[SDAG] Introduce inbounds flag for pointer arithmetic

This patch introduces an inbounds SDNodeFlag, to show that a pointer
addition SDNode implements an inbounds getelementptr operation (i.e.,
the pointer operand is in bounds wrt. the allocated object it is based
on, and the arithmetic does not change that). The flag is set in the DAG
construction when lowering inbounds GEPs.

Inbounds information is useful in the ISel when selecting memory
instructions that perform address computations whose intermediate steps
must be in the same memory region as the final result. A follow-up patch
will start using it for AMDGPU's flat memory instructions, where the
immediate offset must not affect the memory aperture of the address.

A similar patch for gMIR and GlobalISel will follow.

For SWDEV-516125.

Author: ritter-x2a

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

@@ -415,12 +415,15 @@ struct SDNodeFlags {
     Unpredictable = 1 << 13,
     // Compare instructions which may carry the samesign flag.
     SameSign = 1 << 14,
+    // Pointer arithmetic instructions that remain in bounds, e.g., implementing
+    // an inbounds GEP.
+    InBounds = 1 << 15,
 
     // NOTE: Please update LargestValue in LLVM_DECLARE_ENUM_AS_BITMASK below
     // the class definition when adding new flags.
 
     PoisonGeneratingFlags = NoUnsignedWrap | NoSignedWrap | Exact | Disjoint |
-                            NonNeg | NoNaNs | NoInfs | SameSign,
+                            NonNeg | NoNaNs | NoInfs | SameSign | InBounds,
     FastMathFlags = NoNaNs | NoInfs | NoSignedZeros | AllowReciprocal |
                     AllowContract | ApproximateFuncs | AllowReassociation,
   };
@@ -455,6 +458,7 @@ struct SDNodeFlags {
   void setAllowReassociation(bool b) { setFlag<AllowReassociation>(b); }
   void setNoFPExcept(bool b) { setFlag<NoFPExcept>(b); }
   void setUnpredictable(bool b) { setFlag<Unpredictable>(b); }
+  void setInBounds(bool b) { setFlag<InBounds>(b); }
 
   // These are accessors for each flag.
   bool hasNoUnsignedWrap() const { return Flags & NoUnsignedWrap; }
@@ -472,6 +476,7 @@ struct SDNodeFlags {
   bool hasAllowReassociation() const { return Flags & AllowReassociation; }
   bool hasNoFPExcept() const { return Flags & NoFPExcept; }
   bool hasUnpredictable() const { return Flags & Unpredictable; }
+  bool hasInBounds() const { return Flags & InBounds; }
 
   bool operator==(const SDNodeFlags &Other) const {
     return Flags == Other.Flags;
@@ -481,7 +486,7 @@ struct SDNodeFlags {
 };
 
 LLVM_DECLARE_ENUM_AS_BITMASK(decltype(SDNodeFlags::None),
-                             SDNodeFlags::SameSign);
+                             SDNodeFlags::InBounds);
 
 inline SDNodeFlags operator|(SDNodeFlags LHS, SDNodeFlags RHS) {
   LHS |= RHS;

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -4293,6 +4293,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
         if (NW.hasNoUnsignedWrap() ||
             (int64_t(Offset) >= 0 && NW.hasNoUnsignedSignedWrap()))
           Flags |= SDNodeFlags::NoUnsignedWrap;
+        Flags.setInBounds(NW.isInBounds());
 
         N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N,
                         DAG.getConstant(Offset, dl, N.getValueType()), Flags);
@@ -4336,6 +4337,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
         if (NW.hasNoUnsignedWrap() ||
             (Offs.isNonNegative() && NW.hasNoUnsignedSignedWrap()))
           Flags.setNoUnsignedWrap(true);
+        Flags.setInBounds(NW.isInBounds());
 
         OffsVal = DAG.getSExtOrTrunc(OffsVal, dl, N.getValueType());
 
@@ -4398,6 +4400,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
       // pointer index type (add nuw).
       SDNodeFlags AddFlags;
       AddFlags.setNoUnsignedWrap(NW.hasNoUnsignedWrap());
+      AddFlags.setInBounds(NW.isInBounds());
 
       N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N, IdxN, AddFlags);
     }

llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp

@@ -663,6 +663,9 @@ void SDNode::print_details(raw_ostream &OS, const SelectionDAG *G) const {
   if (getFlags().hasSameSign())
     OS << " samesign";
 
+  if (getFlags().hasInBounds())
+    OS << " inbounds";
+
   if (getFlags().hasNonNeg())
     OS << " nneg";
 

llvm/test/CodeGen/X86/merge-store-partially-alias-loads.ll

@@ -16,7 +16,7 @@
 ; DBGDAG-LABEL: Optimized legalized selection DAG: %bb.0 'merge_store_partial_overlap_load:'
 ; DBGDAG: [[ENTRYTOKEN:t[0-9]+]]: ch,glue = EntryToken
 ; DBGDAG-DAG: [[BASEPTR:t[0-9]+]]: i64,ch = CopyFromReg [[ENTRYTOKEN]],
-; DBGDAG-DAG: [[ADDPTR:t[0-9]+]]: i64 = add {{(nuw )?}}[[BASEPTR]], Constant:i64<2>
+; DBGDAG-DAG: [[ADDPTR:t[0-9]+]]: i64 = add {{(nuw )?}}{{(inbounds )?}}[[BASEPTR]], Constant:i64<2>
 
 ; DBGDAG-DAG: [[LD2:t[0-9]+]]: i16,ch = load<(load (s16) from %ir.tmp81, align 1)> [[ENTRYTOKEN]], [[BASEPTR]], undef:i64
 ; DBGDAG-DAG: [[LD1:t[0-9]+]]: i8,ch = load<(load (s8) from %ir.tmp12)> [[ENTRYTOKEN]], [[ADDPTR]], undef:i64