[MC][Bugfix] Remove redundant parameter for relaxInstruction

[skan]

Summary

Before this patch, relaxInstruction takes three arguments, the first
argument refers to the instruction before relaxation and the third
argument is the output instruction after relaxation. There are two quite
strange things:

1. The first argument's type is const MCInst &, the third argument's type is MCInst &, but they may be aliased to the same variable
2. The backends of ARM, AMDGPU, RISC-V, Hexagon assume that the third argument is a fresh uninitialized MCInst even if relaxInstruction may be called like relaxInstruction(Relaxed, STI, Relaxed) in a loop.

In this patch, we drop the thrid argument, and let relaxInstruction
directly modify the given instruction. Also, this patch fixes the bug https://bugs.llvm.org/show_bug.cgi?id=45580, which is introduced by D77851, and
breaks the assumption of ARM, AMDGPU, RISC-V, Hexagon.

[nathanchance]

Just as an FYI, this fixes the issue I reported here: https://bugs.llvm.org/show_bug.cgi?id=45580

[nickdesaulniers]

It would be cool to add a test for RISCV that way we can ensure we don't regress this again. I think @nathanchance 's test case was quite minimal.

[skan]

Feel free to add yourself as a reviewer if you'd like to.

[skan]

Add test for RISCV to prevent future regression

[MaskRay]

llvm/test/MC/RISCV/rv64-relaxation-encoding.s

1

#

9

Omit encodings.

[skan]

Address review comments (only test file changes)

[luismarques]

llvm/test/MC/RISCV/rv64-relax-all.s

9–11 ↗

(On Diff #258503)

I'm confused about this. When you relax the instruction it goes from a compressed instruction to an uncompressed one?

[luismarques]

llvm/test/MC/RISCV/rv64-relax-all.s

9–11 ↗

(On Diff #258503)

Ah, I see which patch this came from. When I think of RISC-V relaxations I associate with going from the more general code pattern to the more restricted one, such as the ones requested by RISC-V relaxation relocations, so I was surprised by this.

[MaskRay]

Seems reasonable. I've only got some nits about using move assignments.

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp

63

Inst = std::move(Res);

llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp

353

Inst = std::move(Res);

llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp

684

Inst = std::move(Res);

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

176

Inst = std::move(Res);

[jrtc27]

llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp

353

Also please make Res scoped to the if. I also note that it would be nice for the code to use ARMCC::AL rather than the hard-coded 14, but that's unrelated to this patch...

[skan]

Address review comments

[MaskRay]

LGTM. As both a bugfix and a cleanup, this looks quite reasonable to me. Probably give RISC-V/Hexagon people one or two days to confirm.

[Razer6]

LGTM. Tested the patch for RISC-V.

[bcain]

LGTM. Tested with Hexagon.

[skan]

Rebase

llvm/include/llvm/MC/MCAsmBackend.h

public:
 
   /// Simple predicate for targets where !Resolved implies requiring relaxation
   virtual bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                                     const MCRelaxableFragment *DF,
                                     const MCAsmLayout &Layout) const = 0;
 
   /// Relax the instruction in the given fragment to the next wider instruction.
   ///
-  /// \param Inst The instruction to relax, which may be the same as the
-  /// output.
+  /// \param [out] Inst The instruction to relax, which is also the relaxed
+  /// instruction.
   /// \param STI the subtarget information for the associated instruction.
-  /// \param [out] Res On return, the relaxed instruction.
-  virtual void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                                MCInst &Res) const = 0;
+  virtual void relaxInstruction(MCInst &Inst,
+                                const MCSubtargetInfo &STI) const {};
 
   /// @}
 
   /// Returns the minimum size of a nop in bytes on this target. The assembler
   /// will use this to emit excess padding in situations where the padding
   /// required for simple alignment would be less than the minimum nop size.
   ///
   virtual unsigned getMinimumNopSize() const { return 1; }

llvm/lib/MC/MCAssembler.cpp

bool MCAssembler::relaxInstruction(MCAsmLayout &Layout,
 
   ++stats::RelaxedInstructions;
 
   // FIXME-PERF: We could immediately lower out instructions if we can tell
   // they are fully resolved, to avoid retesting on later passes.
 
   // Relax the fragment.
 
