[PowerPC][MC] Set 'future' as default CPU type of AIX

[qiucf]

Summary

For 64-bit Power on Linux (both LE and BE), ELFObjectFile returns 'future' as default CPU type if mcpu is not specified, so that all necessary features will be enabled in MC.

While for XCOFF, the default CPU type is always null, which makes tools like llvm-objdump not able to recognize prefixed instructions, unless specifying --mcpu=pwr10 or --mattr=+prefix-instrs manually.

[nemanjai]

LGTM. Whatever the choice is wrt. 32-bit mode, please just add a comment to the function explaining it.

llvm/lib/Object/XCOFFObjectFile.cpp

1223

Why do we restrict this to 64-bit? Just because we do the same for ELF?

[qiucf]

llvm/lib/Object/XCOFFObjectFile.cpp

1223

Yes. I'm also not sure is there some specific reason why PPC ELF does this only for 64-bit (since added by D127824, maybe @MaskRay ). After I tested things on 32-bit is okay, the 64-bit constraint will be removed.

[MaskRay]

llvm/lib/Object/XCOFFObjectFile.cpp

1223

Thanks for the suggestion.

I think I restricted D155089 to 64-bit because I didn't test the 32-bit behavior.. I sent https://reviews.llvm.org/D155114 to extend the behavior to 32-bit.

[qiucf]

Support XCOFF32

llvm/include/llvm/Object/XCOFFObjectFile.h

public:
   StringRef getStringTable() const;
 
   const XCOFF::SymbolAuxType *getSymbolAuxType(uintptr_t AuxEntryAddress) const;
 
   static uintptr_t getAdvancedSymbolEntryAddress(uintptr_t CurrentAddress,
                                                  uint32_t Distance);
 
   static bool classof(const Binary *B) { return B->isXCOFF(); }
+
+  std::optional<StringRef> tryGetCPUName() const override;
 }; // XCOFFObjectFile
 
 typedef struct {
   uint8_t LanguageId;
   uint8_t CpuTypeId;
 } CFileLanguageIdAndTypeIdType;
 
 struct XCOFFSymbolEntry32 {

llvm/lib/Object/XCOFFObjectFile.cpp

 }
 
 Expected<std::unique_ptr<ObjectFile>>
 ObjectFile::createXCOFFObjectFile(MemoryBufferRef MemBufRef,
                                   unsigned FileType) {
   return XCOFFObjectFile::create(FileType, MemBufRef);
 }
 
+std::optional<StringRef> XCOFFObjectFile::tryGetCPUName() const {
+  return StringRef("future");
+}
+
 bool XCOFFSymbolRef::isFunction() const {
   if (!isCsectSymbol())
     return false;
 
   if (getSymbolType() & FunctionSym)
     return true;
 
   Expected<XCOFFCsectAuxRef> ExpCsectAuxEnt = getXCOFFCsectAuxRef();