Changes in directory llvm/lib/Transforms/IPO:
GlobalOpt.cpp updated: 1.66 -> 1.67
---
Log message:
Implement SRA of heap allocations.
---
Diffs of the changes: (+266 -10)
GlobalOpt.cpp | 276 +++++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 files changed, 266 insertions(+), 10 deletions(-)
Index: llvm/lib/Transforms/IPO/GlobalOpt.cpp
diff -u llvm/lib/Transforms/IPO/GlobalOpt.cpp:1.66
llvm/lib/Transforms/IPO/GlobalOpt.cpp:1.67
--- llvm/lib/Transforms/IPO/GlobalOpt.cpp:1.66 Sat Sep 30 14:40:30 2006
+++ llvm/lib/Transforms/IPO/GlobalOpt.cpp Sat Sep 30 18:32:09 2006
@@ -36,6 +36,7 @@
Statistic<> NumMarked ("globalopt", "Number of globals marked constant");
Statistic<> NumSRA ("globalopt", "Number of aggregate globals broken "
"into scalars");
+ Statistic<> NumHeapSRA ("globalopt", "Number of heap objects SRA'd");
Statistic<> NumSubstitute("globalopt",
"Number of globals with initializers stored into
them");
Statistic<> NumDeleted ("globalopt", "Number of globals deleted");
@@ -794,9 +795,235 @@
return false;
}
return true;
+}
+/// ReplaceUsesOfMallocWithGlobal - The Alloc pointer is stored into GV
+/// somewhere. Transform all uses of the allocation into loads from the
+/// global and uses of the resultant pointer. Further, delete the store into
+/// GV. This assumes that these value pass the
+/// 'ValueIsOnlyUsedLocallyOrStoredToOneGlobal' predicate.
+static void ReplaceUsesOfMallocWithGlobal(Instruction *Alloc,
+ GlobalVariable *GV) {
+ while (!Alloc->use_empty()) {
+ Instruction *U = Alloc->use_back();
+ if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
+ // If this is the store of the allocation into the global, remove it.
+ if (SI->getOperand(1) == GV) {
+ SI->eraseFromParent();
+ continue;
+ }
+ }
+
+ // Insert a load from the global, and use it instead of the malloc.
+ Value *NL = new LoadInst(GV, GV->getName()+".val", U);
+ U->replaceUsesOfWith(Alloc, NL);
+ }
}
+/// GlobalLoadUsesSimpleEnoughForHeapSRA - If all users of values loaded from
+/// GV are simple enough to perform HeapSRA, return true.
+static bool GlobalLoadUsesSimpleEnoughForHeapSRA(GlobalVariable *GV) {
+ for (Value::use_iterator UI = GV->use_begin(), E = GV->use_end(); UI != E;
+ ++UI)
+ if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
+ // We permit two users of the load: setcc comparing against the null
+ // pointer, and a getelementptr of a specific form.
+ for (Value::use_iterator UI = LI->use_begin(), E = LI->use_end(); UI !=
E;
+ ++UI) {
+ // Comparison against null is ok.
+ if (SetCondInst *SCI = dyn_cast<SetCondInst>(*UI)) {
+ if (!isa<ConstantPointerNull>(SCI->getOperand(1)))
+ return false;
+ continue;
+ }
+
+ // getelementptr is also ok, but only a simple form.
+ GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*UI);
+ if (!GEPI) return false;
+
+ // Must index into the array and into the struct.
+ if (GEPI->getNumOperands() < 3)
+ return false;
+
+ // Otherwise the GEP is ok.
+ continue;
+ }
+ }
+ return true;
+}
+
+/// RewriteUsesOfLoadForHeapSRoA - We are performing Heap SRoA on a global.
Ptr
+/// is a value loaded from the global. Eliminate all uses of Ptr, making them
+/// use FieldGlobals instead. All uses of loaded values satisfy
+/// GlobalLoadUsesSimpleEnoughForHeapSRA.
+static void RewriteUsesOfLoadForHeapSRoA(LoadInst *Ptr,
+ const std::vector<GlobalVariable*> &FieldGlobals)
{
+ std::vector<Value *> InsertedLoadsForPtr;
+ //InsertedLoadsForPtr.resize(FieldGlobals.size());
+ while (!Ptr->use_empty()) {
+ Instruction *User = Ptr->use_back();
+
+ // If this is a comparison against null, handle it.
+ if (SetCondInst *SCI = dyn_cast<SetCondInst>(User)) {
+ assert(isa<ConstantPointerNull>(SCI->getOperand(1)));
+ // If we have a setcc of the loaded pointer, we can use a setcc of any
+ // field.
+ Value *NPtr;
+ if (InsertedLoadsForPtr.empty()) {
+ NPtr = new LoadInst(FieldGlobals[0], Ptr->getName()+".f0", Ptr);
+ InsertedLoadsForPtr.push_back(Ptr);
+ } else {
+ NPtr = InsertedLoadsForPtr.back();
+ }
+
+ Value *New = new SetCondInst(SCI->getOpcode(), NPtr,
+ Constant::getNullValue(NPtr->getType()),
+ SCI->getName(), SCI);
+ SCI->replaceAllUsesWith(New);
+ SCI->eraseFromParent();
+ continue;
+ }
+
+ // Otherwise, this should be: 'getelementptr Ptr, Idx, uint FieldNo ...'
