Index: lib/Driver/ToolChains/Arch/RISCV.cpp
===================================================================
--- lib/Driver/ToolChains/Arch/RISCV.cpp
+++ lib/Driver/ToolChains/Arch/RISCV.cpp
@@ -365,6 +365,17 @@
     getExtensionFeatures(D, Args, Features, MArch, OtherExts);
   }
 
+  // -mrelax is default, unless -mno-relax is specified.
+  bool Relax = true;
+  if (auto *A = Args.getLastArg(options::OPT_mrelax, options::OPT_mno_relax))
+    if (A->getOption().matches(options::OPT_mno_relax)) {
+      Relax = false;
+      Features.push_back("-relax");
+    }
+
+  if (Relax)
+    Features.push_back("+relax");
+
   // Now add any that the user explicitly requested on the command line,
   // which may override the defaults.
   handleTargetFeaturesGroup(Args, Features, options::OPT_m_riscv_Features_Group);
Index: test/Driver/riscv-features.c
===================================================================
--- test/Driver/riscv-features.c
+++ test/Driver/riscv-features.c
@@ -5,6 +5,8 @@
 
 // RUN: %clang -target riscv32-unknown-elf -### %s -mrelax 2>&1 | FileCheck %s -check-prefix=RELAX
 // RUN: %clang -target riscv32-unknown-elf -### %s -mno-relax 2>&1 | FileCheck %s -check-prefix=NO-RELAX
+// RUN: %clang -target riscv32-unknown-elf -### %s 2>&1 | FileCheck %s -check-prefix=DEFAULT
 
 // RELAX: "-target-feature" "+relax"
 // NO-RELAX: "-target-feature" "-relax"
+// DEFAULT: "-target-feature" "+relax"