[PATCH] D50004: [PowerPC] Emit xscpsgndp instead of xxlor when copying floating point scalar registers

[Nemanja Ivanovic via Phabricator via llvm-commits]

nemanjai added a comment.

In https://reviews.llvm.org/D50004#1182972, @inouehrs wrote:

> XSCPSGNDP has longer latency (6 cycles) than XXLOR (2 cycles) on POWER8 while it has higher throughput with the same latency on POWER9. So XXLOR is preferable for pre-P9.
>
> Also, the two instructions have different behavior for a denormal input value in my understanding; XSCPSGNDP does normalization but XXLOR does not. Does this difference matter?


Yes, I agree that we should limit this to Power9. Does the comment about normalization only pertain to ISA 2.07? The text from ISA 3.0 is:

  Bit 0 of VSR[XT] is set to the contents of bit 0 of VSR[XA].
  Bits 1:63 of VSR[XT] are set to the contents of bits 1:63 of VSR[XB].
  The contents of doubleword element 1 of VSR[XT] are undefined.

There is no mention of normalization.


https://reviews.llvm.org/D50004