diff options
| -rw-r--r-- | lib/Transforms/Scalar/EarlyCSE.cpp | 44 | ||||
| -rw-r--r-- | test/Transforms/EarlyCSE/basic.ll | 10 |
2 files changed, 48 insertions, 6 deletions
diff --git a/lib/Transforms/Scalar/EarlyCSE.cpp b/lib/Transforms/Scalar/EarlyCSE.cpp index 4ff150a..9e7092b 100644 --- a/lib/Transforms/Scalar/EarlyCSE.cpp +++ b/lib/Transforms/Scalar/EarlyCSE.cpp @@ -30,6 +30,7 @@ STATISTIC(NumSimplify, "Number of instructions simplified or DCE'd"); STATISTIC(NumCSE, "Number of instructions CSE'd"); STATISTIC(NumCSELoad, "Number of load instructions CSE'd"); STATISTIC(NumCSECall, "Number of call instructions CSE'd"); +STATISTIC(NumDSE, "Number of trivial dead stores removed"); static unsigned getHash(const void *V) { return DenseMapInfo<const void*>::getHashValue(V); @@ -290,6 +291,12 @@ bool EarlyCSE::processNode(DomTreeNode *Node) { if (BB->getSinglePredecessor() == 0) ++CurrentGeneration; + /// LastStore - Keep track of the last non-volatile store that we saw... for + /// as long as there in no instruction that reads memory. If we see a store + /// to the same location, we delete the dead store. This zaps trivial dead + /// stores which can occur in bitfield code among other things. + StoreInst *LastStore = 0; + bool Changed = false; // See if any instructions in the block can be eliminated. If so, do it. If @@ -337,7 +344,10 @@ bool EarlyCSE::processNode(DomTreeNode *Node) { // If this is a non-volatile load, process it. if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) { // Ignore volatile loads. - if (LI->isVolatile()) continue; + if (LI->isVolatile()) { + LastStore = 0; + continue; + } // If we have an available version of this load, and if it is the right // generation, replace this instruction. @@ -356,9 +366,14 @@ bool EarlyCSE::processNode(DomTreeNode *Node) { // Otherwise, remember that we have this instruction. AvailableLoads->insert(Inst->getOperand(0), std::pair<Value*, unsigned>(Inst, CurrentGeneration)); + LastStore = 0; continue; } + // If this instruction may read from memory, forget LastStore. + if (Inst->mayReadFromMemory()) + LastStore = 0; + // If this is a read-only call, process it. if (CallValue::canHandle(Inst)) { // If we have an available version of this call, and if it is the right @@ -386,14 +401,31 @@ bool EarlyCSE::processNode(DomTreeNode *Node) { if (Inst->mayWriteToMemory()) { ++CurrentGeneration; - // Okay, we just invalidated anything we knew about loaded values. Try to - // salvage *something* by remembering that the stored value is a live - // version of the pointer. It is safe to forward from volatile stores to - // non-volatile loads, so we don't have to check for volatility of the - // store. if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { + // We do a trivial form of DSE if there are two stores to the same + // location with no intervening loads. Delete the earlier store. + if (LastStore && + LastStore->getPointerOperand() == SI->getPointerOperand()) { + DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore << " due to: " + << *Inst << '\n'); + LastStore->eraseFromParent(); + Changed = true; + ++NumDSE; + LastStore = 0; + continue; + } + + // Okay, we just invalidated anything we knew about loaded values. Try + // to salvage *something* by remembering that the stored value is a live + // version of the pointer. It is safe to forward from volatile stores + // to non-volatile loads, so we don't have to check for volatility of + // the store. AvailableLoads->insert(SI->getPointerOperand(), std::pair<Value*, unsigned>(SI->getValueOperand(), CurrentGeneration)); + + // Remember that this was the last store we saw for DSE. + if (!SI->isVolatile()) + LastStore = SI; } } } diff --git a/test/Transforms/EarlyCSE/basic.ll b/test/Transforms/EarlyCSE/basic.ll index 5599a1c..bc152e7 100644 --- a/test/Transforms/EarlyCSE/basic.ll +++ b/test/Transforms/EarlyCSE/basic.ll @@ -96,3 +96,13 @@ define i32 @test6(i32 *%P) { ret i32 %V1 ; CHECK: ret i32 42 } + +;; Trivial dead store elimination. +; CHECK: @test7 +define void @test7(i32 *%P) { + store i32 42, i32* %P + store i32 45, i32* %P + ret void + ; CHECK-NEXT: store i32 45 + ; CHECK-NEXT: ret void +} |