From 708148e41f8f7c1e24c73cfd943645b13a1e05f9 Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Sun, 10 Oct 2004 23:14:11 +0000 Subject: Just because we cannot completely eliminate all uses of a global, we can still optimize away all of the indirect calls and loads, etc from it. This turns code like this: if (G != 0) G(); into if (G != 0) ActualCallee(); This triggers a couple of times in gcc and libstdc++. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16901 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Transforms/IPO/GlobalOpt.cpp | 151 ++++++++++++++++++++++++++++++++------- 1 file changed, 124 insertions(+), 27 deletions(-) (limited to 'lib') diff --git a/lib/Transforms/IPO/GlobalOpt.cpp b/lib/Transforms/IPO/GlobalOpt.cpp index 1732895..fa1d3f3 100644 --- a/lib/Transforms/IPO/GlobalOpt.cpp +++ b/lib/Transforms/IPO/GlobalOpt.cpp @@ -33,6 +33,7 @@ namespace { "into scalars"); Statistic<> NumDeleted ("globalopt", "Number of globals deleted"); Statistic<> NumFnDeleted("globalopt", "Number of functions deleted"); + Statistic<> NumGlobUses ("globalopt", "Number of global uses devirtualized"); struct GlobalOpt : public ModulePass { bool runOnModule(Module &M); @@ -424,12 +425,124 @@ static bool AllUsesOfLoadedValueWillTrapIfNull(GlobalVariable *GV) { return true; } +static bool OptimizeAwayTrappingUsesOfValue(Value *V, Constant *NewV) { + bool Changed = false; + for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ) { + Instruction *I = cast(*UI++); + if (LoadInst *LI = dyn_cast(I)) { + LI->setOperand(0, NewV); + Changed = true; + } else if (StoreInst *SI = dyn_cast(I)) { + if (SI->getOperand(1) == V) { + SI->setOperand(1, NewV); + Changed = true; + } + } else if (isa(I) || isa(I)) { + if (I->getOperand(0) == V) { + // Calling through the pointer! Turn into a direct call, but be careful + // that the pointer is not also being passed as an argument. + I->setOperand(0, NewV); + Changed = true; + bool PassedAsArg = false; + for (unsigned i = 1, e = I->getNumOperands(); i != e; ++i) + if (I->getOperand(i) == V) { + PassedAsArg = true; + I->setOperand(i, NewV); + } + + if (PassedAsArg) { + // Being passed as an argument also. Be careful to not invalidate UI! + UI = V->use_begin(); + } + } + } else if (CastInst *CI = dyn_cast(I)) { + Changed |= OptimizeAwayTrappingUsesOfValue(CI, + ConstantExpr::getCast(NewV, CI->getType())); + if (CI->use_empty()) { + Changed = true; + CI->getParent()->getInstList().erase(CI); + } + } else if (GetElementPtrInst *GEPI = dyn_cast(I)) { + // Should handle GEP here. + std::vector Indices; + Indices.reserve(GEPI->getNumOperands()-1); + for (unsigned i = 1, e = GEPI->getNumOperands(); i != e; ++i) + if (Constant *C = dyn_cast(GEPI->getOperand(i))) + Indices.push_back(C); + else + break; + if (Indices.size() == GEPI->getNumOperands()-1) + Changed |= OptimizeAwayTrappingUsesOfValue(GEPI, + ConstantExpr::getGetElementPtr(NewV, Indices)); + if (GEPI->use_empty()) { + Changed = true; + GEPI->getParent()->getInstList().erase(GEPI); + } + } + } + + return Changed; +} + + +/// OptimizeAwayTrappingUsesOfLoads - The specified global has only one non-null +/// value stored into it. If there are uses of the loaded value that would trap +/// if the loaded value is dynamically null, then we know that they cannot be +/// reachable with a null optimize away the load. +static bool OptimizeAwayTrappingUsesOfLoads(GlobalVariable *GV, Constant *LV) { + std::vector Loads; + bool Changed = false; + + // Replace all uses of loads with uses of uses of the stored value. + for (Value::use_iterator