[CodeGen] [ARM] Make RISC-V Init Undef Pass Target Independent and add support for the ARM Architecture. (#77770)

When using Greedy Register Allocation, there are times where
early-clobber values are ignored, and assigned the same register. This
is illeagal behaviour for these intructions. To get around this, using
Pseudo instructions for early-clobber registers gives them a definition
and allows Greedy to assign them to a different register. This then
meets the ARM Architecture Reference Manual and matches the defined
behaviour.

This patch takes the existing RISC-V patch and makes it target
independent, then adds support for the ARM Architecture. Doing this will
ensure early-clobber restraints are followed when using the ARM
Architecture. Making the pass target independent will also open up
possibility that support other architectures can be added in the future.

Author: Stylie777

llvm/include/llvm/CodeGen/Passes.h

@@ -196,6 +196,11 @@ namespace llvm {
   /// This pass reads flow sensitive profile.
   extern char &MIRProfileLoaderPassID;
 
+  // This pass gives undef values a Pseudo Instruction definition for
+  // Instructions to ensure early-clobber is followed when using the greedy
+  // register allocator.
+  extern char &InitUndefID;
+
   /// FastRegisterAllocation Pass - This pass register allocates as fast as
   /// possible. It is best suited for debug code where live ranges are short.
   ///

llvm/include/llvm/CodeGen/TargetInstrInfo.h

@@ -2223,6 +2223,15 @@ class TargetInstrInfo : public MCInstrInfo {
     llvm_unreachable("unknown number of operands necessary");
   }
 
+  /// Gets the opcode for the Pseudo Instruction used to initialize
+  /// the undef value. If no Instruction is available, this will
+  /// fail compilation.
+  virtual unsigned getUndefInitOpcode(unsigned RegClassID) const {
+    (void)RegClassID;
+
+    llvm_unreachable("Unexpected register class.");
+  }
+
 private:
   mutable std::unique_ptr<MIRFormatter> Formatter;
   unsigned CallFrameSetupOpcode, CallFrameDestroyOpcode;

llvm/include/llvm/CodeGen/TargetRegisterInfo.h

@@ -1172,6 +1172,28 @@ class TargetRegisterInfo : public MCRegisterInfo {
   virtual bool isNonallocatableRegisterCalleeSave(MCRegister Reg) const {
     return false;
   }
+
+  /// Returns the Largest Super Class that is being initialized. There
+  /// should be a Pseudo Instruction implemented for the super class
+  /// that is being returned to ensure that Init Undef can apply the
+  /// initialization correctly.
+  virtual const TargetRegisterClass *
+  getLargestSuperClass(const TargetRegisterClass *RC) const {
+    llvm_unreachable("Unexpected target register class.");
+  }
+
+  /// Returns if the architecture being targeted has the required Pseudo
+  /// Instructions for initializing the register. By default this returns false,
+  /// but where it is overriden for an architecture, the behaviour will be
+  /// different. This can either be a check to ensure the Register Class is
+  /// present, or to return true as an indication the architecture supports the
+  /// pass. If using the method that does not check for the Register Class, it
+  /// is imperative to ensure all required Pseudo Instructions are implemented,
+  /// otherwise compilation may fail with an `Unexpected register class` error.
+  virtual bool
+  doesRegClassHavePseudoInitUndef(const TargetRegisterClass *RC) const {
+    return false;
+  }
 };
 
 //===----------------------------------------------------------------------===//

llvm/include/llvm/CodeGen/TargetSubtargetInfo.h

@@ -327,6 +327,12 @@ class TargetSubtargetInfo : public MCSubtargetInfo {
 
   /// Get the list of MacroFusion predicates.
   virtual std::vector<MacroFusionPredTy> getMacroFusions() const { return {}; };
+
+  /// supportsInitUndef is used to determine if an architecture supports
+  /// the Init Undef Pass. By default, it is assumed that it will not support
+  /// the pass, with architecture specific overrides providing the information
+  /// where they are implemented.
+  virtual bool supportsInitUndef() const { return false; }
 };
 
 } // end namespace llvm

llvm/include/llvm/InitializePasses.h

@@ -300,6 +300,7 @@ void initializeTLSVariableHoistLegacyPassPass(PassRegistry &);
 void initializeTwoAddressInstructionPassPass(PassRegistry&);
 void initializeTypeBasedAAWrapperPassPass(PassRegistry&);
 void initializeTypePromotionLegacyPass(PassRegistry&);
+void initializeInitUndefPass(PassRegistry &);
 void initializeUniformityInfoWrapperPassPass(PassRegistry &);
 void initializeUnifyLoopExitsLegacyPassPass(PassRegistry &);
 void initializeUnpackMachineBundlesPass(PassRegistry&);

llvm/include/llvm/Passes/CodeGenPassBuilder.h

@@ -1066,6 +1066,8 @@ void CodeGenPassBuilder<Derived>::addOptimizedRegAlloc(
     AddMachinePass &addPass) const {
   addPass(DetectDeadLanesPass());
 
+  addPass(InitUndefPass());
+
   addPass(ProcessImplicitDefsPass());
 
   // Edge splitting is smarter with machine loop info.

llvm/include/llvm/Passes/MachinePassRegistry.def

@@ -173,6 +173,7 @@ DUMMY_MACHINE_FUNCTION_PASS("fs-profile-loader", MIRProfileLoaderNewPass)
 DUMMY_MACHINE_FUNCTION_PASS("funclet-layout", FuncletLayoutPass)
 DUMMY_MACHINE_FUNCTION_PASS("gc-empty-basic-blocks", GCEmptyBasicBlocksPass)
 DUMMY_MACHINE_FUNCTION_PASS("implicit-null-checks", ImplicitNullChecksPass)
+DUMMY_MACHINE_FUNCTION_PASS("init-undef-pass", InitUndefPass)
 DUMMY_MACHINE_FUNCTION_PASS("instruction-select", InstructionSelectPass)
 DUMMY_MACHINE_FUNCTION_PASS("irtranslator", IRTranslatorPass)
 DUMMY_MACHINE_FUNCTION_PASS("kcfi", MachineKCFIPass)

llvm/lib/CodeGen/CMakeLists.txt

@@ -75,6 +75,7 @@ add_llvm_component_library(LLVMCodeGen
   IfConversion.cpp
   ImplicitNullChecks.cpp
   IndirectBrExpandPass.cpp
+  InitUndef.cpp
   InlineSpiller.cpp
   InterferenceCache.cpp
   InterleavedAccessPass.cpp

llvm/lib/CodeGen/CodeGen.cpp

@@ -54,6 +54,7 @@ void llvm::initializeCodeGen(PassRegistry &Registry) {
   initializeIfConverterPass(Registry);
   initializeImplicitNullChecksPass(Registry);
   initializeIndirectBrExpandLegacyPassPass(Registry);
+  initializeInitUndefPass(Registry);
   initializeInterleavedLoadCombinePass(Registry);
   initializeInterleavedAccessPass(Registry);
   initializeJMCInstrumenterPass(Registry);