[LoongArch] Add "32bit" target feature

[XiaodongLoong]

Summary

There are a few intrinsics or instructions on LoongArch
that are only appropriate for loongarch32 target. So the
feature "32bit" is added to implement it.

[XiaodongLoong]

rebase code and improve error message.

[SixWeining]

LGTM.

llvm/lib/Target/LoongArch/LoongArch.td

 // LoongArch subtarget features and instruction predicates.
 //===----------------------------------------------------------------------===//
 
 // LoongArch is divided into two versions, the 32-bit version (LA32) and the
 // 64-bit version (LA64).
 def Feature64Bit
     : SubtargetFeature<"64bit", "HasLA64", "true",
                        "LA64 Basic Integer and Privilege Instruction Set">;
+def Feature32Bit
+    : SubtargetFeature<"32bit", "HasLA32", "true",
+                       "LA32 Basic Integer and Privilege Instruction Set">;
 def IsLA64
     : Predicate<"Subtarget->is64Bit()">,
       AssemblerPredicate<(all_of Feature64Bit),
                          "LA64 Basic Integer and Privilege Instruction Set">;
 def IsLA32
     : Predicate<"!Subtarget->is64Bit()">,
       AssemblerPredicate<(all_of(not Feature64Bit)),
                          "LA32 Basic Integer and Privilege Instruction Set">;

 include "LoongArchRegisterInfo.td"
 include "LoongArchCallingConv.td"
 include "LoongArchInstrInfo.td"
 
 //===----------------------------------------------------------------------===//
 // LoongArch processors supported.
 //===----------------------------------------------------------------------===//
 
-def : ProcessorModel<"generic-la32", NoSchedModel, []>;
+def : ProcessorModel<"generic-la32", NoSchedModel, [Feature32Bit]>;
 def : ProcessorModel<"generic-la64", NoSchedModel, [Feature64Bit]>;
 
 // Support generic for compatibility with other targets. The triple will be used
 // to change to the appropriate la32/la64 version.
 def : ProcessorModel<"generic", NoSchedModel, []>;
 
 def : ProcessorModel<"la464", NoSchedModel, [Feature64Bit,
                                              FeatureExtLASX,

llvm/lib/Target/LoongArch/LoongArchSubtarget.h

 #define GET_SUBTARGETINFO_HEADER
 #include "LoongArchGenSubtargetInfo.inc"
 
 namespace llvm {
 class StringRef;
 
 class LoongArchSubtarget : public LoongArchGenSubtargetInfo {
   virtual void anchor();
+  bool HasLA32 = false;
   bool HasLA64 = false;
   bool HasBasicF = false;
   bool HasBasicD = false;
   bool HasExtLSX = false;
   bool HasExtLASX = false;
   bool HasExtLVZ = false;
   bool HasExtLBT = false;
   bool HasLaGlobalWithPcrel = false;

llvm/lib/Target/LoongArch/LoongArchSubtarget.cpp

if (TuneCPU.empty())
     TuneCPU = CPU;
 
   ParseSubtargetFeatures(CPU, TuneCPU, FS);
   if (Is64Bit) {
     GRLenVT = MVT::i64;
     GRLen = 64;
   }
 
+  if (HasLA32 == HasLA64)
+    report_fatal_error("Please use one feature of 32bit and 64bit.");
+
+  if (Is64Bit && HasLA32)
+    report_fatal_error("Feature 32bit should be used for loongarch32 target.");
+
+  if (!Is64Bit && HasLA64)
+    report_fatal_error("Feature 64bit should be used for loongarch64 target.");
+
   // TODO: ILP32{S,F} LP64{S,F}
   TargetABI = Is64Bit ? LoongArchABI::ABI_LP64D : LoongArchABI::ABI_ILP32D;
   return *this;
 }
 
 LoongArchSubtarget::LoongArchSubtarget(const Triple &TT, StringRef CPU,
                                        StringRef TuneCPU, StringRef FS,
                                        StringRef ABIName,
                                        const TargetMachine &TM)
     : LoongArchGenSubtargetInfo(TT, CPU, TuneCPU, FS),
       FrameLowering(
           initializeSubtargetDependencies(TT, CPU, TuneCPU, FS, ABIName)),
       InstrInfo(*this), RegInfo(getHwMode()), TLInfo(TM, *this) {}

llvm/test/CodeGen/LoongArch/feature-32bit.ll

+; RUN: llc --mtriple=loongarch64 --mattr=help 2>&1 | FileCheck %s
+; RUN: llc --mtriple=loongarch32 --mattr=help 2>&1 | FileCheck %s
+
+; CHECK: Available features for this target:
+; CHECK: 32bit - LA32 Basic Integer and Privilege Instruction Set.