[Object] Add some more LoongArch support

[xen0n]

Summary

Add ELFObjectFileBase::getLoongArchFeatures, and return the proper ELF
relative reloc type for LoongArch.

[SixWeining]

llvm/lib/Object/ELF.cpp

226–227

Seems this can be tested in llvm/unittests/Object/ELFObjectFileTest.cpp:

// ELF relative relocation type test.
TEST(ELFObjectFileTest, RelativeRelocationTypeTest) {
  EXPECT_EQ(ELF::R_CKCORE_RELATIVE, getELFRelativeRelocationType(ELF::EM_CSKY));
}

or in llvm/unittests/Object/ELFTest.cpp:

TEST(ELFTest, getELFRelativeRelocationType) {
  EXPECT_EQ(ELF::R_VE_RELATIVE, getELFRelativeRelocationType(EM_VE));
}

llvm/lib/Object/ELFObjectFile.cpp

344

Maybe similar tests could be added like RISCV in D42629.

[xen0n]

rebase and add to unit test case

[xen0n]

Remove llvm-objdump's now unnecessary --mattr arguments in LoongArch
MC tests' RUN lines, like what D42629 did and @SixWeining suggested.

[SixWeining]

LGTM. Thanks.

[MaskRay]

LGTM.

llvm/lib/Object/ELFObjectFile.cpp

353

It's only two features so don't bother with [[fallthrough]] at present. this comment is unnecessary.

[xen0n]

Rebase and shorten code to avoid explanatory comment on why duplicating one line of code.

[xen0n]

llvm/lib/Object/ELFObjectFile.cpp

353

The comment was there explaining why code was duplicated instead of the shorter and arguably equally easy-to-understand approach. If it is removed but the duplicated AddFeature remained, then different people's coding style preferences could clash and I wanted to avoid such unnecessary debates.

Since you asked, I've thus removed the comment and chose to [[fallthrough]] instead. It's only one line after all, and I believe even programmers so used to functional style code must retain some control-flow deciphering skills to navigate in large C++ codebase in the first place...

llvm/include/llvm/Object/ELFObjectFile.h

 class ELFObjectFileBase : public ObjectFile {
   friend class ELFRelocationRef;
   friend class ELFSectionRef;
   friend class ELFSymbolRef;
 
   SubtargetFeatures getMIPSFeatures() const;
   SubtargetFeatures getARMFeatures() const;
   SubtargetFeatures getRISCVFeatures() const;
+  SubtargetFeatures getLoongArchFeatures() const;
 
   StringRef getAMDGPUCPUName() const;
 
 protected:
   ELFObjectFileBase(unsigned int Type, MemoryBufferRef Source);
 
   virtual uint64_t getSymbolSize(DataRefImpl Symb) const = 0;
   virtual uint8_t getSymbolBinding(DataRefImpl Symb) const = 0;

llvm/lib/Object/ELF.cpp

uint32_t llvm::object::getELFRelativeRelocationType(uint32_t Machine) {
   case ELF::EM_CSKY:
     return ELF::R_CKCORE_RELATIVE;
   case ELF::EM_VE:
     return ELF::R_VE_RELATIVE;
   case ELF::EM_AMDGPU:
     break;
   case ELF::EM_BPF:
     break;
+  case ELF::EM_LOONGARCH:
+    return ELF::R_LARCH_RELATIVE;
   default:
     break;
   }
   return 0;
 }
 
 StringRef llvm::object::getELFSectionTypeName(uint32_t Machine, unsigned Type) {
   switch (Machine) {

llvm/lib/Object/ELFObjectFile.cpp

while (!Arch.empty()) {
       Arch = Arch.drop_until([](char c) { return c == '_' || c == '\0'; });
       Arch = Arch.drop_while([](char c) { return c == '_'; });
     }
   }
 
   return Features;
 }
 
+SubtargetFeatures ELFObjectFileBase::getLoongArchFeatures() const {
+  SubtargetFeatures Features;
+
+  switch (getPlatformFlags() & ELF::EF_LOONGARCH_ABI_MODIFIER_MASK) {
+  case ELF::EF_LOONGARCH_ABI_SOFT_FLOAT:
+    break;
+  case ELF::EF_LOONGARCH_ABI_DOUBLE_FLOAT:
+    Features.AddFeature("d");
+    // D implies F according to LoongArch ISA spec.
+    // It's only two features so don't bother with [[fallthrough]] at present.
+    Features.AddFeature("f");
+    break;
+  case ELF::EF_LOONGARCH_ABI_SINGLE_FLOAT:
+    Features.AddFeature("f");
+    break;
+  }
+
+  return Features;
+}
+
 SubtargetFeatures ELFObjectFileBase::getFeatures() const {
   switch (getEMachine()) {
   case ELF::EM_MIPS:
     return getMIPSFeatures();
   case ELF::EM_ARM:
     return getARMFeatures();
   case ELF::EM_RISCV:
     return getRISCVFeatures();
+  case ELF::EM_LOONGARCH:
+    return getLoongArchFeatures();
   default:
     return SubtargetFeatures();
   }
 }
 
 Optional<StringRef> ELFObjectFileBase::tryGetCPUName() const {
   switch (getEMachine()) {
   case ELF::EM_AMDGPU: