Re: GCC-generated code and i386 condition codes behavior

[Robert Dewar <dewar at adacore dot com>]

Andi Kleen wrote:
> Robert Dewar <dewar@adacore.com> writes:
>
> > I must say I am a bit surprised that gcc never takes advantage
> > of the fact that inc and dec do not destroy the carry flag, this
> > is quite significant for some loops.
>
>
> A lot of this old wisdom is no longer true.
>
> inc and dec are to be generally avoided these days, because the
> partial changing of EFLAGS causes too strict dependencies on Intel P4
> cores and threefore slower execution. It's better to use add/sub
> $1,... instead, except if you're optimizing for code size.

actually as Geert points out, probably LEA is the right choice
when you don't want to affect the flags, though in this case
I guess you need some otherway to test for loop termination.
Not clear how you do that without destroying the carry flag.
I will do some benchmarks in this area to see how the old
wisdom holds :-)
> -Andi