[PATCH] D33720: [PowerPC] Eliminate compares - add i64 sext/zext handling for SETNE

[Guozhi Wei via Phabricator via llvm-commits]

Carrot added a comment.

In https://reviews.llvm.org/D33720#789251, @Carrot wrote:

> In https://reviews.llvm.org/D33720#788925, @nemanjai wrote:
>
> > That's actually not what the SUBFE instruction does. As per the ISA:
> >
> >   subfe RT,RA,RB
> >   computes
> >   RT = ~(RA) + (RB) + CA
> >
> >
> > So in your example, it will be:
> >
> >   ~AC + XOR + CARRY
> >   ~(~0) + 0 + 0
> >
> >
> > Which of course will just be a zero.
>
>
> Then the document is wrong.
>
> Mathematically, subtraction is
>  RB - RA = RB + ~RA + 1
>  just like the description in instruction subf.
>
> A subtraction when considering borrowing from lower bit is
>  RB - RA - CA = RB + ~RA + 1 - CA = RB + ~RA + !CA


I wrote a small program to test it, you are right, the instruction subfe does
RT = ~(RA) + (RB) + CA

so
RT = (RB) + ~(RA) + CA = (RB) + ~(RA) + 1 - 1 + CA = RB - RA + CA - 1
I can't understand the mathematical meaning of the right side. And how can it be used to implement high precision integer substraction.


Repository:
  rL LLVM

https://reviews.llvm.org/D33720