[RISCV] Define FeatureRelax and shouldForceRelocation for RISCV linker relaxation

[shiva0217]

Summary

1. Deine FeatureRelax to enable/disable linker relaxation

2. Define shouldForceRelocation to preserve relocation types when linker relaxation enabled. So then linker could fixup branches offsets while relaxation.

[asb]

Thanks Shiva. I've suggested an alternate phrasing of the comment on shouldForceRelocation but this is looking good to me.

lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

46–47

I think this would be clearer rephrased with something like:

"If linker relaxation is enabled, always emit relocations even if the fixup can be resolved. This is necessary for correctness as offsets may change during relaxation."

[shiva0217]

update comments for shouldForceRelocation.

lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

public:
 
   void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                   const MCValue &Target, MutableArrayRef<char> Data,
                   uint64_t Value, bool IsResolved) const override;
 
   std::unique_ptr<MCObjectWriter>
   createObjectWriter(raw_pwrite_stream &OS) const override;
 
+  // If linker relaxation is enabled, always emit relocations even if the fixup
+  // can be resolved. This is necessary for correctness as offsets may change
+  // during relaxation.
+  bool shouldForceRelocation(const MCAssembler &Asm, const MCFixup &Fixup,
+                             const MCValue &Target) override {
+    return STI.getFeatureBits()[RISCV::FeatureRelax];
+  }
+
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override;
 
   unsigned getNumFixupKinds() const override {
     return RISCV::NumTargetFixupKinds;
   }
 

lib/Target/RISCV/RISCV.td

 def IsRV64 : Predicate<"Subtarget->is64Bit()">,
                        AssemblerPredicate<"Feature64Bit">;
 def IsRV32 : Predicate<"!Subtarget->is64Bit()">,
                        AssemblerPredicate<"!Feature64Bit">;
 
 def RV64           : HwMode<"+64bit">;
 def RV32           : HwMode<"-64bit">;
 
+def FeatureRelax
+    : SubtargetFeature<"relax", "EnableLinkerRelax", "true",
+                       "Enable Linker relaxation.">;
+
 //===----------------------------------------------------------------------===//
 // Registers, calling conventions, instruction descriptions.
 //===----------------------------------------------------------------------===//
 
 include "RISCVRegisterInfo.td"
 include "RISCVCallingConv.td"
 include "RISCVInstrInfo.td"
 

lib/Target/RISCV/RISCVSubtarget.h

 class RISCVSubtarget : public RISCVGenSubtargetInfo {
   virtual void anchor();
   bool HasStdExtM = false;
   bool HasStdExtA = false;
   bool HasStdExtF = false;
   bool HasStdExtD = false;
   bool HasStdExtC = false;
   bool HasRV64 = false;
+  bool EnableLinkerRelax = false;
   unsigned XLen = 32;
   MVT XLenVT = MVT::i32;
   RISCVFrameLowering FrameLowering;
   RISCVInstrInfo InstrInfo;
   RISCVRegisterInfo RegInfo;
   RISCVTargetLowering TLInfo;
   SelectionDAGTargetInfo TSInfo;
 
const SelectionDAGTargetInfo *getSelectionDAGInfo() const override {
     return &TSInfo;
   }
   bool hasStdExtM() const { return HasStdExtM; }
   bool hasStdExtA() const { return HasStdExtA; }
   bool hasStdExtF() const { return HasStdExtF; }
   bool hasStdExtD() const { return HasStdExtD; }
   bool hasStdExtC() const { return HasStdExtC; }
   bool is64Bit() const { return HasRV64; }
+  bool enableLinkerRelax() const { return EnableLinkerRelax; }
   MVT getXLenVT() const { return XLenVT; }
   unsigned getXLen() const { return XLen; }
 };
 } // End llvm namespace
 
 #endif