[clang] [PowerPC] Add an alias for -mregnames so that full register names used in assembly. (PR #70255)

[Chen Zheng via cfe-commits]

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@@ -80,6 +80,7 @@ class LLVM_LIBRARY_VISIBILITY PPCTargetInfo : public TargetInfo {
   bool IsISA3_0 = false;
   bool IsISA3_1 = false;
   bool HasQuadwordAtomics = false;
+  bool FullRegisterNames = false;
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chenzheng1030 wrote:

Using a target feature bit for assembly printing seems not match other bits. IIUC, all the bits here should control the available instructions on a subtarget.

Could we use an option in `TargetMachine::Options::MCOptions`? this looks like the way where clang can accept an option and control the code generation in the backend. The current option `Options.MCOptions.PreserveAsmComments` seems a little similar with this functionality.

https://github.com/llvm/llvm-project/pull/70255