-  MCInst Relaxed;
-  getBackend().relaxInstruction(F.getInst(), *F.getSubtargetInfo(), Relaxed);
+  MCInst Relaxed = F.getInst();
+  getBackend().relaxInstruction(Relaxed, *F.getSubtargetInfo());
 
   // Encode the new instruction.
   //
   // FIXME-PERF: If it matters, we could let the target do this. It can
   // probably do so more efficiently in many cases.
   SmallVector<MCFixup, 4> Fixups;
   SmallString<256> Code;
   raw_svector_ostream VecOS(Code);

llvm/lib/MC/MCObjectStreamer.cpp

void MCObjectStreamer::emitInstructionImpl(const MCInst &Inst,
   // - The RelaxAll flag was passed
   // - Bundling is enabled and this instruction is inside a bundle-locked
   //   group. We want to emit all such instructions into the same data
   //   fragment.
   if (Assembler.getRelaxAll() ||
       (Assembler.isBundlingEnabled() && Sec->isBundleLocked())) {
     MCInst Relaxed = Inst;
     while (getAssembler().getBackend().mayNeedRelaxation(Relaxed, STI))
-      getAssembler().getBackend().relaxInstruction(Relaxed, STI, Relaxed);
+      getAssembler().getBackend().relaxInstruction(Relaxed, STI);
     EmitInstToData(Relaxed, STI);
     return;
   }
 
   // Otherwise emit to a separate fragment.
   EmitInstToFragment(Inst, STI);
 }
 

llvm/lib/MCA/CodeEmitter.cpp

CodeEmitter::getOrCreateEncodingInfo(unsigned MCID) {
   EncodingInfo &EI = Encodings[MCID];
   if (EI.second)
     return EI;
 
   SmallVector<llvm::MCFixup, 2> Fixups;
   const MCInst &Inst = Sequence[MCID];
   MCInst Relaxed(Sequence[MCID]);
   if (MAB.mayNeedRelaxation(Inst, STI))
-    MAB.relaxInstruction(Inst, STI, Relaxed);
+    MAB.relaxInstruction(Relaxed, STI);
 
   EI.first = Code.size();
   MCE.encodeInstruction(Relaxed, VecOS, Fixups, STI);
   EI.second = Code.size() - EI.first;
   return EI;
 }
 
 } // namespace mca
 } // namespace llvm

llvm/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp

void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                   uint64_t Value, bool IsResolved,
                   const MCSubtargetInfo *STI) const override;
 
   bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override;
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override;
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override;
+  void relaxInstruction(MCInst &Inst,
+                        const MCSubtargetInfo &STI) const override;
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 
   void HandleAssemblerFlag(MCAssemblerFlag Flag) {}
 
   unsigned getPointerSize() const { return 8; }
 
   unsigned getFixupKindContainereSizeInBytes(unsigned Kind) const;
 
bool AArch64AsmBackend::fixupNeedsRelaxation(const MCFixup &Fixup,
                                              const MCAsmLayout &Layout) const {
   // FIXME:  This isn't correct for AArch64. Just moving the "generic" logic
   // into the targets for now.
   //
   // Relax if the value is too big for a (signed) i8.
   return int64_t(Value) != int64_t(int8_t(Value));
 }
 
-void AArch64AsmBackend::relaxInstruction(const MCInst &Inst,
-                                         const MCSubtargetInfo &STI,
-                                         MCInst &Res) const {
+void AArch64AsmBackend::relaxInstruction(MCInst &Inst,
+                                         const MCSubtargetInfo &STI) const {
   llvm_unreachable("AArch64AsmBackend::relaxInstruction() unimplemented");
 }
 
 bool AArch64AsmBackend::writeNopData(raw_ostream &OS, uint64_t Count) const {
   // If the count is not 4-byte aligned, we must be writing data into the text
   // section (otherwise we have unaligned instructions, and thus have far
   // bigger problems), so just write zeros instead.
   OS.write_zeros(Count % 4);

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp

public:
   void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                   const MCValue &Target, MutableArrayRef<char> Data,
                   uint64_t Value, bool IsResolved,
                   const MCSubtargetInfo *STI) const override;
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override;
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override;
+  void relaxInstruction(MCInst &Inst,
+                        const MCSubtargetInfo &STI) const override;
 
   bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override;
 
   unsigned getMinimumNopSize() const override;
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 
   const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const override;
 };
 
 } //End anonymous namespace
 
-void AMDGPUAsmBackend::relaxInstruction(const MCInst &Inst,
-                                        const MCSubtargetInfo &STI,
-                                        MCInst &Res) const {
+void AMDGPUAsmBackend::relaxInstruction(MCInst &Inst,
+                                        const MCSubtargetInfo &STI) const {
+  MCInst Res;
   unsigned RelaxedOpcode = AMDGPU::getSOPPWithRelaxation(Inst.getOpcode());
   Res.setOpcode(RelaxedOpcode);
   Res.addOperand(Inst.getOperand(0));
+  Inst = Res;
   return;
 }
 
 bool AMDGPUAsmBackend::fixupNeedsRelaxation(const MCFixup &Fixup,
                                             uint64_t Value,
                                             const MCRelaxableFragment *DF,
                                             const MCAsmLayout &Layout) const {
   // if the branch target has an offset of x3f this needs to be relaxed to

llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h

public:
 
   const char *reasonForFixupRelaxation(const MCFixup &Fixup,
                                        uint64_t Value) const;
 
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override;
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override;
+  void relaxInstruction(MCInst &Inst,
+                        const MCSubtargetInfo &STI) const override;
 
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 
   void handleAssemblerFlag(MCAssemblerFlag Flag) override;
 
   unsigned getPointerSize() const { return 4; }
   bool isThumb() const { return isThumbMode; }
   void setIsThumb(bool it) { isThumbMode = it; }
 };
 } // end namespace llvm
 
 #endif

llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp

 }
 
 bool ARMAsmBackend::fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                                          const MCRelaxableFragment *DF,
                                          const MCAsmLayout &Layout) const {
   return reasonForFixupRelaxation(Fixup, Value);
 }
 
-void ARMAsmBackend::relaxInstruction(const MCInst &Inst,
-                                     const MCSubtargetInfo &STI,
-                                     MCInst &Res) const {
+void ARMAsmBackend::relaxInstruction(MCInst &Inst,
+                                     const MCSubtargetInfo &STI) const {
   unsigned RelaxedOp = getRelaxedOpcode(Inst.getOpcode(), STI);
 
   // Sanity check w/ diagnostic if we get here w/ a bogus instruction.
   if (RelaxedOp == Inst.getOpcode()) {
     SmallString<256> Tmp;
     raw_svector_ostream OS(Tmp);
     Inst.dump_pretty(OS);
     OS << "\n";
     report_fatal_error("unexpected instruction to relax: " + OS.str());
   }
 
+  MCInst Res;
   // If we are changing Thumb CBZ or CBNZ instruction to a NOP, aka tHINT, we
   // have to change the operands too.
   if ((Inst.getOpcode() == ARM::tCBZ || Inst.getOpcode() == ARM::tCBNZ) &&
       RelaxedOp == ARM::tHINT) {
     Res.setOpcode(RelaxedOp);
     Res.addOperand(MCOperand::createImm(0));
     Res.addOperand(MCOperand::createImm(14));
     Res.addOperand(MCOperand::createReg(0));
+    Inst = Res;
     return;
   }
 
   // The rest of instructions we're relaxing have the same operands.
   // We just need to update to the proper opcode.
-  Res = Inst;
-  Res.setOpcode(RelaxedOp);
+  Inst.setOpcode(RelaxedOp);
 }
 
 bool ARMAsmBackend::writeNopData(raw_ostream &OS, uint64_t Count) const {
   const uint16_t Thumb1_16bitNopEncoding = 0x46c0; // using MOV r8,r8
   const uint16_t Thumb2_16bitNopEncoding = 0xbf00; // NOP
   const uint32_t ARMv4_NopEncoding = 0xe1a00000;   // using MOV r0,r0
   const uint32_t ARMv6T2_NopEncoding = 0xe320f000; // NOP
   if (isThumb()) {

llvm/lib/Target/AVR/MCTargetDesc/AVRAsmBackend.h

public:
 
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override {
     llvm_unreachable("RelaxInstruction() unimplemented");
     return false;
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 
   bool shouldForceRelocation(const MCAssembler &Asm, const MCFixup &Fixup,
                              const MCValue &Target) override;
 
 private:
   Triple::OSType OSType;
 };
 
 } // end namespace llvm
 
 #endif // LLVM_AVR_ASM_BACKEND_H
 

llvm/lib/Target/BPF/MCTargetDesc/BPFAsmBackend.cpp

public:
 
   unsigned getNumFixupKinds() const override { return 1; }
 
   bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override {
     return false;
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 };
 
