[Backend] Add clearSubtargetMap API for TargetMachine. (#112383)

- [x] Add `clearSubtargetInfo` API to TargetMachine and each backend to
make it possible to release memory used in each backend's
`SubtargetInfo` map if needed. Keep this API as `protected` so that it
will be used with precautions.

Author: weiweichen

llvm/include/llvm/Target/TargetMachine.h

@@ -448,6 +448,9 @@ class LLVMTargetMachine : public TargetMachine {
 
   void initAsmInfo();
 
+  /// Reset internal state.
+  virtual void reset() {};
+
 public:
   /// Get a TargetTransformInfo implementation for the target.
   ///

llvm/lib/Target/AArch64/AArch64TargetMachine.cpp

@@ -272,6 +272,8 @@ extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeAArch64Target() {
   initializeAArch64GlobalsTaggingPass(*PR);
 }
 
+void AArch64TargetMachine::reset() { SubtargetMap.clear(); }
+
 //===----------------------------------------------------------------------===//
 // AArch64 Lowering public interface.
 //===----------------------------------------------------------------------===//

llvm/lib/Target/AArch64/AArch64TargetMachine.h

@@ -26,6 +26,9 @@ class AArch64TargetMachine : public LLVMTargetMachine {
   std::unique_ptr<TargetLoweringObjectFile> TLOF;
   mutable StringMap<std::unique_ptr<AArch64Subtarget>> SubtargetMap;
 
+  /// Reset internal state.
+  void reset() override;
+
 public:
   AArch64TargetMachine(const Target &T, const Triple &TT, StringRef CPU,
                        StringRef FS, const TargetOptions &Options,

llvm/lib/Target/ARM/ARMTargetMachine.cpp

@@ -642,3 +642,5 @@ bool ARMBaseTargetMachine::parseMachineFunctionInfo(
   MF.getInfo<ARMFunctionInfo>()->initializeBaseYamlFields(YamlMFI);
   return false;
 }
+
+void ARMBaseTargetMachine::reset() { SubtargetMap.clear(); }

llvm/lib/Target/ARM/ARMTargetMachine.h

@@ -38,6 +38,9 @@ class ARMBaseTargetMachine : public LLVMTargetMachine {
   bool isLittle;
   mutable StringMap<std::unique_ptr<ARMSubtarget>> SubtargetMap;
 
+  /// Reset internal state.
+  void reset() override;
+
 public:
   ARMBaseTargetMachine(const Target &T, const Triple &TT, StringRef CPU,
                        StringRef FS, const TargetOptions &Options,

llvm/lib/Target/X86/X86TargetMachine.cpp

@@ -376,6 +376,8 @@ bool X86TargetMachine::isNoopAddrSpaceCast(unsigned SrcAS,
   return SrcAS < 256 && DestAS < 256;
 }
 
+void X86TargetMachine::reset() { SubtargetMap.clear(); }
+
 //===----------------------------------------------------------------------===//
 // X86 TTI query.
 //===----------------------------------------------------------------------===//

llvm/lib/Target/X86/X86TargetMachine.h

@@ -31,6 +31,9 @@ class X86TargetMachine final : public LLVMTargetMachine {
   // True if this is used in JIT.
   bool IsJIT;
 
+  /// Reset internal state.
+  void reset() override;
+
 public:
   X86TargetMachine(const Target &T, const Triple &TT, StringRef CPU,
                    StringRef FS, const TargetOptions &Options,