Index: include/llvm/MC/MCAsmBackend.h
===================================================================
--- include/llvm/MC/MCAsmBackend.h
+++ include/llvm/MC/MCAsmBackend.h
@@ -198,6 +198,8 @@
   ///
   /// \returns true iff any relaxation occurred.
   bool relaxFragment(MCPaddingFragment *PF, MCAsmLayout &Layout);
+
+  virtual void setSTI(const MCSubtargetInfo &STI) {}
 };
 
 } // end namespace llvm
Index: lib/MC/MCAsmStreamer.cpp
===================================================================
--- lib/MC/MCAsmStreamer.cpp
+++ lib/MC/MCAsmStreamer.cpp
@@ -79,7 +79,7 @@
   }
 
   MCAssembler &getAssembler() { return *Assembler; }
-  MCAssembler *getAssemblerPtr() override { return nullptr; }
+  MCAssembler *getAssemblerPtr() override { return Assembler.get(); }
 
   inline void EmitEOL() {
     // Dump Explicit Comments here.
Index: lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp
===================================================================
--- lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp
+++ lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp
@@ -13,6 +13,8 @@
 #include "MCTargetDesc/RISCVTargetStreamer.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/MC/MCAsmBackend.h"
+#include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCInst.h"
@@ -1117,9 +1119,35 @@
     return false;
   }
 
+  if (Option == "relax") {
+    getTargetStreamer().emitDirectiveOptionRelax();
+
+    Parser.Lex();
+    if (Parser.getTok().isNot(AsmToken::EndOfStatement))
+      return Error(Parser.getTok().getLoc(),
+                   "unexpected token, expected end of statement");
+
+    setFeatureBits(RISCV::FeatureRelax, "relax");
+    getTargetStreamer().getStreamer().getAssemblerPtr()->getBackend().setSTI(
+        getSTI());
+    return false;
+  }
+
+  if (Option == "norelax") {
+    getTargetStreamer().emitDirectiveOptionNoRelax();
+
+    Parser.Lex();
+    if (Parser.getTok().isNot(AsmToken::EndOfStatement))
+      return Error(Parser.getTok().getLoc(),
+                   "unexpected token, expected end of statement");
+
+    clearFeatureBits(RISCV::FeatureRelax, "relax");
+    return false;
+  }
+
   // Unknown option.
   Warning(Parser.getTok().getLoc(),
-          "unknown option, expected 'rvc' or 'norvc'");
+          "unknown option, expected 'rvc', 'norvc', 'relax' or 'norelax'");
   Parser.eatToEndOfStatement();
   return false;
 }
Index: lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
+++ lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
@@ -27,26 +27,28 @@
 
 namespace {
 class RISCVAsmBackend : public MCAsmBackend {
-  const MCSubtargetInfo &STI;
+  const MCSubtargetInfo *STI;
   uint8_t OSABI;
   bool Is64Bit;
 
 public:
   RISCVAsmBackend(const MCSubtargetInfo &STI, uint8_t OSABI, bool Is64Bit)
-      : MCAsmBackend(support::little), STI(STI), OSABI(OSABI),
+      : MCAsmBackend(support::little), STI(&STI), OSABI(OSABI),
         Is64Bit(Is64Bit) {}
   ~RISCVAsmBackend() override {}
 
   // Generate diff expression relocations if the relax feature is enabled,
   // otherwise it is safe for the assembler to calculate these internally.
   bool requiresDiffExpressionRelocations() const override {
-    return STI.getFeatureBits()[RISCV::FeatureRelax];
+    return STI->getFeatureBits()[RISCV::FeatureRelax];
   }
   void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                   const MCValue &Target, MutableArrayRef<char> Data,
                   uint64_t Value, bool IsResolved,
                   const MCSubtargetInfo *STI) const override;
 
+  void setSTI(const MCSubtargetInfo &_STI) override { STI = &_STI; }
+
   std::unique_ptr<MCObjectTargetWriter>
   createObjectTargetWriter() const override;
 
