From: Michael Ellerman <mpe@ellerman•id.au>
To: Segher Boessenkool <segher@kernel•crashing.org>
Cc: linuxppc-dev@ozlabs•org, chandan@linux•ibm.com
Subject: Re: [PATCH v3] powerpc/64: Fix memcmp reading past the end of src/dest
Date: Tue, 26 Mar 2019 20:18:07 +1100 [thread overview]
Message-ID: <87va06ngds.fsf@concordia.ellerman.id.au> (raw)
In-Reply-To: <20190325175431.GJ3969@gate.crashing.org>
Segher Boessenkool <segher@kernel•crashing.org> writes:
> On Mon, Mar 25, 2019 at 11:33:56PM +1100, Michael Ellerman wrote:
>> Segher Boessenkool <segher@kernel•crashing.org> writes:
>> > On Fri, Mar 22, 2019 at 11:37:24PM +1100, Michael Ellerman wrote:
>> >> + clrldi r6,r4,(64-12) // r6 = r4 & 0xfff
>> >
>> > You can just write
>> > rlwinm r6,r4,0,0x0fff
>>
>> > if that is clearer? Or do you still want a comment with that :-)
>>
>> I don't think it's clearer doing a rotate of zero bits :)
>>
>> And yeah I'd probably still leave the comment, so I'm inclined to stick
>> with the clrldi?
>
> I always have to think what the clrldi etc. do exactly, while with rlwinm
> it is obvious. But yeah this may be different for other people who are
> used to different idiom.
Interesting, I'm the opposite. You know ppc assembler better than me so
I guess I just need to spend more time on it and embrace the zen of the
rotate instructions.
>> Would be nice if the assembler could support:
>>
>> andi r6, r4, 0x0fff
>>
>> And turn it into the rlwinm, or rldicl :)
>
> The extended mnemonics are *simple*, *one-to-one* mappings.
It would still be simple and 1:1, but would only be valid for certain
constants :)
> Having "andi. 6,4,0x0f0f" a valid insn, but an extended mnemonic "andi 6,4,0x0f0f"
> that is not (and the other way around for say 0xff0000ff) would violate that.
I agree that's a bit of a foot gun.
I'm not sure it's vastly more hostile though than `andi. 6,4,0xffff`
being valid but `andi. 6,4,0x1ffff` being not valid.
The assembler could print a nice error saying you need to use a
contiguous mask. And I mean how often do you andi. with a mask that
isn't contiguous?
> You could do some assembler macro, that can also expand to multiple insns
> where that is useful. Also one for loading constants, etc. The downside
> to that is you often do care how many insns are generated.
>
> Instead you could do a macro for only those cases that can be done with *one*
> insn. But that then is pretty restricted in use, and people have to learn
> what values are valid.
>
> I don't see a perfect solution.
Yeah, I guess a new `andi` instruction is the only real answer :)
cheers
next prev parent reply other threads:[~2019-03-26 9:19 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-22 12:37 [PATCH v3] powerpc/64: Fix memcmp reading past the end of src/dest Michael Ellerman
2019-03-22 18:29 ` Segher Boessenkool
2019-03-25 12:33 ` Michael Ellerman
2019-03-25 17:54 ` Segher Boessenkool
2019-03-26 9:18 ` Michael Ellerman [this message]
2019-03-26 18:39 ` Segher Boessenkool
2019-03-25 6:38 ` Chandan Rajendra
2019-03-25 6:39 ` Chandan Rajendra
2019-03-31 10:13 ` [v3] " Michael Ellerman
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