+ GetElementPtrInst *GEPI = cast<GetElementPtrInst>(User);
+ assert(GEPI->getNumOperands() >= 3 &&
isa<ConstantUInt>(GEPI->getOperand(2))
+ && "Unexpected GEPI!");
+
+ // Load the pointer for this field.
+ unsigned FieldNo = cast<ConstantUInt>(GEPI->getOperand(2))->getValue();
+ if (InsertedLoadsForPtr.size() <= FieldNo)
+ InsertedLoadsForPtr.resize(FieldNo+1);
+ if (InsertedLoadsForPtr[FieldNo] == 0)
+ InsertedLoadsForPtr[FieldNo] = new LoadInst(FieldGlobals[FieldNo],
+ Ptr->getName()+".f" +
+ utostr(FieldNo), Ptr);
+ Value *NewPtr = InsertedLoadsForPtr[FieldNo];
+
+ // Create the new GEP idx vector.
+ std::vector<Value*> GEPIdx;
+ GEPIdx.push_back(GEPI->getOperand(1));
+ GEPIdx.insert(GEPIdx.end(), GEPI->op_begin()+3, GEPI->op_end());
+
+ Value *NGEPI = new GetElementPtrInst(NewPtr, GEPIdx, GEPI->getName(),
GEPI);
+ GEPI->replaceAllUsesWith(NGEPI);
+ GEPI->eraseFromParent();
+ }
+}
+
+/// PerformHeapAllocSRoA - MI is an allocation of an array of structures.
Break
+/// it up into multiple allocations of arrays of the fields.
+static GlobalVariable *PerformHeapAllocSRoA(GlobalVariable *GV, MallocInst
*MI){
+ /*DEBUG*/(std::cerr << "SROA HEAP ALLOC: " << *GV << " MALLOC = " << *MI);
+ const StructType *STy = cast<StructType>(MI->getAllocatedType());
+
+ // There is guaranteed to be at least one use of the malloc (storing
+ // it into GV). If there are other uses, change them to be uses of
+ // the global to simplify later code. This also deletes the store
+ // into GV.
+ ReplaceUsesOfMallocWithGlobal(MI, GV);
+
+ // Okay, at this point, there are no users of the malloc. Insert N
+ // new mallocs at the same place as MI, and N globals.
+ std::vector<GlobalVariable*> FieldGlobals;
+ std::vector<MallocInst*> FieldMallocs;
+
+ for (unsigned FieldNo = 0, e = STy->getNumElements(); FieldNo !=
e;++FieldNo){
+ const Type *FieldTy = STy->getElementType(FieldNo);
+ const Type *PFieldTy = PointerType::get(FieldTy);
+
+ GlobalVariable *NGV =
+ new GlobalVariable(PFieldTy, false, GlobalValue::InternalLinkage,
+ Constant::getNullValue(PFieldTy),
+ GV->getName() + ".f" + utostr(FieldNo), GV);
+ FieldGlobals.push_back(NGV);
+
+ MallocInst *NMI = new MallocInst(FieldTy, MI->getArraySize(),
+ MI->getName() + ".f" +
utostr(FieldNo),MI);
+ FieldMallocs.push_back(NMI);
+ new StoreInst(NMI, NGV, MI);
+ }
+
+ // The tricky aspect of this transformation is handling the case when malloc
+ // fails. In the original code, malloc failing would set the result pointer
+ // of malloc to null. In this case, some mallocs could succeed and others
+ // could fail. As such, we emit code that looks like this:
+ // F0 = malloc(field0)
+ // F1 = malloc(field1)
+ // F2 = malloc(field2)
+ // if (F0 == 0 || F1 == 0 || F2 == 0) {
+ // if (F0) { free(F0); F0 = 0; }
+ // if (F1) { free(F1); F1 = 0; }
+ // if (F2) { free(F2); F2 = 0; }
+ // }
+ Value *RunningOr = 0;
+ for (unsigned i = 0, e = FieldMallocs.size(); i != e; ++i) {
+ Value *Cond = new SetCondInst(Instruction::SetEQ, FieldMallocs[i],
+
Constant::getNullValue(FieldMallocs[i]->getType()),
+ "isnull", MI);
+ if (!RunningOr)
+ RunningOr = Cond; // First seteq
+ else
+ RunningOr = BinaryOperator::createOr(RunningOr, Cond, "tmp", MI);
+ }
+
+ // Split the basic block at the old malloc.
+ BasicBlock *OrigBB = MI->getParent();
+ BasicBlock *ContBB = OrigBB->splitBasicBlock(MI, "malloc_cont");
+
+ // Create the block to check the first condition. Put all these blocks at
the
+ // end of the function as they are unlikely to be executed.
