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Re: PowerPC suboptimal "add with carry" optimization
David Edelsohn <dje.gcc@gmail.com> wrote on 2010/04/26 14:54:55:
>
> On Mon, Apr 26, 2010 at 8:21 AM, Joakim Tjernlund
> <joakim.tjernlund@transmode.se> wrote:
> > Manuel López-Ibáñez <lopezibanez@gmail.com> wrote on 2010/04/26 13:59:04:
> >> On 26 April 2010 09:13, Joakim Tjernlund <joakim.tjernlund@transmode.se> wrote:
> >> > Ian Lance Taylor <iant@google.com> wrote on 2010/04/25 20:07:03:
> >> >> Joakim Tjernlund <joakim.tjernlund@transmode.se> writes:
> >> >>
> >> >> > Noticed that gcc 4.3.4 doesn't optimize "add with carry" properly:
> >> >>
> >> >> Please file a missed-optimization report according to
> >> >> http://gcc.gnu.org/bugs/ . ?Thanks.
> >> >
> >> > I rather not, going through all that just for one odd report. I was hoping
> >> > an interested part would pick it up from my email.
> >>
> >> What is to be done besides what you have done here but in a more
> >> useful, structured manner? I am asking because we want to make things
> >> simple but not simpler than they become more complex for us.
> >
> > Lots of stuff to read on that page, lots of info to supply,
> > registering an account, specify all that system info and so on.
>
> You do not need to register an account to report a bug or request a
Just tried that and don't see that.
> feature enhancement. Some of the information can be left blank, but
> if we do not have information about the system and an example, we may
> not understand the request or may not be able to reproduce the problem
> and fix it.
Just go look at that page, it is so much info that if the bug isn't very importat
you quickly decide it is not worth it. How I am supposed to know what is need
or not without reading all that?
>
> >> Aren't you interested in someone fixing the bug?
> >
> > Sure or better yet, show me some way to restructure the C code so proper
> > asm code is generated.
>
> You referred to PowerPC patterns in GCC that use the carry
> instructions. Those patterns are optimizations for materializing a
> truthvalue in a GPR or performance a 64 bit arithmetic operation in 32
> bit mode. The PowerPC port of GCC does not recognize general carry
> arithmetic operations yet and track the carry bit.
OK, thanks. I know x86 do I had hope there was some way to do that i
PowerPC too.