 } // end anonymous namespace
 
 bool BPFAsmBackend::writeNopData(raw_ostream &OS, uint64_t Count) const {
   if ((Count % 8) != 0)
     return false;

llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp

public:
 
   /// Simple predicate for targets where !Resolved implies requiring relaxation
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override {
     llvm_unreachable("Handled by fixupNeedsRelaxationAdvanced");
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {
+  void relaxInstruction(MCInst &Inst,
+                        const MCSubtargetInfo &STI) const override {
     assert(HexagonMCInstrInfo::isBundle(Inst) &&
            "Hexagon relaxInstruction only works on bundles");
 
+    MCInst Res;
     Res.setOpcode(Hexagon::BUNDLE);
     Res.addOperand(MCOperand::createImm(Inst.getOperand(0).getImm()));
     // Copy the results into the bundle.
     bool Update = false;
     for (auto &I : HexagonMCInstrInfo::bundleInstructions(Inst)) {
       MCInst &CrntHMI = const_cast<MCInst &>(*I.getInst());
 
       // if immediate extender needed, add it in
       if (*RelaxTarget == &CrntHMI) {
         Update = true;
         assert((HexagonMCInstrInfo::bundleSize(Res) < HEXAGON_PACKET_SIZE) &&
                "No room to insert extender for relaxation");
 
         MCInst *HMIx = takeExtender();
         *HMIx = HexagonMCInstrInfo::deriveExtender(
                 *MCII, CrntHMI,
                 HexagonMCInstrInfo::getExtendableOperand(*MCII, CrntHMI));
         Res.addOperand(MCOperand::createInst(HMIx));
         *RelaxTarget = nullptr;
       }
       // now copy over the original instruction(the one we may have extended)
       Res.addOperand(MCOperand::createInst(I.getInst()));
     }
+
+    Inst = Res;
     (void)Update;
     assert(Update && "Didn't find relaxation target");
   }
 
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override {
     static const uint32_t Nopcode  = 0x7f000000, // Hard-coded NOP.
                           ParseIn  = 0x00004000, // In packet parse-bits.
                           ParseEnd = 0x0000c000; // End of packet parse-bits.

llvm/lib/Target/Lanai/MCTargetDesc/LanaiAsmBackend.cpp

unsigned getNumFixupKinds() const override {
     return Lanai::NumTargetFixupKinds;
   }
 
   bool mayNeedRelaxation(const MCInst & /*Inst*/,
                          const MCSubtargetInfo &STI) const override {
     return false;
   }
 
-  void relaxInstruction(const MCInst & /*Inst*/,
-                        const MCSubtargetInfo & /*STI*/,
-                        MCInst & /*Res*/) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 };
 
 bool LanaiAsmBackend::writeNopData(raw_ostream &OS, uint64_t Count) const {
   if ((Count % 4) != 0)
     return false;
 
   for (uint64_t i = 0; i < Count; i += 4)

llvm/lib/Target/MSP430/MCTargetDesc/MSP430AsmBackend.cpp

const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const override {
     return Infos[Kind - FirstTargetFixupKind];
   }
 
   bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override {
     return false;
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 };
 
 uint64_t MSP430AsmBackend::adjustFixupValue(const MCFixup &Fixup,
                                             uint64_t Value,
                                             MCContext &Ctx) const {
   unsigned Kind = Fixup.getKind();
   switch (Kind) {

llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.h

public:
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override {
     // FIXME.
     llvm_unreachable("RelaxInstruction() unimplemented");
     return false;
   }
 
-  /// RelaxInstruction - Relax the instruction in the given fragment
-  /// to the next wider instruction.
-  ///
-  /// \param Inst - The instruction to relax, which may be the same
-  /// as the output.
-  /// \param [out] Res On return, the relaxed instruction.
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {}
-
-  /// @}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 
   bool shouldForceRelocation(const MCAssembler &Asm, const MCFixup &Fixup,
                              const MCValue &Target) override;
 
   bool isMicroMips(const MCSymbol *Sym) const override;
 }; // class MipsAsmBackend
 
 } // namespace
 
 #endif

llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp

public:
   bool fixupNeedsRelaxation(const MCFixup &Fixup,
                             uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override {
     // FIXME.
     llvm_unreachable("relaxInstruction() unimplemented");
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {
+  void relaxInstruction(MCInst &Inst,
+                        const MCSubtargetInfo &STI) const override {
     // FIXME.
     llvm_unreachable("relaxInstruction() unimplemented");
   }
 
