BI != BB->end(); ++BI) {
Modified: llvm/trunk/test/Transforms/JumpThreading/basic.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/JumpThreading/basic.ll?rev=93221&r1=93220&r2=93221&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/JumpThreading/basic.ll (original)
+++ llvm/trunk/test/Transforms/JumpThreading/basic.ll Mon Jan 11 20:07:17 2010
@@ -383,11 +383,11 @@
}
-;;; Duplicate condition to avoid xor of cond.
-;;; TODO: Make this happen.
-define i32 @testXX(i1 %cond, i1 %cond2) {
+;; Duplicate condition to avoid xor of cond.
+;; rdar://7391699
+define i32 @test13(i1 %cond, i1 %cond2) {
Entry:
-; CHECK: @testXX
+; CHECK: @test13
%v1 = call i32 @f1()
br i1 %cond, label %Merge, label %F1
@@ -396,7 +396,8 @@
Merge:
%B = phi i1 [true, %Entry], [%cond2, %F1]
- %M = icmp eq i32 %v1, 192
+ %C = phi i32 [192, %Entry], [%v1, %F1]
+ %M = icmp eq i32 %C, 192
%N = xor i1 %B, %M
br i1 %N, label %T2, label %F2
@@ -405,6 +406,18 @@
F2:
ret i32 %v1
+
+;; FIXME: CONSTANT FOLD on clone and when phi gets eliminated.
+
+; CHECK: Entry.Merge_crit_edge:
+; CHECK-NEXT: %M1 = icmp eq i32 192, 192
+; CHECK-NEXT: %N2 = xor i1 true, %M1
+; CHECK-NEXT: br i1 %N2, label %T2, label %F2
+
+; CHECK: Merge:
+; CHECK-NEXT: %M = icmp eq i32 %v1, 192
+; CHECK-NEXT: %N = xor i1 %cond2, %M
+; CHECK-NEXT: br i1 %N, label %T2, label %F2
}
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