Re: [PATCH][RTL-ifcvt] PR rtl-optimization/68841: Make sure one basic block doesn't clobber CC reg usage of the other

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

On 18/12/15 13:16, Bernd Schmidt wrote:
> On 12/18/2015 02:07 PM, Kyrill Tkachov wrote:
> > In this PR we have a THEN and an ELSE block where one uses the condition
> > reg from the preceeding comparison
> > but the other block has an arithmetic operation that clobbers the CC reg.
> > ifcvt emits the latter first and dead code elimination later sees this
> > and eliminates the first comparison
> > because it sees that the CC reg is clobbered between the comparison and
> > its usage.
>
> >      (noce_try_cmove_arith): Check CC reg usage in both blocks
> >      and emit them in such an order so as not to clobber the CC reg
> >      before its use, if possible.
>
> Why is this done here? It looks to me like bbs_ok_for_cmove_arith is the function that already tries to sort out issues like this. Does it maybe just need to be extended to see clobbers?

I can do that. A prototype of that approach works for this testacase.
I'll test more extensively...

Thanks,
Kyrill

> Bernd