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

[Lei Mou]

Thank you very much for the information!

Yours Sincerely,
Lei Mou

On Wed, Apr 18, 2012 at 11:03 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:

>
> On Apr 17, 2012, at 9:09 PM, Lei Mou wrote:
>
> 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!
>
>
> Registers are partitioned into three disjoint sets:
>
> 1. Allocatable registers.
> 2. Reserved registers.
> 3. The rest: Unallocatable, unreserved registers.
>
> The set of reserved registers is not known to TableGen, and it can vary
> for different sub-targets, and even different functions. The reserved
> registers for a function are computed by the TRI::getReservedRegs(MF) hook.
>
> A register is allocatable if it belongs to at least one allocatable
> register class AND it is not reserved.
>
> See RegisterClassInfo.h, in particular the isAllocatable() function.
>
> The third set of unallocatable, unreserved registers usually only contains
> the flags. It is different from the reserved registers because the register
> allocator is still tracking liveness of these registers. The reserved
> registers are mostly ignored for liveness tracking.
>
> /jakob
>
>
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