[clang] [clang] fix uefi target for aarch64 & x86_64 (PR #120632)

[Roland McGrath via cfe-commits]

================
@@ -820,43 +820,6 @@ class LLVM_LIBRARY_VISIBILITY X86_64TargetInfo : public X86TargetInfo {
   }
 };
 
-// x86-64 UEFI target
-class LLVM_LIBRARY_VISIBILITY UEFIX86_64TargetInfo
-    : public UEFITargetInfo<X86_64TargetInfo> {
-public:
-  UEFIX86_64TargetInfo(const llvm::Triple &Triple, const TargetOptions &Opts)
-      : UEFITargetInfo<X86_64TargetInfo>(Triple, Opts) {
-    this->TheCXXABI.set(TargetCXXABI::Microsoft);
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frobtech wrote:

UEFI has nothing to say about C++. It is an ABI spec that is described in terms of C. It says nothing about what code not directly interacting with UEFI ABIs might do.

That said, heretofore the only way to compile code directly for the UEFI ABI is to use the Windows target.  Thus, the expectations for existing source code that targets UEFI directly via Clang is to get all the ABI and API details of whatever language as the Windows target does them (except of course for the actual OS-specific aspects per se).  So for all "ABI choice" things, the sensible default for UEFI target on $ARCH is what the Windows target for $ARCH does.  This goes for what type long is, what C++ ABI to use, alignment quirks, etc.  It also goes for language API details/extensions that are not directly related to Windows per se, such as the various pragmas for PECOFF-specific features or Windows spellings of generic extensions (e.g. `-fms-extensions` stuff).

Once we have the UEFI targets on their feet, we'll want to look into offering flexibility to make non-default choices (i.e. choices different from Windows) for various ABI and API things.  But we can get to that later.

https://github.com/llvm/llvm-project/pull/120632