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

[Kyrill Tkachov]

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
>