[RISCV] Fix logic to check if frame pointer should be used

[mgrang]

Summary

The logic was broken for Linux triples as it returns true in the switch for Triple.isOSLinux().

[mgrang]

@asb Should we keep frame pointer enabled for debug builds? Internally, for ARM/Thumb we leave frame pointer ON if debug is enabled.

[asb]

Could you please add a test, perhaps to test/Driver/frame-pointer.c?

My understanding is that in general there is a desire for codegen to remain the same or as close as possible whether it's a -g build or not, so I think disabling fp elimination for debug builds wouldn't be the right route. Or did I misunderstand you?

[mgrang]

Added unit tests.

[asb]

Thanks, looks good to me.

lib/Driver/ToolChains/Clang.cpp

static bool useFramePointerForTargetByDefault(const ArgList &Args,
                                               const llvm::Triple &Triple) {
   switch (Triple.getArch()) {
   case llvm::Triple::xcore:
   case llvm::Triple::wasm32:
   case llvm::Triple::wasm64:
     // XCore never wants frame pointers, regardless of OS.
     // WebAssembly never wants frame pointers.
     return false;
+  case llvm::Triple::riscv32:
+  case llvm::Triple::riscv64:
+    return !areOptimizationsEnabled(Args);
   default:
     break;
   }
 
   if (Triple.isOSLinux() || Triple.getOS() == llvm::Triple::CloudABI) {
     switch (Triple.getArch()) {
     // Don't use a frame pointer on linux if optimizing for certain targets.
     case llvm::Triple::mips64:
     case llvm::Triple::mips64el:
     case llvm::Triple::mips:
     case llvm::Triple::mipsel:
     case llvm::Triple::ppc:
     case llvm::Triple::ppc64:
     case llvm::Triple::ppc64le:
     case llvm::Triple::systemz:
     case llvm::Triple::x86:
     case llvm::Triple::x86_64:
       return !areOptimizationsEnabled(Args);
     default:
       return true;
     }
   }
 
-  switch (Triple.getArch()) {
-    case llvm::Triple::riscv32:
-    case llvm::Triple::riscv64:
-      return !areOptimizationsEnabled(Args);
-    default:
-      break;
-  }
-
   if (Triple.isOSWindows()) {
     switch (Triple.getArch()) {
     case llvm::Triple::x86:
       return !areOptimizationsEnabled(Args);
     case llvm::Triple::x86_64:
       return Triple.isOSBinFormatMachO();
     case llvm::Triple::arm:
     case llvm::Triple::thumb: