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

[Bernd Schmidt <bschmidt@redhat.com>]

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?

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?


Bernd