GUI = GV->use_begin(), E = GV->use_end(); + GUI != E; ++GUI) + if (LoadInst *LI = dyn_cast(*GUI)) { + Loads.push_back(LI); + Changed |= OptimizeAwayTrappingUsesOfValue(LI, LV); + } else { + assert(isa(*GUI) && "Only expect load and stores!"); + } + + if (Changed) { + DEBUG(std::cerr << "OPTIMIZED LOADS FROM STORED ONCE POINTER: " << *GV); + ++NumGlobUses; + } + + // Delete all of the loads we can, keeping track of whether we nuked them all! + bool AllLoadsGone = true; + while (!Loads.empty()) { + LoadInst *L = Loads.back(); + if (L->use_empty()) { + L->getParent()->getInstList().erase(L); + Changed = true; + } else { + AllLoadsGone = false; + } + Loads.pop_back(); + } + + // If we nuked all of the loads, then none of the stores are needed either, + // nor is the global. + if (AllLoadsGone) { + DEBUG(std::cerr << " *** GLOBAL NOW DEAD!\n"); + CleanupConstantGlobalUsers(GV, 0); + if (GV->use_empty()) { + GV->getParent()->getGlobalList().erase(GV); + ++NumDeleted; + } + Changed = true; + } + return Changed; +} + + // OptimizeOnceStoredGlobal - Try to optimize globals based on the knowledge // that only one value (besides its initializer) is ever stored to the global. static bool OptimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal) { if (CastInst *CI = dyn_cast(StoredOnceVal)) StoredOnceVal = CI->getOperand(0); else if (GetElementPtrInst *GEPI =dyn_cast(StoredOnceVal)){ + // "getelementptr Ptr, 0, 0, 0" is really just a cast. bool IsJustACast = true; for (unsigned i = 1, e = GEPI->getNumOperands(); i != e; ++i) if (!isa(GEPI->getOperand(i)) || @@ -441,36 +554,20 @@ static bool OptimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal) { StoredOnceVal = GEPI->getOperand(0); } - // If we are dealing with a pointer global that is initialized to null, only - // has one (non-null) value stored into, and if we know that all users of the - // loaded value trap if null, then the load users must never get the - // initializer. Instead, replace all of the loads with the stored value. + // If we are dealing with a pointer global that is initialized to null and + // only has one (non-null) value stored into it, then we can optimize any + // users of the loaded value (often calls and loads) that would trap if the + // value was null. if (isa(GV->getInitializer()->getType()) && GV->getInitializer()->isNullValue()) { - if (Constant *SOVC = dyn_cast(StoredOnceVal)) - if (AllUsesOfLoadedValueWillTrapIfNull(GV)) { - DEBUG(std::cerr << "REPLACING STORED GLOBAL POINTER: " << *GV); - - if (GV->getInitializer()->getType() != SOVC->getType()) - SOVC = ConstantExpr::getCast(SOVC, GV->getInitializer()->getType()); - - //std::cerr << " Stored Value: " << *SOVC << "\n"; - - // Replace all uses of loads with uses of uses of the stored value. - while (!GV->use_empty()) - if (LoadInst *LI = dyn_cast(GV->use_back())) { - LI->replaceAllUsesWith(SOVC); - LI->getParent()->getInstList().erase(LI); // Nuke the load. - } else if (StoreInst *SI = dyn_cast(GV->use_back())) { - SI->getParent()->getInstList().erase(SI); // Nuke the store - } else { - assert(0 && "Unknown user of stored once global!"); - } - - // Nuke the now-dead global. - GV->getParent()->getGlobalList().erase(GV); + if (Constant *SOVC = dyn_cast(StoredOnceVal)) { + if (GV->getInitializer()->getType() != SOVC->getType()) + SOVC = ConstantExpr::getCast(SOVC, GV->getInitializer()->getType()); + + // Optimize away any trapping uses of the loaded value. + if (OptimizeAwayTrappingUsesOfLoads(GV, SOVC)) return true; - } + } //if (isa(StoredOnceValue)) } return false; -- cgit v1.1