@@ -55,7 +57,7 @@
   // during relaxation.
   bool shouldForceRelocation(const MCAssembler &Asm, const MCFixup &Fixup,
                              const MCValue &Target) override {
-    return STI.getFeatureBits()[RISCV::FeatureRelax];
+    return STI->getFeatureBits()[RISCV::FeatureRelax];
   }
 
   bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
@@ -202,7 +204,7 @@
 }
 
 bool RISCVAsmBackend::writeNopData(raw_ostream &OS, uint64_t Count) const {
-  bool HasStdExtC = STI.getFeatureBits()[RISCV::FeatureStdExtC];
+  bool HasStdExtC = STI->getFeatureBits()[RISCV::FeatureStdExtC];
   unsigned MinNopLen = HasStdExtC ? 2 : 4;
 
   if ((Count % MinNopLen) != 0)
Index: lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.h
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.h
+++ lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.h
@@ -22,6 +22,8 @@
 
   virtual void emitDirectiveOptionRVC();
   virtual void emitDirectiveOptionNoRVC();
+  virtual void emitDirectiveOptionRelax();
+  virtual void emitDirectiveOptionNoRelax();
 };
 }
 #endif
Index: lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
+++ lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
@@ -40,3 +40,5 @@
 
 void RISCVTargetELFStreamer::emitDirectiveOptionRVC() {}
 void RISCVTargetELFStreamer::emitDirectiveOptionNoRVC() {}
+void RISCVTargetELFStreamer::emitDirectiveOptionRelax() {}
+void RISCVTargetELFStreamer::emitDirectiveOptionNoRelax() {}
Index: lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.h
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.h
+++ lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.h
@@ -20,6 +20,8 @@
 
   virtual void emitDirectiveOptionRVC() = 0;
   virtual void emitDirectiveOptionNoRVC() = 0;
+  virtual void emitDirectiveOptionRelax() = 0;
+  virtual void emitDirectiveOptionNoRelax() = 0;
 };
 
 // This part is for ascii assembly output
@@ -31,6 +33,8 @@
 
   void emitDirectiveOptionRVC() override;
   void emitDirectiveOptionNoRVC() override;
+  void emitDirectiveOptionRelax() override;
+  void emitDirectiveOptionNoRelax() override;
 };
 
 }
Index: lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.cpp
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.cpp
+++ lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.cpp
@@ -30,3 +30,11 @@
 void RISCVTargetAsmStreamer::emitDirectiveOptionNoRVC() {
   OS << "\t.option\tnorvc\n";
 }
+
+void RISCVTargetAsmStreamer::emitDirectiveOptionRelax() {
+  OS << "\t.option\trelax\n";
+}
+
+void RISCVTargetAsmStreamer::emitDirectiveOptionNoRelax() {
+  OS << "\t.option\tnorelax\n";
+}
Index: test/MC/RISCV/option-invalid.s
===================================================================
--- test/MC/RISCV/option-invalid.s
+++ test/MC/RISCV/option-invalid.s
@@ -13,5 +13,5 @@
 # CHECK: error: unexpected token, expected end of statement
 .option rvc foo
 
-# CHECK: warning: unknown option, expected 'rvc' or 'norvc'
+# CHECK: warning: unknown option, expected 'rvc', 'norvc', 'relax' or 'norelax'
 .option bar
Index: test/MC/RISCV/option-relax.s
===================================================================
--- /dev/null
+++ test/MC/RISCV/option-relax.s
@@ -0,0 +1,29 @@
+# RUN: llvm-mc -triple riscv32 < %s \
+# RUN:     | FileCheck -check-prefix=CHECK-INST %s
+# RUN: llvm-mc -filetype=obj -triple riscv32 < %s \
+# RUN:     | llvm-readobj -r | FileCheck -check-prefix=CHECK-RELOC %s
+
+# RUN: llvm-mc -triple riscv64 < %s \
+# RUN:     | FileCheck -check-prefix=CHECK-INST %s
+# RUN: llvm-mc -filetype=obj -triple riscv64 < %s \
+# RUN:     | llvm-readobj -r | FileCheck -check-prefix=CHECK-RELOC %s
+
+.option norelax
+# CHECK-INST: call foo
+# CHECK-RELOC: R_RISCV_CALL foo 0x0
+# CHECK-RELOC-NOT: R_RISCV_RELAX foo 0x0
+call foo
+
+.option relax
+# CHECK-INST: .option relax
+# CHECK-INST: call bar
+# CHECK-RELOC-NEXT: R_RISCV_CALL bar 0x0
+# CHECK-RELOC-NEXT: R_RISCV_RELAX bar 0x0
+call bar
+
+.option norelax
+# CHECK-INST: .option norelax
+# CHECK-INST: call baz
+# CHECK-RELOC-NEXT: R_RISCV_CALL baz 0x0
+# CHECK-RELOC-NOT: R_RISCV_RELAX baz 0x0
+call baz
\ No newline at end of file