forked from OSchip/llvm-project
Teach SimplifyCFG how to eliminate duplicate PHI nodes within a block.
This reduces codesize on a variety of codes by 1-2% on x86-64. It also helps clean up after SSAUpdater. llvm-svn: 85626
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@ -24,6 +24,7 @@
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/Statistic.h"
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@ -1748,6 +1749,63 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
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return true;
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return true;
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}
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}
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/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI
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/// nodes in this block. This doesn't try to be clever about PHI nodes
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/// which differ only in the order of the incoming values, but instcombine
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/// orders them so it usually won't matter.
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static bool EliminateDuplicatePHINodes(BasicBlock *BB) {
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bool Changed = false;
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// Map from PHI hash values to PHI nodes. If multiple PHIs have
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// the same hash value, the element is the first PHI in the
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// linked list in CollisionMap.
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DenseMap<uintptr_t, PHINode *> HashMap;
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// Maintain linked lists of PHI nodes with common hash values.
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DenseMap<PHINode *, PHINode *> CollisionMap;
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// Examine each PHI.
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for (BasicBlock::iterator I = BB->begin();
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PHINode *PN = dyn_cast<PHINode>(I++); ) {
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// Compute a hash value on the operands. Instcombine will likely have sorted
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// them, which helps expose duplicates, but we have to check all the
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// operands to be safe in case instcombine hasn't run.
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uintptr_t Hash = 0;
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for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) {
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// This hash algorithm is quite weak as hash functions go, but it seems
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// to do a good enough job for this particular purpose, and is very quick.
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Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I));
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Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
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}
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// If we've never seen this hash value before, it's a unique PHI.
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std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair =
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HashMap.insert(std::make_pair(Hash, PN));
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if (Pair.second) continue;
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// Otherwise it's either a duplicate or a hash collision.
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for (PHINode *OtherPN = Pair.first->second; ; ) {
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if (OtherPN->isIdenticalTo(PN)) {
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// A duplicate. Replace this PHI with its duplicate.
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PN->replaceAllUsesWith(OtherPN);
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PN->eraseFromParent();
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Changed = true;
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break;
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}
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// A non-duplicate hash collision.
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DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN);
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if (I == CollisionMap.end()) {
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// Set this PHI to be the head of the linked list of colliding PHIs.
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PHINode *Old = Pair.first->second;
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Pair.first->second = PN;
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CollisionMap[PN] = Old;
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break;
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}
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// Procede to the next PHI in the list.
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OtherPN = I->second;
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}
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}
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return Changed;
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}
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/// SimplifyCFG - This function is used to do simplification of a CFG. For
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/// SimplifyCFG - This function is used to do simplification of a CFG. For
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/// example, it adjusts branches to branches to eliminate the extra hop, it
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/// example, it adjusts branches to branches to eliminate the extra hop, it
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@ -1777,6 +1835,9 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
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// away...
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// away...
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Changed |= ConstantFoldTerminator(BB);
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Changed |= ConstantFoldTerminator(BB);
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// Check for and eliminate duplicate PHI nodes in this block.
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Changed |= EliminateDuplicatePHINodes(BB);
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// If there is a trivial two-entry PHI node in this basic block, and we can
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// If there is a trivial two-entry PHI node in this basic block, and we can
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// eliminate it, do so now.
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// eliminate it, do so now.
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if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
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if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
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