[llvm-commits] [llvm] r85626 - /llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
Dan Gohman
gohman at apple.com
Fri Oct 30 15:39:05 PDT 2009
Author: djg
Date: Fri Oct 30 17:39:04 2009
New Revision: 85626
URL: http://llvm.org/viewvc/llvm-project?rev=85626&view=rev
Log:
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.
Modified:
llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=85626&r1=85625&r2=85626&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Fri Oct 30 17:39:04 2009
@@ -24,6 +24,7 @@
#include "llvm/Support/raw_ostream.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
@@ -1748,6 +1749,63 @@
return true;
}
+/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI
+/// nodes in this block. This doesn't try to be clever about PHI nodes
+/// which differ only in the order of the incoming values, but instcombine
+/// orders them so it usually won't matter.
+static bool EliminateDuplicatePHINodes(BasicBlock *BB) {
+ bool Changed = false;
+
+ // Map from PHI hash values to PHI nodes. If multiple PHIs have
+ // the same hash value, the element is the first PHI in the
+ // linked list in CollisionMap.
+ DenseMap<uintptr_t, PHINode *> HashMap;
+
+ // Maintain linked lists of PHI nodes with common hash values.
+ DenseMap<PHINode *, PHINode *> CollisionMap;
+
+ // Examine each PHI.
+ for (BasicBlock::iterator I = BB->begin();
+ PHINode *PN = dyn_cast<PHINode>(I++); ) {
+ // Compute a hash value on the operands. Instcombine will likely have sorted
+ // them, which helps expose duplicates, but we have to check all the
+ // operands to be safe in case instcombine hasn't run.
+ uintptr_t Hash = 0;
+ for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) {
+ // This hash algorithm is quite weak as hash functions go, but it seems
+ // to do a good enough job for this particular purpose, and is very quick.
+ Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I));
+ Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
+ }
+ // If we've never seen this hash value before, it's a unique PHI.
+ std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair =
+ HashMap.insert(std::make_pair(Hash, PN));
+ if (Pair.second) continue;
+ // Otherwise it's either a duplicate or a hash collision.
+ for (PHINode *OtherPN = Pair.first->second; ; ) {
+ if (OtherPN->isIdenticalTo(PN)) {
+ // A duplicate. Replace this PHI with its duplicate.
+ PN->replaceAllUsesWith(OtherPN);
+ PN->eraseFromParent();
+ Changed = true;
+ break;
+ }
+ // A non-duplicate hash collision.
+ DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN);
+ if (I == CollisionMap.end()) {
+ // Set this PHI to be the head of the linked list of colliding PHIs.
+ PHINode *Old = Pair.first->second;
+ Pair.first->second = PN;
+ CollisionMap[PN] = Old;
+ break;
+ }
+ // Procede to the next PHI in the list.
+ OtherPN = I->second;
+ }
+ }
+
+ return Changed;
+}
/// SimplifyCFG - This function is used to do simplification of a CFG. For
/// example, it adjusts branches to branches to eliminate the extra hop, it
@@ -1777,6 +1835,9 @@
// away...
Changed |= ConstantFoldTerminator(BB);
+ // Check for and eliminate duplicate PHI nodes in this block.
+ Changed |= EliminateDuplicatePHINodes(BB);
+
// If there is a trivial two-entry PHI node in this basic block, and we can
// eliminate it, do so now.
if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
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