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Re: [PATCH][RTL-ree] PR rtl-optimization/68194: Restrict copy instruction in presence of conditional moves
- From: Kyrill Tkachov <kyrylo dot tkachov at arm dot com>
- To: Bernd Schmidt <bschmidt at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Cc: Jeff Law <law at redhat dot com>
- Date: Tue, 17 Nov 2015 09:08:36 +0000
- Subject: Re: [PATCH][RTL-ree] PR rtl-optimization/68194: Restrict copy instruction in presence of conditional moves
- Authentication-results: sourceware.org; auth=none
- References: <5649E333 dot 4090904 at arm dot com> <564A2339 dot 3030308 at redhat dot 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