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split critical edges more carefully and intelligently. In particular, critical
edges whose destinations are not phi nodes don't bother us. Also, share split edges, since the split edge can't have a phi. This significantly reduces the complexity of generated code in some cases. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31274 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -44,8 +44,6 @@
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#include "llvm/Support/MathExtras.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/Compiler.h"
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#include <map>
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#include <set>
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#include <iostream>
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#include <algorithm>
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using namespace llvm;
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@ -3490,6 +3488,63 @@ static bool OptimizeGEPExpression(GetElementPtrInst *GEPI,
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return true;
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}
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/// SplitEdgeNicely - Split the critical edge from TI to it's specified
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/// successor if it will improve codegen. We only do this if the successor has
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/// phi nodes (otherwise critical edges are ok). If there is already another
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/// predecessor of the succ that is empty (and thus has no phi nodes), use it
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/// instead of introducing a new block.
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static void SplitEdgeNicely(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
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BasicBlock *TIBB = TI->getParent();
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BasicBlock *Dest = TI->getSuccessor(SuccNum);
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assert(isa<PHINode>(Dest->begin()) &&
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"This should only be called if Dest has a PHI!");
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/// TIPHIValues - This array is lazily computed to determine the values of
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/// PHIs in Dest that TI would provide.
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std::vector<Value*> TIPHIValues;
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// Check to see if Dest has any blocks that can be used as a split edge for
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// this terminator.
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for (pred_iterator PI = pred_begin(Dest), E = pred_end(Dest); PI != E; ++PI) {
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BasicBlock *Pred = *PI;
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// To be usable, the pred has to end with an uncond branch to the dest.
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BranchInst *PredBr = dyn_cast<BranchInst>(Pred->getTerminator());
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if (!PredBr || !PredBr->isUnconditional() ||
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// Must be empty other than the branch.
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&Pred->front() != PredBr)
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continue;
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// Finally, since we know that Dest has phi nodes in it, we have to make
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// sure that jumping to Pred will have the same affect as going to Dest in
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// terms of PHI values.
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PHINode *PN;
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unsigned PHINo = 0;
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bool FoundMatch = true;
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for (BasicBlock::iterator I = Dest->begin();
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(PN = dyn_cast<PHINode>(I)); ++I, ++PHINo) {
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if (PHINo == TIPHIValues.size())
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TIPHIValues.push_back(PN->getIncomingValueForBlock(TIBB));
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// If the PHI entry doesn't work, we can't use this pred.
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if (TIPHIValues[PHINo] != PN->getIncomingValueForBlock(Pred)) {
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FoundMatch = false;
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break;
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}
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}
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// If we found a workable predecessor, change TI to branch to Succ.
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if (FoundMatch) {
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Dest->removePredecessor(TIBB);
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TI->setSuccessor(SuccNum, Pred);
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return;
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}
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}
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SplitCriticalEdge(TI, SuccNum, P, true);
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}
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bool SelectionDAGISel::runOnFunction(Function &Fn) {
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MachineFunction &MF = MachineFunction::construct(&Fn, TLI.getTargetMachine());
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RegMap = MF.getSSARegMap();
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@ -3505,11 +3560,13 @@ bool SelectionDAGISel::runOnFunction(Function &Fn) {
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while (MadeChange) {
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MadeChange = false;
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for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
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// Split all critical edges.
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// Split all critical edges where the dest block has a PHI.
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TerminatorInst *BBTI = BB->getTerminator();
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if (BBTI->getNumSuccessors() > 1) {
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for (unsigned i = 0, e = BBTI->getNumSuccessors(); i != e; ++i)
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SplitCriticalEdge(BBTI, i, this, true);
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if (isa<PHINode>(BBTI->getSuccessor(i)->begin()) &&
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isCriticalEdge(BBTI, i, true))
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SplitEdgeNicely(BBTI, i, this);
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}
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