+ BasicBlock *NullPtrBlock = new BasicBlock("malloc_ret_null",
+ OrigBB->getParent());
+
+ // Remove the uncond branch from OrigBB to ContBB, turning it into a cond
+ // branch on RunningOr.
+ OrigBB->getTerminator()->eraseFromParent();
+ new BranchInst(NullPtrBlock, ContBB, RunningOr, OrigBB);
+
+ // Within the NullPtrBlock, we need to emit a comparison and branch for each
+ // pointer, because some may be null while others are not.
+ for (unsigned i = 0, e = FieldGlobals.size(); i != e; ++i) {
+ Value *GVVal = new LoadInst(FieldGlobals[i], "tmp", NullPtrBlock);
+ Value *Cmp = new SetCondInst(Instruction::SetNE, GVVal,
+ Constant::getNullValue(GVVal->getType()),
+ "tmp", NullPtrBlock);
+ BasicBlock *FreeBlock = new BasicBlock("free_it", OrigBB->getParent());
+ BasicBlock *NextBlock = new BasicBlock("next", OrigBB->getParent());
+ new BranchInst(FreeBlock, NextBlock, Cmp, NullPtrBlock);
+
+ // Fill in FreeBlock.
+ new FreeInst(GVVal, FreeBlock);
+ new StoreInst(Constant::getNullValue(GVVal->getType()), FieldGlobals[i],
+ FreeBlock);
+ new BranchInst(NextBlock, FreeBlock);
+
+ NullPtrBlock = NextBlock;
+ }
+
+ new BranchInst(ContBB, NullPtrBlock);
+
+
+ // MI is no longer needed, remove it.
+ MI->eraseFromParent();
+
+
+ // Okay, the malloc site is completely handled. All of the uses of GV are
now
+ // loads, and all uses of those loads are simple. Rewrite them to use loads
+ // of the per-field globals instead.
+ while (!GV->use_empty()) {
+ LoadInst *LI = cast<LoadInst>(GV->use_back());
+ RewriteUsesOfLoadForHeapSRoA(LI, FieldGlobals);
+ LI->eraseFromParent();
+ }
+
+ // The old global is now dead, remove it.
+ GV->eraseFromParent();
+
+ ++NumHeapSRA;
+ return FieldGlobals[0];
+}
+
+
// 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,
@@ -835,23 +1062,52 @@
if (!MI->getAllocatedType()->isSized())
return false;
+ // We can't optimize this global unless all uses of it are *known* to be
+ // of the malloc value, not of the null initializer value (consider a use
+ // that compares the global's value against zero to see if the malloc has
+ // been reached). To do this, we check to see if all uses of the global
+ // would trap if the global were null: this proves that they must all
+ // happen after the malloc.
+ if (!AllUsesOfLoadedValueWillTrapIfNull(GV))
+ return false;
+
+ // We can't optimize this if the malloc itself is used in a complex way,
+ // for example, being stored into multiple globals. This allows the
+ // malloc to be stored into the specified global, loaded setcc'd, and
+ // GEP'd. These are all things we could transform to using the global
+ // for.
+ if (!ValueIsOnlyUsedLocallyOrStoredToOneGlobal(MI, GV))
+ return false;
+
+
// If we have a global that is only initialized with a fixed size malloc,
- // and if all users of the malloc trap, and if the malloc'd address is
not
- // put anywhere else, transform the program to use global memory instead
- // of malloc'd memory. This eliminates dynamic allocation (good) and
- // exposes the resultant global to further GlobalOpt (even better). Note
- // that we restrict this transformation to only working on small
- // allocations (2048 bytes currently), as we don't want to introduce a
16M
- // global or something.
+ // transform the program to use global memory instead of malloc'd memory.
+ // This eliminates dynamic allocation, avoids an indirection accessing
the
+ // data, and exposes the resultant global to further GlobalOpt.
if (ConstantInt *NElements = dyn_cast<ConstantInt>(MI->getArraySize())) {
+ // Restrict this transformation to only working on small allocations
+ // (2048 bytes currently), as we don't want to introduce a 16M global
or
+ // something.
if (NElements->getRawValue()*
- TD.getTypeSize(MI->getAllocatedType()) < 2048 &&
- AllUsesOfLoadedValueWillTrapIfNull(GV) &&
- ValueIsOnlyUsedLocallyOrStoredToOneGlobal(MI, GV)) {
+ TD.getTypeSize(MI->getAllocatedType()) < 2048) {
GVI = OptimizeGlobalAddressOfMalloc(GV, MI);
return true;
}
}
+
+ // If the allocation is an array of structures, consider transforming
this
+ // into multiple malloc'd arrays, one for each field. This is basically
+ // SRoA for malloc'd memory.
+ if (const StructType *AllocTy =
+ dyn_cast<StructType>(MI->getAllocatedType())) {
+ // This the structure has an unreasonable number of fields, leave it
+ // alone.
+ if (AllocTy->getNumElements() <= 16 && AllocTy->getNumElements() > 0 &&
+ GlobalLoadUsesSimpleEnoughForHeapSRA(GV)) {
+ GVI = PerformHeapAllocSRoA(GV, MI);
+ return true;
+ }
+ }
}
}
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