[SelectionDAGBuilder] Set NoUnsignedWrap for inbounds gep and load/store offsets.

In an inbounds getelementptr, when an index produces a constant non-negative
offset to add to the base, the add can be assumed to not have unsigned overflow.

This relies on the assumption that addresses can't occupy more than half the
address space, which isn't possible in C because it wouldn't be possible to
represent the difference between the start of the object and one-past-the-end
in a ptrdiff_t.

Setting the NoUnsignedWrap flag is theoretically useful in general, and is
specifically useful to the WebAssembly backend, since it permits stronger
constant offset folding.

Differential Revision: http://reviews.llvm.org/D15544

llvm-svn: 256890

2 files changed, 52 insertions, 9 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 0872d7a9a22..bc2405b952a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6843,9 +6843,13 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
   uint64_t PtrOff = ShAmt / 8;
   unsigned NewAlign = MinAlign(LN0->getAlignment(), PtrOff);
   SDLoc DL(LN0);
+  // The original load itself didn't wrap, so an offset within it doesn't.
+  SDNodeFlags Flags;
+  Flags.setNoUnsignedWrap(true);
   SDValue NewPtr = DAG.getNode(ISD::ADD, DL,
                                PtrType, LN0->getBasePtr(),
-                               DAG.getConstant(PtrOff, DL, PtrType));
+                               DAG.getConstant(PtrOff, DL, PtrType),
+                               &Flags);
   AddToWorklist(NewPtr.getNode());
 
   SDValue Load;
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 94550d60b36..e446a934554 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -1329,12 +1329,18 @@ void SelectionDAGBuilder::visitRet(const ReturnInst &I) {
     ComputeValueVTs(TLI, DL, I.getOperand(0)->getType(), ValueVTs, &Offsets);
     unsigned NumValues = ValueVTs.size();
 
+    // An aggregate return value cannot wrap around the address space, so
+    // offsets to its parts don't wrap either.
+    SDNodeFlags Flags;
+    Flags.setNoUnsignedWrap(true);
+
     SmallVector<SDValue, 4> Chains(NumValues);
     for (unsigned i = 0; i != NumValues; ++i) {
       SDValue Add = DAG.getNode(ISD::ADD, getCurSDLoc(),
                                 RetPtr.getValueType(), RetPtr,
                                 DAG.getIntPtrConstant(Offsets[i],
-                                                      getCurSDLoc()));
+                                                      getCurSDLoc()),
+                                &Flags);
       Chains[i] =
         DAG.getStore(Chain, getCurSDLoc(),
                      SDValue(RetOp.getNode(), RetOp.getResNo() + i),
@@ -2994,8 +3000,15 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
       if (Field) {
         // N = N + Offset
         uint64_t Offset = DL->getStructLayout(StTy)->getElementOffset(Field);
+
+        // In an inbouds GEP with an offset that is nonnegative even when
+        // interpreted as signed, assume there is no unsigned overflow.
+        SDNodeFlags Flags;
+        if (int64_t(Offset) >= 0 && cast<GEPOperator>(I).isInBounds())
+          Flags.setNoUnsignedWrap(true);
+
         N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N,
-                        DAG.getConstant(Offset, dl, N.getValueType()));
+                        DAG.getConstant(Offset, dl, N.getValueType()), &Flags);
       }
 
       Ty = StTy->getElementType(Field);
@@ -3020,7 +3033,14 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
         SDValue OffsVal = VectorWidth ?
           DAG.getConstant(Offs, dl, MVT::getVectorVT(PtrTy, VectorWidth)) :
           DAG.getConstant(Offs, dl, PtrTy);
-        N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N, OffsVal);
+
+        // In an inbouds GEP with an offset that is nonnegative even when
+        // interpreted as signed, assume there is no unsigned overflow.
+        SDNodeFlags Flags;
+        if (Offs.isNonNegative() && cast<GEPOperator>(I).isInBounds())
+          Flags.setNoUnsignedWrap(true);
+
+        N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N, OffsVal, &Flags);
         continue;
       }
 
@@ -3092,10 +3112,13 @@ void SelectionDAGBuilder::visitAlloca(const AllocaInst &I) {
     Align = 0;
 
