[RISCV][GISEL] Do not initialize GlobalISel objects unless needed (#98233)

Prior to this commit, we created the GlobalISel objects in the
RISCVSubtarget constructor, even if we are not running GlobalISel. This
patch moves creation of the GlobalISel objects into their getters, which
ensures that we only create these objects if they are actually needed.
This helps since some of the constructors of the GlobalISel objects have
a significant amount of code.

We make the `unique_ptr`s `mutable` since GlobalISel passes only have
access to `const TargetSubtargetInfo` through `MF.getSubtarget()`.

This patch is tested by the fact that all existing RISC-V GlobalISel
tests remain passing.

Author: michaelmaitland

llvm/lib/Target/RISCV/RISCVSubtarget.cpp

@@ -97,29 +97,32 @@ RISCVSubtarget::RISCVSubtarget(const Triple &TT, StringRef CPU,
       RVVVectorBitsMin(RVVVectorBitsMin), RVVVectorBitsMax(RVVVectorBitsMax),
       FrameLowering(
           initializeSubtargetDependencies(TT, CPU, TuneCPU, FS, ABIName)),
-      InstrInfo(*this), RegInfo(getHwMode()), TLInfo(TM, *this) {
-  CallLoweringInfo.reset(new RISCVCallLowering(*getTargetLowering()));
-  Legalizer.reset(new RISCVLegalizerInfo(*this));
-
-  auto *RBI = new RISCVRegisterBankInfo(getHwMode());
-  RegBankInfo.reset(RBI);
-  InstSelector.reset(createRISCVInstructionSelector(
-      *static_cast<const RISCVTargetMachine *>(&TM), *this, *RBI));
-}
+      InstrInfo(*this), RegInfo(getHwMode()), TLInfo(TM, *this) {}
 
 const CallLowering *RISCVSubtarget::getCallLowering() const {
+  if (!CallLoweringInfo)
+    CallLoweringInfo.reset(new RISCVCallLowering(*getTargetLowering()));
   return CallLoweringInfo.get();
 }
 
 InstructionSelector *RISCVSubtarget::getInstructionSelector() const {
+  if (!InstSelector) {
+    InstSelector.reset(createRISCVInstructionSelector(
+        *static_cast<const RISCVTargetMachine *>(&TLInfo.getTargetMachine()),
+        *this, *static_cast<const RISCVRegisterBankInfo *>(getRegBankInfo())));
+  }
   return InstSelector.get();
 }
 
 const LegalizerInfo *RISCVSubtarget::getLegalizerInfo() const {
+  if (!Legalizer)
+    Legalizer.reset(new RISCVLegalizerInfo(*this));
   return Legalizer.get();
 }
 
 const RegisterBankInfo *RISCVSubtarget::getRegBankInfo() const {
+  if (!RegBankInfo)
+    RegBankInfo.reset(new RISCVRegisterBankInfo(getHwMode()));
   return RegBankInfo.get();
 }
 

llvm/lib/Target/RISCV/RISCVSubtarget.h

@@ -245,10 +245,10 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
 
 protected:
   // GlobalISel related APIs.
-  std::unique_ptr<CallLowering> CallLoweringInfo;
-  std::unique_ptr<InstructionSelector> InstSelector;
-  std::unique_ptr<LegalizerInfo> Legalizer;
-  std::unique_ptr<RegisterBankInfo> RegBankInfo;
+  mutable std::unique_ptr<CallLowering> CallLoweringInfo;
+  mutable std::unique_ptr<InstructionSelector> InstSelector;
+  mutable std::unique_ptr<LegalizerInfo> Legalizer;
+  mutable std::unique_ptr<RegisterBankInfo> RegBankInfo;
 
   // Return the known range for the bit length of RVV data registers as set
   // at the command line. A value of 0 means nothing is known about that particular