[RISCV] Generate MIPS load/store pair instructions (#124717)

Introduce RISCVLoadStoreOptimizer MIR Pass that will do the
optimization. The load/store pairing pass identifies adjacent load/store
instructions operating on consecutive memory locations and merges them
into a single paired instruction.

This is part of MIPS extensions for the p8700 CPU.

Production of ldp/sdp instructions is OFF by default, since it is
beneficial for -Os only in the case of p8700 CPU.

Author: djtodoro

llvm/lib/Target/RISCV/CMakeLists.txt

@@ -48,6 +48,7 @@ add_llvm_target(RISCVCodeGen
   RISCVISelDAGToDAG.cpp
   RISCVISelLowering.cpp
   RISCVLandingPadSetup.cpp
+  RISCVLoadStoreOptimizer.cpp
   RISCVMachineFunctionInfo.cpp
   RISCVMergeBaseOffset.cpp
   RISCVOptWInstrs.cpp

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -2767,6 +2767,42 @@ MachineInstr *RISCVInstrInfo::emitLdStWithAddr(MachineInstr &MemI,
       .setMIFlags(MemI.getFlags());
 }
 
+// TODO: At the moment, MIPS introduced paring of instructions operating with
+// word or double word. This should be extended with more instructions when more
+// vendors support load/store pairing.
+bool RISCVInstrInfo::isPairableLdStInstOpc(unsigned Opc) {
+  switch (Opc) {
+  default:
+    return false;
+  case RISCV::SW:
+  case RISCV::SD:
+  case RISCV::LD:
+  case RISCV::LW:
+    return true;
+  }
+}
+
+bool RISCVInstrInfo::isLdStSafeToPair(const MachineInstr &LdSt,
+                                      const TargetRegisterInfo *TRI) {
+  // If this is a volatile load/store, don't mess with it.
+  if (LdSt.hasOrderedMemoryRef() || LdSt.getNumExplicitOperands() != 3)
+    return false;
+
+  if (LdSt.getOperand(1).isFI())
+    return true;
+
+  assert(LdSt.getOperand(1).isReg() && "Expected a reg operand.");
+  // Can't cluster if the instruction modifies the base register
+  // or it is update form. e.g. ld x5,8(x5)
+  if (LdSt.modifiesRegister(LdSt.getOperand(1).getReg(), TRI))
+    return false;
+
+  if (!LdSt.getOperand(2).isImm())
+    return false;
+
+  return true;
+}
+
 bool RISCVInstrInfo::getMemOperandsWithOffsetWidth(
     const MachineInstr &LdSt, SmallVectorImpl<const MachineOperand *> &BaseOps,
     int64_t &Offset, bool &OffsetIsScalable, LocationSize &Width,

llvm/lib/Target/RISCV/RISCVInstrInfo.h

@@ -301,6 +301,12 @@ class RISCVInstrInfo : public RISCVGenInstrInfo {
 
   bool isHighLatencyDef(int Opc) const override;
 
+  /// Return true if pairing the given load or store may be paired with another.
+  static bool isPairableLdStInstOpc(unsigned Opc);
+
+  static bool isLdStSafeToPair(const MachineInstr &LdSt,
+                               const TargetRegisterInfo *TRI);
+
 protected:
   const RISCVSubtarget &STI;