Re: [PATCH][RTL-ree] PR rtl-optimization/68194: Restrict copy instruction in presence of conditional moves

[Kyrill Tkachov <kyrylo.tkachov@arm.com>]

Hi Bernd,

On 16/11/15 18:40, Bernd Schmidt wrote:
> On 11/16/2015 03:07 PM, Kyrill Tkachov wrote:
>
>> I've explained in the comments in the patch what's going on but the
>> short version is trying to change the destination of a defining insn
>> that feeds into an extend insn is not valid if the defining insn
>> doesn't feed directly into the extend insn. In the ree pass the only
>> way this can happen is if there is an intermediate conditional move
>> that the pass tries to handle in a special way. An equivalent fix
>> would have been to check on that path (when copy_needed in
>> combine_reaching_defs is true) that the state->copies_list vector
>> (that contains the conditional move insns feeding into the extend
>> insn) is empty.
>
> I ran this through gdb, and I think I see what's going on. For reference, here's a comment from the source:
>
>       /* Considering transformation of
>          (set (reg1) (expression))
>          ...
>          (set (reg2) (any_extend (reg1)))
>
>          into
>
>          (set (reg2) (any_extend (expression)))
>          (set (reg1) (reg2))
>          ...  */
>
> I was thinking that another possible fix would be to also check !reg_used_between_p for reg1 to ensure it's not used. I'm thinking this might be a little clearer - what is your opinion?

Yes, I had considered that as well. It should be equivalent. I didn't use !reg_used_between_p because I thought
it'd be more expensive than checking reg_overlap_mentioned_p since we must iterate over a number of instructions
and call reg_overlap_mentioned_p on each one. But I suppose this case is rare enough that it wouldn't make any
measurable difference.

Would you prefer to use !reg_used_between_p here?

>
> The added comment could lead to some confusion since it's placed in front of an existing if statement that also tests a different condition. Also, if we go with your fix,
>
>> +      || !reg_overlap_mentioned_p (tmp_reg, SET_SRC (PATTERN (cand->insn))))
>
> Shouldn't this really be !rtx_equal_p?
>

Maybe, will it behave the right way if the two regs have different modes or when subregs are involved?
(will we even hit such a case in this path?)

Thanks,
Kyrill
>
> Bernd
>