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override {
     uint64_t NumNops = Count / 4;
     for (uint64_t i = 0; i != NumNops; ++i)
       support::endian::write<uint32_t>(OS, 0x60000000, Endian);

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h

public:
   Optional<MCFixupKind> getFixupKind(StringRef Name) const override;
 
   const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const override;
 
   bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override;
   unsigned getRelaxedOpcode(unsigned Op) const;
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override;
-
+  void relaxInstruction(MCInst &Inst,
+                        const MCSubtargetInfo &STI) const override;
 
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 
   const MCTargetOptions &getTargetOptions() const { return TargetOptions; }
   RISCVABI::ABI getTargetABI() const { return TargetABI; }
 };
 }
 
 #endif

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

case RISCV::fixup_riscv_rvc_branch:
     return Offset > 254 || Offset < -256;
   case RISCV::fixup_riscv_rvc_jump:
     // For compressed jump instructions the immediate must be
     // in the range [-2048, 2046].
     return Offset > 2046 || Offset < -2048;
   }
 }
 
-void RISCVAsmBackend::relaxInstruction(const MCInst &Inst,
-                                       const MCSubtargetInfo &STI,
-                                       MCInst &Res) const {
+void RISCVAsmBackend::relaxInstruction(MCInst &Inst,
+                                       const MCSubtargetInfo &STI) const {
   // TODO: replace this with call to auto generated uncompressinstr() function.
+  MCInst Res;
   switch (Inst.getOpcode()) {
   default:
     llvm_unreachable("Opcode not expected!");
   case RISCV::C_BEQZ:
     // c.beqz $rs1, $imm -> beq $rs1, X0, $imm.
     Res.setOpcode(RISCV::BEQ);
     Res.addOperand(Inst.getOperand(0));
     Res.addOperand(MCOperand::createReg(RISCV::X0));
case RISCV::C_J:
     break;
   case RISCV::C_JAL:
     // c.jal $imm -> jal X1, $imm.
     Res.setOpcode(RISCV::JAL);
     Res.addOperand(MCOperand::createReg(RISCV::X1));
     Res.addOperand(Inst.getOperand(0));
     break;
   }
+  Inst = Res;
 }
 
 // Given a compressed control flow instruction this function returns
 // the expanded instruction.
 unsigned RISCVAsmBackend::getRelaxedOpcode(unsigned Op) const {
   switch (Op) {
   default:
     return Op;

llvm/lib/Target/Sparc/MCTargetDesc/SparcAsmBackend.cpp

public:
     bool fixupNeedsRelaxation(const MCFixup &Fixup,
                               uint64_t Value,
                               const MCRelaxableFragment *DF,
                               const MCAsmLayout &Layout) const override {
       // FIXME.
       llvm_unreachable("fixupNeedsRelaxation() unimplemented");
       return false;
     }
-    void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                          MCInst &Res) const override {
+    void relaxInstruction(MCInst &Inst,
+                          const MCSubtargetInfo &STI) const override {
       // FIXME.
       llvm_unreachable("relaxInstruction() unimplemented");
     }
 
     bool writeNopData(raw_ostream &OS, uint64_t Count) const override {
       // Cannot emit NOP with size not multiple of 32 bits.
       if (Count % 4 != 0)
         return false;

llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmBackend.cpp

bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override {
     return false;
   }
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *Fragment,
                             const MCAsmLayout &Layout) const override {
     return false;
   }
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {
-    llvm_unreachable("SystemZ does do not have assembler relaxation");
-  }
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
   std::unique_ptr<MCObjectTargetWriter>
   createObjectTargetWriter() const override {
     return createSystemZObjectWriter(OSABI);
   }
 };
 } // end anonymous namespace
 

llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyAsmBackend.cpp

bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
     return false;
   }
 
   bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override {
     return false;
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 };
 
 const MCFixupKindInfo &
 WebAssemblyAsmBackend::getFixupKindInfo(MCFixupKind Kind) const {
   const static MCFixupKindInfo Infos[WebAssembly::NumTargetFixupKinds] = {
       // This table *must* be in the order that the fixup_* kinds are defined in
       // WebAssemblyFixupKinds.h.

llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

public:
 
   bool mayNeedRelaxation(const MCInst &Inst,
                          const MCSubtargetInfo &STI) const override;
 
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
                             const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const override;
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override;
+  void relaxInstruction(MCInst &Inst,
+                        const MCSubtargetInfo &STI) const override;
 
   bool padInstructionViaRelaxation(MCRelaxableFragment &RF,
                                    MCCodeEmitter &Emitter,
                                    unsigned &RemainingSize) const;
 
   bool padInstructionViaPrefix(MCRelaxableFragment &RF, MCCodeEmitter &Emitter,
                                unsigned &RemainingSize) const;
 
bool X86AsmBackend::fixupNeedsRelaxation(const MCFixup &Fixup,
                                          const MCRelaxableFragment *DF,
                                          const MCAsmLayout &Layout) const {
   // Relax if the value is too big for a (signed) i8.
   return !isInt<8>(Value);
 }
 
 // FIXME: Can tblgen help at all here to verify there aren't other instructions
 // we can relax?
-void X86AsmBackend::relaxInstruction(const MCInst &Inst,
-                                     const MCSubtargetInfo &STI,
-                                     MCInst &Res) const {
+void X86AsmBackend::relaxInstruction(MCInst &Inst,
+                                     const MCSubtargetInfo &STI) const {
   // The only relaxations X86 does is from a 1byte pcrel to a 4byte pcrel.
   bool Is16BitMode = STI.getFeatureBits()[X86::Mode16Bit];
   unsigned RelaxedOp = getRelaxedOpcode(Inst, Is16BitMode);
 
   if (RelaxedOp == Inst.getOpcode()) {
     SmallString<256> Tmp;
     raw_svector_ostream OS(Tmp);
     Inst.dump_pretty(OS);
     OS << "\n";
     report_fatal_error("unexpected instruction to relax: " + OS.str());
   }
 
-  Res = Inst;
-  Res.setOpcode(RelaxedOp);
+  Inst.setOpcode(RelaxedOp);
 }
 
 /// Return true if this instruction has been fully relaxed into it's most
 /// general available form.
 static bool isFullyRelaxed(const MCRelaxableFragment &RF) {
   auto &Inst = RF.getInst();
   auto &STI = *RF.getSubtargetInfo();
   bool Is16BitMode = STI.getFeatureBits()[X86::Mode16Bit];

 bool X86AsmBackend::padInstructionViaRelaxation(MCRelaxableFragment &RF,
                                                 MCCodeEmitter &Emitter,
                                                 unsigned &RemainingSize) const {
   if (isFullyRelaxed(RF))
     // TODO: There are lots of other tricks we could apply for increasing
     // encoding size without impacting performance.
     return false;
 
-  MCInst Relaxed;
-  relaxInstruction(RF.getInst(), *RF.getSubtargetInfo(), Relaxed);
+  MCInst Relaxed = RF.getInst();
+  relaxInstruction(Relaxed, *RF.getSubtargetInfo());
 
   SmallVector<MCFixup, 4> Fixups;
   SmallString<15> Code;
   raw_svector_ostream VecOS(Code);
   Emitter.encodeInstruction(Relaxed, VecOS, Fixups, *RF.getSubtargetInfo());
   const unsigned OldSize = RF.getContents().size();
   const unsigned NewSize = Code.size();
   assert(NewSize >= OldSize && "size decrease during relaxation?");

llvm/test/MC/RISCV/rv64-relaxation-encoding.s

+// RUN: llvm-mc -filetype=obj -triple riscv64 -mattr=+c %s | llvm-objdump -d -M no-aliases - | FileCheck %s --check-prefix=INSTR
+
+// RUN: llvm-mc -filetype=obj -triple riscv64 -mattr=+c %s --mc-relax-all | llvm-objdump -d -M no-aliases - | FileCheck %s --check-prefix=RELAX-INSTR
+
+## Check the instructions are relaxed correctly
+
+NEAR:
+
+# INSTR:       01 c1                            c.beqz    a0, 0 <NEAR>
+# RELAX-INSTR: 63 00 05 00                      beq    a0, zero, 0 <NEAR>
+c.beqz a0, NEAR
+
+# INSTR:       fd bf                            c.j    -2 <NEAR>
+# RELAX-INSTR: 6f f0 df ff                      jal    zero, -4 <NEAR>
+c.j NEAR