[RISCV] Add -march=unset to cancel and ignore a previous -march. (#148321)

-mcpu is used to determine the ISA string if an explicit -march is not
present on the command line. If there is a -march present it always has
priority over -mcpu regardless of where it appears in the command line.

This can cause issues if -march appears in a Makefile and a user wants
to override it with an -mcpu on the command line. The user would need to
provide a potentially long ISA string to -march that matches the -mcpu
in order to override the MakeFile.

This issue can also be seen on Compiler Explorer where the rv64gc
toolchain is passed -march=rv64gc before any user command line options
are added. If you pass -mcpu=spacemit-x60, vectors will not be enabled
due to the hidden -march.

This patch adds a new option for -march, "unset" that makes the -march
ignored for purposes of prioritizing over -mcpu. Now a user can write
-march=unset -mcpu=<cpu_name>. This is also implemented by gcc for
ARM32.

An alternative would be to allow -march to take a cpu name, but that
requires "-march=<cpu_name> -mcpu=<cpu_name>" or "-march=<cpu_name>
-mtune=<cpu_name>" to ensure the tune cpu also gets updated. IMHO,
needing to repeat the CPU name twice isn't friendly and invites
mistakes.

gcc has implemented this as well.

Author: topperc

clang/docs/ReleaseNotes.rst

@@ -234,6 +234,9 @@ RISC-V Support
 - Add support for `__attribute__((interrupt("rnmi")))` to be used with the `Smrnmi` extension.
   With this the `Smrnmi` extension is fully supported.
 
+- 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.
 //