[clang] [llvm] [clang][llvm][PPC] Implement NoCallerSavedRegs attribute (PR #174320)

[via cfe-commits]

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@@ -6007,10 +6007,13 @@ The user can call functions specified with the 'no_caller_saved_registers'
 attribute from an interrupt handler without saving and restoring all
 call-clobbered registers.
 
-Functions specified with the 'no_caller_saved_registers' attribute should only
+On X86, functions specified with the 'no_caller_saved_registers' attribute should only
 call other functions with the 'no_caller_saved_registers' attribute, or should be
 compiled with the '-mgeneral-regs-only' flag to avoid saving unused non-GPR registers.
 
+On PowerPC, functions specified with the 'no_caller_saved_registers' attribute may not
+return a value as the return register will be restored and thus cannot hold a value.
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Luminyx1 wrote:

The intent was to allow for use cases where even modification of `r3` is considered intrusive (similar to an interrupt handler, but with a standard return routine such that `__attribute__((interrupt))` is not appropriate). What are the semantics of `preserve_most`/`preserve_all`? Would it be appropriate to implement `preserve_most` as saving general registers but not float registers (as x86 and ARM currently do), then `preserve_all` additionally saves float registers + the return register?

https://github.com/llvm/llvm-project/pull/174320