Improve isSafeToLoadUnconditionally to recognize that GEPs with constant
indices are safe if the result is known to be within the bounds of the
underlying object.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94829 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index ae728dd..306ed67 100644
--- a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -199,8 +199,8 @@
//
if (SelectInst *SI = dyn_cast<SelectInst>(Op)) {
// load (select (Cond, &V1, &V2)) --> select(Cond, load &V1, load &V2).
- if (isSafeToLoadUnconditionally(SI->getOperand(1), SI) &&
- isSafeToLoadUnconditionally(SI->getOperand(2), SI)) {
+ if (isSafeToLoadUnconditionally(SI->getOperand(1), SI, TD) &&
+ isSafeToLoadUnconditionally(SI->getOperand(2), SI, TD)) {
Value *V1 = Builder->CreateLoad(SI->getOperand(1),
SI->getOperand(1)->getName()+".val");
Value *V2 = Builder->CreateLoad(SI->getOperand(2),
diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index 292a4b3..9c18452 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -1650,7 +1650,8 @@
// put anywhere; this can be improved, but should be conservatively safe.
if (!allSingleSucc &&
// FIXME: REEVALUTE THIS.
- !isSafeToLoadUnconditionally(LoadPtr, UnavailablePred->getTerminator())) {
+ !isSafeToLoadUnconditionally(LoadPtr,
+ UnavailablePred->getTerminator(), TD)) {
assert(NewInsts.empty() && "Should not have inserted instructions");
return false;
}
diff --git a/lib/Transforms/Utils/Local.cpp b/lib/Transforms/Utils/Local.cpp
index 92bdf2d..f0097d0 100644
--- a/lib/Transforms/Utils/Local.cpp
+++ b/lib/Transforms/Utils/Local.cpp
@@ -38,20 +38,73 @@
// Local analysis.
//
+/// getUnderlyingObjectWithOffset - Strip off up to MaxLookup GEPs and
+/// bitcasts to get back to the underlying object being addressed, keeping
+/// track of the offset in bytes from the GEPs relative to the result.
+/// This is closely related to Value::getUnderlyingObject but is located
+/// here to avoid making VMCore depend on TargetData.
+static Value *getUnderlyingObjectWithOffset(Value *V, const TargetData *TD,
+ unsigned &ByteOffset,
+ unsigned MaxLookup = 6) {
+ if (!isa<PointerType>(V->getType()))
+ return V;
+ for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) {
+ if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
+ if (!GEP->hasAllConstantIndices())
+ return V;
+ SmallVector<Value*, 8> Indices(GEP->op_begin() + 1, GEP->op_end());
+ ByteOffset += TD->getIndexedOffset(GEP->getPointerOperandType(),
+ &Indices[0], Indices.size());
+ V = GEP->getPointerOperand();
+ } else if (Operator::getOpcode(V) == Instruction::BitCast) {
+ V = cast<Operator>(V)->getOperand(0);
+ } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
+ if (GA->mayBeOverridden())
+ return V;
+ V = GA->getAliasee();
+ } else {
+ return V;
+ }
+ assert(isa<PointerType>(V->getType()) && "Unexpected operand type!");
+ }
+ return V;
+}
+
/// isSafeToLoadUnconditionally - Return true if we know that executing a load
/// from this value cannot trap. If it is not obviously safe to load from the
/// specified pointer, we do a quick local scan of the basic block containing
/// ScanFrom, to determine if the address is already accessed.
-bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) {
- // If it is an alloca it is always safe to load from.
- if (isa<AllocaInst>(V)) return true;
+bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
+ const TargetData *TD) {
+ unsigned ByteOffset = 0;
+ Value *Base = V;
+ if (TD)
+ Base = getUnderlyingObjectWithOffset(V, TD, ByteOffset);
- // If it is a global variable it is mostly safe to load from.
- if (const GlobalValue *GV = dyn_cast<GlobalVariable>(V))
- // Don't try to evaluate aliases. External weak GV can be null.
- return !isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage();
+ const Type *BaseType = 0;
+ if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base))
+ // If it is an alloca it is always safe to load from.
+ BaseType = AI->getAllocatedType();
+ else if (const GlobalValue *GV = dyn_cast<GlobalValue>(Base)) {
+ // Global variables are safe to load from but their size cannot be
+ // guaranteed if they are overridden.
+ if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden())
+ BaseType = GV->getType()->getElementType();
+ }
- // Otherwise, be a little bit agressive by scanning the local block where we
+ if (BaseType) {
+ if (!TD)
+ return true; // Loading directly from an alloca or global is OK.
+ if (BaseType->isSized()) {
+ // Check if the load is within the bounds of the underlying object.
+ const PointerType *AddrTy = cast<PointerType>(V->getType());
+ unsigned LoadSize = TD->getTypeStoreSize(AddrTy->getElementType());
+ if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType))
+ return true;
+ }
+ }
+
+ // Otherwise, be a little bit aggressive by scanning the local block where we
// want to check to see if the pointer is already being loaded or stored
// from/to. If so, the previous load or store would have already trapped,
// so there is no harm doing an extra load (also, CSE will later eliminate