   // Round the size of the allocation up to the stack alignment size
-  // by add SA-1 to the size.
+  // by add SA-1 to the size. This doesn't overflow because we're computing
+  // an address inside an alloca.
+  SDNodeFlags Flags;
+  Flags.setNoUnsignedWrap(true);
   AllocSize = DAG.getNode(ISD::ADD, dl,
                           AllocSize.getValueType(), AllocSize,
-                          DAG.getIntPtrConstant(StackAlign - 1, dl));
+                          DAG.getIntPtrConstant(StackAlign - 1, dl), &Flags);
 
   // Mask out the low bits for alignment purposes.
   AllocSize = DAG.getNode(ISD::AND, dl,
@@ -3168,6 +3191,11 @@ void SelectionDAGBuilder::visitLoad(const LoadInst &I) {
   if (isVolatile)
     Root = TLI.prepareVolatileOrAtomicLoad(Root, dl, DAG);
 
+  // An aggregate load cannot wrap around the address space, so offsets to its
+  // parts don't wrap either.
+  SDNodeFlags Flags;
+  Flags.setNoUnsignedWrap(true);
+
   SmallVector<SDValue, 4> Values(NumValues);
   SmallVector<SDValue, 4> Chains(std::min(MaxParallelChains, NumValues));
   EVT PtrVT = Ptr.getValueType();
@@ -3188,7 +3216,8 @@ void SelectionDAGBuilder::visitLoad(const LoadInst &I) {
     }
     SDValue A = DAG.getNode(ISD::ADD, dl,
                             PtrVT, Ptr,
-                            DAG.getConstant(Offsets[i], dl, PtrVT));
+                            DAG.getConstant(Offsets[i], dl, PtrVT),
+                            &Flags);
     SDValue L = DAG.getLoad(ValueVTs[i], dl, Root,
                             A, MachinePointerInfo(SV, Offsets[i]), isVolatile,
                             isNonTemporal, isInvariant, Alignment, AAInfo,
@@ -3243,6 +3272,11 @@ void SelectionDAGBuilder::visitStore(const StoreInst &I) {
   AAMDNodes AAInfo;
   I.getAAMetadata(AAInfo);
 
+  // An aggregate load cannot wrap around the address space, so offsets to its
+  // parts don't wrap either.
+  SDNodeFlags Flags;
+  Flags.setNoUnsignedWrap(true);
+
   unsigned ChainI = 0;
   for (unsigned i = 0; i != NumValues; ++i, ++ChainI) {
     // See visitLoad comments.
@@ -3253,7 +3287,7 @@ void SelectionDAGBuilder::visitStore(const StoreInst &I) {
       ChainI = 0;
     }
     SDValue Add = DAG.getNode(ISD::ADD, dl, PtrVT, Ptr,
-                              DAG.getConstant(Offsets[i], dl, PtrVT));
+                              DAG.getConstant(Offsets[i], dl, PtrVT), &Flags);
     SDValue St = DAG.getStore(Root, dl,
                               SDValue(Src.getNode(), Src.getResNo() + i),
                               Add, MachinePointerInfo(PtrV, Offsets[i]),
@@ -7202,10 +7236,15 @@ TargetLowering::LowerCallTo(TargetLowering::CallLoweringInfo &CLI) const {
     ReturnValues.resize(NumValues);
     SmallVector<SDValue, 4> Chains(NumValues);
 
+    // An aggregate return value cannot wrap around the address space, so
+    // offsets to its parts don't wrap either.
+    SDNodeFlags Flags;
+    Flags.setNoUnsignedWrap(true);
+
     for (unsigned i = 0; i < NumValues; ++i) {
       SDValue Add = CLI.DAG.getNode(ISD::ADD, CLI.DL, PtrVT, DemoteStackSlot,
                                     CLI.DAG.getConstant(Offsets[i], CLI.DL,
-                                                        PtrVT));
+                                                        PtrVT), &Flags);
       SDValue L = CLI.DAG.getLoad(
           RetTys[i], CLI.DL, CLI.Chain, Add,
           MachinePointerInfo::getFixedStack(CLI.DAG.getMachineFunction(),