[RISCV] Remove incomplete PRE_DEC/POST_DEC code for XTHeadMemIdx. (#76922)

As far as I can tell if getIndexedAddressParts received an ISD::SUB, the
constant would be negated. So `IsInc` should be set to true since the
SUB was effectively converted to ADD. This means we should never use
PRE_DEC/POST_DEC.

No tests are affected because DAGCombine aggressively turns SUB with
constant into ADD so no lit test has a SUB reach getIndexedAddressParts.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -763,14 +763,12 @@ bool RISCVDAGToDAGISel::tryIndexedLoad(SDNode *Node) {
     return false;
 
   EVT LoadVT = Ld->getMemoryVT();
-  bool IsPre = (AM == ISD::PRE_INC || AM == ISD::PRE_DEC);
-  bool IsPost = (AM == ISD::POST_INC || AM == ISD::POST_DEC);
+  assert(AM == ISD::PRE_INC ||
+         AM == ISD::POST_INC && "Unexpected addressing mode");
+  bool IsPre = AM == ISD::PRE_INC;
+  bool IsPost = AM == ISD::POST_INC;
   int64_t Offset = C->getSExtValue();
 
-  // Convert decrements to increments by a negative quantity.
-  if (AM == ISD::PRE_DEC || AM == ISD::POST_DEC)
-    Offset = -Offset;
-
   // The constants that can be encoded in the THeadMemIdx instructions
   // are of the form (sign_extend(imm5) << imm2).
   int64_t Shift;

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -1350,8 +1350,7 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
   }
 
   if (Subtarget.hasVendorXTHeadMemIdx()) {
-    for (unsigned im = (unsigned)ISD::PRE_INC; im != (unsigned)ISD::POST_DEC;
-         ++im) {
+    for (unsigned im : {ISD::PRE_INC, ISD::POST_INC}) {
       setIndexedLoadAction(im, MVT::i8, Legal);
       setIndexedStoreAction(im, MVT::i8, Legal);
       setIndexedLoadAction(im, MVT::i16, Legal);
@@ -19269,7 +19268,6 @@ bool RISCVTargetLowering::isVScaleKnownToBeAPowerOfTwo() const {
 bool RISCVTargetLowering::getIndexedAddressParts(SDNode *Op, SDValue &Base,
                                                  SDValue &Offset,
                                                  ISD::MemIndexedMode &AM,
-                                                 bool &IsInc,
                                                  SelectionDAG &DAG) const {
   // Target does not support indexed loads.
   if (!Subtarget.hasVendorXTHeadMemIdx())
@@ -19296,7 +19294,6 @@ bool RISCVTargetLowering::getIndexedAddressParts(SDNode *Op, SDValue &Base,
     if (!isLegalIndexedOffset)
       return false;
 
-    IsInc = (Op->getOpcode() == ISD::ADD);
     Offset = Op->getOperand(1);
     return true;
   }
@@ -19319,11 +19316,10 @@ bool RISCVTargetLowering::getPreIndexedAddressParts(SDNode *N, SDValue &Base,
   } else
     return false;
 
-  bool IsInc;
-  if (!getIndexedAddressParts(Ptr.getNode(), Base, Offset, AM, IsInc, DAG))
+  if (!getIndexedAddressParts(Ptr.getNode(), Base, Offset, AM, DAG))
     return false;
 
-  AM = IsInc ? ISD::PRE_INC : ISD::PRE_DEC;
+  AM = ISD::PRE_INC;
   return true;
 }
 
@@ -19343,15 +19339,14 @@ bool RISCVTargetLowering::getPostIndexedAddressParts(SDNode *N, SDNode *Op,
   } else
     return false;
 
-  bool IsInc;
-  if (!getIndexedAddressParts(Op, Base, Offset, AM, IsInc, DAG))
+  if (!getIndexedAddressParts(Op, Base, Offset, AM, DAG))
     return false;
   // Post-indexing updates the base, so it's not a valid transform
   // if that's not the same as the load's pointer.
   if (Ptr != Base)
     return false;
 
-  AM = IsInc ? ISD::POST_INC : ISD::POST_DEC;
+  AM = ISD::POST_INC;
   return true;
 }
 

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -774,8 +774,7 @@ class RISCVTargetLowering : public TargetLowering {
   bool isVScaleKnownToBeAPowerOfTwo() const override;
 
   bool getIndexedAddressParts(SDNode *Op, SDValue &Base, SDValue &Offset,
-                              ISD::MemIndexedMode &AM, bool &IsInc,
-                              SelectionDAG &DAG) const;
+                              ISD::MemIndexedMode &AM, SelectionDAG &DAG) const;
   bool getPreIndexedAddressParts(SDNode *N, SDValue &Base, SDValue &Offset,
                                  ISD::MemIndexedMode &AM,
                                  SelectionDAG &DAG) const override;