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Have GVN do a pre-pass over the CFG that folds away unconditional branches where possible. This allows local PRE to be more aggressive.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53615 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -41,6 +41,7 @@ using namespace llvm;
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STATISTIC(NumGVNInstr, "Number of instructions deleted");
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STATISTIC(NumGVNLoad, "Number of loads deleted");
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STATISTIC(NumGVNPRE, "Number of instructions PRE'd");
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STATISTIC(NumGVNBlocks, "Number of blocks merged");
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static cl::opt<bool> EnablePRE("enable-pre",
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cl::init(false), cl::Hidden);
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@ -749,6 +750,7 @@ namespace {
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bool isSafeReplacement(PHINode* p, Instruction* inst);
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bool performPRE(Function& F);
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Value* lookupNumber(BasicBlock* BB, uint32_t num);
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bool mergeBlockIntoPredecessor(BasicBlock* BB);
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};
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char GVN::ID = 0;
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@ -1324,12 +1326,71 @@ bool GVN::performPRE(Function& F) {
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return changed;
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}
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// GVN::iterateOnFunction - Executes one iteration of GVN
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// mergeBlockIntoPredecessor - If this block is the only successor
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// of its predecessor, and the edge is non-critical,
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// fold it into that predecessor.
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bool GVN::mergeBlockIntoPredecessor(BasicBlock* BB) {
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// Can't merge the entry block.
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if (pred_begin(BB) == pred_end(BB)) return false;
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// Can't merge if there are multiple preds.
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if (++pred_begin(BB) != pred_end(BB)) return false;
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BasicBlock* PredBB = *pred_begin(BB);
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// Can't merge if the edge is critical.
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if (PredBB->getTerminator()->getNumSuccessors() != 1) return false;
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// Begin by getting rid of unneeded PHIs.
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while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
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PN->replaceAllUsesWith(PN->getIncomingValue(0));
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BB->getInstList().pop_front(); // Delete the phi node...
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}
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// Delete the unconditional branch from the predecessor...
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PredBB->getInstList().pop_back();
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// Move all definitions in the successor to the predecessor...
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PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
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// Make all PHI nodes that referred to BB now refer to Pred as their
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// source...
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BB->replaceAllUsesWith(PredBB);
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// Finally, erase the old block and update dominator info.
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DominatorTree& DT = getAnalysis<DominatorTree>();
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DomTreeNode* DTN = DT[BB];
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DomTreeNode* PredDTN = DT[PredBB];
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if (DTN) {
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SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end());
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for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(),
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DE = Children.end(); DI != DE; ++DI)
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DT.changeImmediateDominator(*DI, PredDTN);
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DT.eraseNode(BB);
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}
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BB->eraseFromParent();
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NumGVNBlocks++;
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return true;
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}
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// iterateOnFunction - Executes one iteration of GVN
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bool GVN::iterateOnFunction(Function &F) {
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// Clean out global sets from any previous functions
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VN.clear();
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phiMap.clear();
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// Merge unconditional branches, allowing PRE to catch more
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// optimization opportunities.
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bool mergedBlocks = false;
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for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ) {
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BasicBlock* BB = FI;
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++FI;
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mergedBlocks |= mergeBlockIntoPredecessor(BB);
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}
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for (DenseMap<BasicBlock*, ValueNumberScope*>::iterator
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I = localAvail.begin(), E = localAvail.end(); I != E; ++I)
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delete I->second;
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@ -1346,5 +1407,5 @@ bool GVN::iterateOnFunction(Function &F) {
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if (EnablePRE)
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changed |= performPRE(F);
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return changed;
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return changed || mergedBlocks;
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}
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