[RISCV] Support 'generic' as a valid CPU name.

Most other targets support 'generic', but RISCV issues an error.
This can require a special case in tools that use LLVM that aren't
clang.

This patch treats "generic" the same as an empty string and remaps
it to generic-rv/rv64 based on the triple. Unfortunately, it has to
be added to RISCV.td because MCSubtargetInfo is constructed and
parses the CPU before RISCVSubtarget's constructor gets a chance
to remap it. The CPU will then reparsed and the state in the
MCSubtargetInfo subclass will be updated again.

Fixes PR54146.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D121149

llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCTargetDesc.cpp

@@ -77,11 +77,8 @@ createRISCVMCObjectFileInfo(MCContext &Ctx, bool PIC,
 
 static MCSubtargetInfo *createRISCVMCSubtargetInfo(const Triple &TT,
                                                    StringRef CPU, StringRef FS) {
-  if (CPU.empty())
+  if (CPU.empty() || CPU == "generic")
     CPU = TT.isArch64Bit() ? "generic-rv64" : "generic-rv32";
-  if (CPU == "generic")
-    report_fatal_error(Twine("CPU 'generic' is not supported. Use ") +
-                       (TT.isArch64Bit() ? "generic-rv64" : "generic-rv32"));
   return createRISCVMCSubtargetInfoImpl(TT, CPU, /*TuneCPU*/ CPU, FS);
 }
 

llvm/lib/Target/RISCV/RISCV.td

@@ -452,6 +452,9 @@ include "RISCVSchedSiFive7.td"
 
 def : ProcessorModel<"generic-rv32", NoSchedModel, []>;
 def : ProcessorModel<"generic-rv64", NoSchedModel, [Feature64Bit]>;
+// Support generic for compatibility with other targets. The triple will be used
+// to change to the appropriate rv32/rv64 version.
+def : ProcessorModel<"generic", NoSchedModel, []>;
 
 def : ProcessorModel<"rocket-rv32", RocketModel, []>;
 def : ProcessorModel<"rocket-rv64", RocketModel, [Feature64Bit]>;

llvm/lib/Target/RISCV/RISCVSubtarget.cpp

@@ -69,11 +69,8 @@ RISCVSubtarget::initializeSubtargetDependencies(const Triple &TT, StringRef CPU,
                                                 StringRef ABIName) {
   // Determine default and user-specified characteristics
   bool Is64Bit = TT.isArch64Bit();
-  if (CPU.empty())
+  if (CPU.empty() || CPU == "generic")
     CPU = Is64Bit ? "generic-rv64" : "generic-rv32";
-  if (CPU == "generic")
-    report_fatal_error(Twine("CPU 'generic' is not supported. Use ") +
-                       (Is64Bit ? "generic-rv64" : "generic-rv32"));
 
   if (TuneCPU.empty())
     TuneCPU = CPU;