From: Kyrill Tkachov <kyrylo.tkachov@arm.com>
To: Bernd Schmidt <bschmidt@redhat.com>,
GCC Patches <gcc-patches@gcc.gnu.org>
Cc: Jeff Law <law@redhat.com>
Subject: Re: [PATCH][RTL-ree] PR rtl-optimization/68194: Restrict copy instruction in presence of conditional moves
Date: Tue, 17 Nov 2015 09:49:00 -0000 [thread overview]
Message-ID: <564AF80C.3010605@arm.com> (raw)
In-Reply-To: <564AEE94.3070708@arm.com>
On 17/11/15 09:08, Kyrill Tkachov wrote:
> 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.
Actually, I tried it out. And while a check reg_used_between_p fixed the testcase, it caused code quality regressions
on aarch64. Seems it's too aggressive in restricting ree.
I'll have a closer look.
Kyrill
>
> 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
>>
>
next prev parent reply other threads:[~2015-11-17 9:49 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-16 14:08 Kyrill Tkachov
2015-11-16 18:41 ` Bernd Schmidt
2015-11-17 9:08 ` Kyrill Tkachov
2015-11-17 9:49 ` Kyrill Tkachov [this message]
2015-11-17 10:17 ` Kyrill Tkachov
2015-11-17 12:10 ` Bernd Schmidt
2015-11-17 13:03 ` Kyrill Tkachov
2015-11-17 23:11 ` Bernd Schmidt
2015-11-18 9:11 ` Kyrill Tkachov
2015-11-19 10:28 ` Kyrill Tkachov
2015-11-20 1:41 ` Bernd Schmidt
2015-11-20 9:16 ` Kyrill Tkachov
2015-11-23 15:12 ` Kyrill Tkachov
2015-11-24 13:33 ` Kyrill Tkachov
2015-11-24 13:42 ` Bernd Schmidt
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