diff --git a/llvm/include/llvm/MC/MCAsmBackend.h b/llvm/include/llvm/MC/MCAsmBackend.h
--- a/llvm/include/llvm/MC/MCAsmBackend.h
+++ b/llvm/include/llvm/MC/MCAsmBackend.h
@@ -161,12 +161,11 @@
 
   /// 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 {};
 
   /// @}
 
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -926,8 +926,8 @@
 
   // 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.
   //
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -408,7 +408,7 @@
       (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;
   }
diff --git a/llvm/lib/MCA/CodeEmitter.cpp b/llvm/lib/MCA/CodeEmitter.cpp
--- a/llvm/lib/MCA/CodeEmitter.cpp
+++ b/llvm/lib/MCA/CodeEmitter.cpp
@@ -25,7 +25,7 @@
   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);
diff --git a/llvm/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp b/llvm/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp
--- a/llvm/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp
+++ b/llvm/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp
@@ -93,8 +93,8 @@
   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) {}
@@ -467,9 +467,8 @@
   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");
 }
 
diff --git a/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp b/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp
--- a/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp
+++ b/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp
@@ -40,8 +40,8 @@
                             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;
@@ -54,12 +54,13 @@
 
 } //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;
 }
 
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.h
@@ -66,8 +66,8 @@
                             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;
 
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
@@ -328,9 +328,8 @@
   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.
@@ -342,6 +341,7 @@
     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) &&
@@ -350,13 +350,13 @@
     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 {
diff --git a/llvm/lib/Target/AVR/MCTargetDesc/AVRAsmBackend.h b/llvm/lib/Target/AVR/MCTargetDesc/AVRAsmBackend.h
--- a/llvm/lib/Target/AVR/MCTargetDesc/AVRAsmBackend.h
+++ b/llvm/lib/Target/AVR/MCTargetDesc/AVRAsmBackend.h
@@ -62,9 +62,6 @@
     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,
diff --git a/llvm/lib/Target/BPF/MCTargetDesc/BPFAsmBackend.cpp b/llvm/lib/Target/BPF/MCTargetDesc/BPFAsmBackend.cpp
--- a/llvm/lib/Target/BPF/MCTargetDesc/BPFAsmBackend.cpp
+++ b/llvm/lib/Target/BPF/MCTargetDesc/BPFAsmBackend.cpp
@@ -48,9 +48,6 @@
     return false;
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 };
 
diff --git a/llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp b/llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp
--- a/llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp
+++ b/llvm/lib/Target/Hexagon/MCTargetDesc/HexagonAsmBackend.cpp
@@ -651,11 +651,12 @@
     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.
@@ -679,6 +680,8 @@
       // 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");
   }
diff --git a/llvm/lib/Target/Lanai/MCTargetDesc/LanaiAsmBackend.cpp b/llvm/lib/Target/Lanai/MCTargetDesc/LanaiAsmBackend.cpp
--- a/llvm/lib/Target/Lanai/MCTargetDesc/LanaiAsmBackend.cpp
+++ b/llvm/lib/Target/Lanai/MCTargetDesc/LanaiAsmBackend.cpp
@@ -74,10 +74,6 @@
     return false;
   }
 
-  void relaxInstruction(const MCInst & /*Inst*/,
-                        const MCSubtargetInfo & /*STI*/,
-                        MCInst & /*Res*/) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 };
 
diff --git a/llvm/lib/Target/MSP430/MCTargetDesc/MSP430AsmBackend.cpp b/llvm/lib/Target/MSP430/MCTargetDesc/MSP430AsmBackend.cpp
--- a/llvm/lib/Target/MSP430/MCTargetDesc/MSP430AsmBackend.cpp
+++ b/llvm/lib/Target/MSP430/MCTargetDesc/MSP430AsmBackend.cpp
@@ -95,9 +95,6 @@
     return false;
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 };
 
diff --git a/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.h b/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.h
--- a/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.h
+++ b/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.h
@@ -74,17 +74,6 @@
     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,
diff --git a/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp b/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
--- a/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
+++ b/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
@@ -192,8 +192,8 @@
     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");
   }
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
@@ -105,9 +105,8 @@
                          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;
 
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
@@ -139,10 +139,10 @@
   }
 }
 
-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!");
@@ -173,6 +173,7 @@
     Res.addOperand(Inst.getOperand(0));
     break;
   }
+  Inst = Res;
 }
 
 // Given a compressed control flow instruction this function returns
diff --git a/llvm/lib/Target/Sparc/MCTargetDesc/SparcAsmBackend.cpp b/llvm/lib/Target/Sparc/MCTargetDesc/SparcAsmBackend.cpp
--- a/llvm/lib/Target/Sparc/MCTargetDesc/SparcAsmBackend.cpp
+++ b/llvm/lib/Target/Sparc/MCTargetDesc/SparcAsmBackend.cpp
@@ -271,8 +271,8 @@
       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");
     }
diff --git a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmBackend.cpp b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmBackend.cpp
--- a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmBackend.cpp
+++ b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmBackend.cpp
@@ -63,10 +63,6 @@
                             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 {
diff --git a/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyAsmBackend.cpp b/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyAsmBackend.cpp
--- a/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyAsmBackend.cpp
+++ b/llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyAsmBackend.cpp
@@ -64,9 +64,6 @@
     return false;
   }
 
-  void relaxInstruction(const MCInst &Inst, const MCSubtargetInfo &STI,
-                        MCInst &Res) const override {}
-
   bool writeNopData(raw_ostream &OS, uint64_t Count) const override;
 };
 
diff --git a/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp b/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
--- a/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
+++ b/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
@@ -192,8 +192,8 @@
                             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,
@@ -821,9 +821,8 @@
 
 // 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);
@@ -836,8 +835,7 @@
     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
@@ -911,8 +909,8 @@
     // 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;
diff --git a/llvm/test/MC/RISCV/rv64-relaxation-encoding.s b/llvm/test/MC/RISCV/rv64-relaxation-encoding.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/RISCV/rv64-relaxation-encoding.s
@@ -0,0 +1,15 @@
+// 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