bug in longlong.h for aarch64 sub_ddmmss

[Torbjörn Granlund]

Vincent Lefevre <vincent at vinc17.net> writes:

  On 2020-06-17 11:04:26 +0200, Niels Möller wrote:
  > tg at gmplib.org (Torbjörn Granlund) writes:
  > 
  > > The two - signs ought to be ~, I think.  Let me think a buit more about that.
  > 
  > If I remember ARM conventions correctly, subtract with carry is defined
  > as
  > 
  >   {cout, r} = a + ~b + cin

  This is a - b - borrow, where the borrow is the complement of the
  carry bit.

Niels' definition is important as it captures the similarity with
addition.  It is indeed how the instructions are described in the vendor
manuals.

-- 
Torbjörn
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