[LLVMdev] Conceptual difference between "Unallocatable" and "Reserved" registers.

[Lei Mou]

Hi,

I'm writing to ask the differences between a "reserved" register and an
"unallocable" register. In X86 backend, for example, the stack pointer
register and instruction pointer are reserved but allocatable. In the
Doxygen document of function llvm::TargetRegisterInfo::getReservedRegs,
it says that a reserved register is one that *has particular uses and
should be considered unavailable at all times. *If it is unavailable at all
times, why not make such a reserved register unallocatable? Thanks!

Regards,
Lei Mou
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