[RISCV] Add -march=unset to cancel and ignore a previous -march.

clang/docs/ReleaseNotes.rst

@@ -1082,6 +1082,9 @@ RISC-V Support
 
 - Add support for the `__builtin_riscv_pause()` intrinsic from the `Zihintpause` extension.
 
+- Add `-march=unset` to clear any previous `-march=` value. This ISA string will
+  be computed from `-mcpu` or the platform default.
+
 CUDA/HIP Language Changes
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Driver/ToolChains/Arch/RISCV.cpp

@@ -273,9 +273,12 @@ std::string riscv::getRISCVArch(const llvm::opt::ArgList &Args,
   // Clang does not yet support MULTILIB_REUSE, so we use `rv{XLEN}imafdc`
   // instead of `rv{XLEN}gc` though they are (currently) equivalent.
 
-  // 1. If `-march=` is specified, use it.
-  if (const Arg *A = Args.getLastArg(options::OPT_march_EQ))
-    return A->getValue();
+  // 1. If `-march=` is specified, use it unless the value is "unset".
+  if (const Arg *A = Args.getLastArg(options::OPT_march_EQ)) {
+    StringRef MArch = A->getValue();
+    if (MArch != "unset")
+      return MArch.str();
+  }
 
   // 2. Get march (isa string) based on `-mcpu=`
   if (const Arg *A = Args.getLastArg(options::OPT_mcpu_EQ)) {
@@ -300,7 +303,7 @@ std::string riscv::getRISCVArch(const llvm::opt::ArgList &Args,
 
     StringRef MArch = llvm::RISCV::getMArchFromMcpu(CPU);
     // Bypass if target cpu's default march is empty.
-    if (MArch != "")
+    if (!MArch.empty())
       return MArch.str();
   }
 

clang/test/Driver/riscv-cpus.c

@@ -403,6 +403,11 @@
 // MCPU-MARCH: "-nostdsysteminc" "-target-cpu" "sifive-e31" "-target-feature" "+m" "-target-feature" "+c"
 // MCPU-MARCH: "-target-abi" "ilp32"
 
+// -march=unset erases previous march
+// RUN: %clang --target=riscv32 -### -c %s 2>&1 -march=rv32imc -march=unset -mcpu=sifive-e31 | FileCheck -check-prefix=MARCH-UNSET %s
+// MARCH-UNSET: "-nostdsysteminc" "-target-cpu" "sifive-e31" "-target-feature" "+m" "-target-feature" "+a" "-target-feature" "+c"
+// MARCH-UNSET-SAME: "-target-abi" "ilp32"
+
 // Check interaction between mcpu and mtune, mtune won't affect arch related
 // target feature, but mcpu will.
 //