From: "Carlos O'Donell" <carlos@redhat.com>
To: Steve Ellcey <sellcey@mips.com>,
"Carlos O'Donell" <carlos@systemhalted.org>
Cc: "libc-ports@sourceware.org" <libc-ports@sourceware.org>
Subject: Re: [Patch, mips] Faster strcmp for mips
Date: Fri, 15 Nov 2013 18:59:00 -0000 [thread overview]
Message-ID: <52866F20.6070505@redhat.com> (raw)
In-Reply-To: <1384541575.2484.110.camel@ubuntu-sellcey>
On 11/15/2013 01:52 PM, Steve Ellcey wrote:
> On Fri, 2013-11-15 at 13:47 -0500, Carlos O'Donell wrote:
>> On Thu, Nov 14, 2013 at 4:23 PM, Steve Ellcey <sellcey@mips.com> wrote:
>>> This means it could be loading bytes beyond the end of the strings being
>>> compared but it looks like other architecture specific strcmp functions
>>> are also doing this optimization and the newlib version of strcmp also does
>>> this.
>>
>> I thought that doing so was dangerous? I'm pretty sure we've been trying
>> to fix such "load bytes beyond the end of the string" issues because you
>> could have a string that straddles a page boundary with the next page
>> unmapped and such an optimized routine would fault on a read from the
>> unmapped page.
>>
>> How do you plan to fix that?
>>
>> Cheers,
>> Carlos.
>
> I think the assumption has been that if an aligned word contains at
> least one byte of the string, it is OK to read that entire word. I
> don't think a single aligned word (4 or 8 bytes depending on the
> architecture) can straddle a page boundary. If the word was unaligned,
> then I think it could straddle a page boundary and you could get into
> trouble.
That is obviously correct because the aligned word itself wouldn't
cross the the page boundary. My only comment here is that reading
past the end of the string is a dangerous operation unless you take
care to ensure it doesn't cross a page boundary. Given that you've
clarified that it's only ever a an aligned word load, that can't
possibly cross a page boundary so you're safe.
Sorry for the noise.
Cheers,
Carlos.
next prev parent reply other threads:[~2013-11-15 18:59 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-14 21:25 Steve Ellcey
2013-11-14 23:14 ` Ondřej Bílka
2013-11-15 18:21 ` Steve Ellcey
2013-11-15 19:02 ` Ondřej Bílka
2013-11-18 23:50 ` Steve Ellcey
2013-11-19 2:32 ` Ondřej Bílka
2013-11-15 18:48 ` Carlos O'Donell
2013-11-15 18:53 ` Steve Ellcey
2013-11-15 18:59 ` Carlos O'Donell [this message]
2015-02-19 7:25 ` Matt Turner
2015-02-19 18:12 ` Steve Ellcey
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