[PATCH] D100050: [AIX] Remove unused vector registers from allocation order in the default AltiVec ABI

[Jinsong Ji via Phabricator via llvm-commits]

jsji added a comment.

Thanks.

> 1. Applying the fix in getReservedRegs //should// work, however, removing from the allocation order is technically more correct since they are not actually reserved for any use. Instead they cannot be used at all.

I am not sure why it is *technically more correct* to change allocation order instead of *reservering reserved reg in designed API*.

I believe these regs are exactly what *Reserved Register* defines.

According to AIX ABI: https://www.ibm.com/docs/en/aix/7.2?topic=concepts-aix-vector-programming

  VR20:31	Reserved (default mode)
  Nonvolatile (extended ABI mode)
  
  When the default Vector enabled mode is used, these registers are reserved and must not be used.
  In the extended ABI vector enabled mode, these registers are nonvolatile and their values are preserved across function calls

And the `getReservedRegs` API is:

  /// Returns a bitset indexed by physical register number indicating if a
  /// register is a special register that has particular uses and should be
  /// considered unavailable at all times, e.g. stack pointer, return address.
  /// A reserved register:
  /// - is not allocatable
  /// - is considered always live
  /// - is ignored by liveness tracking
  /// It is often necessary to reserve the super registers of a reserved
  /// register as well, to avoid them getting allocated indirectly. You may use
  /// markSuperRegs() and checkAllSuperRegsMarked() in this case.
  virtual BitVector getReservedRegs(const MachineFunction &MF) const = 0;

Not that my understanding of `reserved for any use` is that compiler will work well if there is some special (or unexpected) use of these regs,
but it does not mean that you *HAVE* to have some use in compiler...

> 2. I am concerned about the different register subclasses that we can accidentally omit using the `getReservedRegs`

I don't think this is not a good reason. We should be able to reserve what we want here. If there is bug preventing us to do so, we should fix it.
For missing VFRC in original implementation, that is a bug , we should be able to fix it by marking all alias regs as reserved.


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https://reviews.llvm.org/D100050