On Fri, May 13, 2011 at 07:55:44AM +0200, Piotr Wyderski wrote:
> Jakub Jelinek wrote:
>
> > /* X86_TUNE_USE_INCDEC */
> > ~(m_PENT4 | m_NOCONA | m_CORE2I7 | m_GENERIC | m_ATOM),
> >
> > So, if you say -mtune=bdver1 or -mtune=k8, it will generate incl,
> > if addl is better (e.g. on Atom incl is very bad compared to addl $1),
> > it will generate it.
>
> Why is lock inc/dec worse than add/sub on Core2I7?
> The only difference I know of is the way the carry flag
> is handled.
Yeah, and that is exactly the problem, instructions that only sets a subset
of the flags is problematic.
See e.g. Intel's 248966.pdf, 3.5.1.1 "Use of the INC and DEC Instructions".
Jakub
