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From: Tim Deegan <tim@xen.org>
To: Jan Beulich <JBeulich@suse.com>
Cc: Ian Campbell <Ian.Campbell@eu.citrix.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Subject: Re: [PATCH 2/2] arm64: fix fls()
Date: Thu, 22 Jan 2015 15:29:07 +0100	[thread overview]
Message-ID: <20150122142907.GG97314@deinos.phlegethon.org> (raw)
In-Reply-To: <54C111A802000078000582E3@mail.emea.novell.com>

At 14:05 +0000 on 22 Jan (1421931912), Jan Beulich wrote:
> >>> On 22.01.15 at 15:01, <tim@xen.org> wrote:
> > At 13:38 +0000 on 22 Jan (1421930334), Jan Beulich wrote:
> >> It using CLZ on a 64-bit register while specifying the input operand as
> >> only 32 bits wide is wrong: An operand intentionally shrunk down to 32
> >> bits at the source level doesn't imply respective zero extension also
> >> happens at the machine instruction level, and hence the wrong result
> >> could get returned.
> >> 
> >> Add suitable inline assembly abstraction so that the function can
> >> remain shared between arm32 and arm64.
> > 
> > Would casting the asm arguments to unsigned long DTRT withuot
> > needing #ifdefs?
> 
> The input one - yes, I think so. But it wouldn't be me to introduce
> such (suspicious) casts, and I also think it is better to use 32-bit
> instruction variants where possible (which quite certainly won't
> perform worse than the 64-bit ones, but may perform better).

Ah, I had misunderstood - I thought the new runes were just enforcing
correct extension of the arguments.  Using a 32-bit instruction does
indeed sound better.

Tim.

  reply	other threads:[~2015-01-22 14:29 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-22 13:26 [PATCH 0/2] fls() / ffs() adjustments Jan Beulich
2015-01-22 13:38 ` [PATCH 1/2] make fls() and ffs() consistent across architectures Jan Beulich
2015-01-23 14:38   ` Andrew Cooper
2015-01-23 14:53     ` Jan Beulich
2015-01-22 13:38 ` [PATCH 2/2] arm64: fix fls() Jan Beulich
2015-01-22 14:01   ` Tim Deegan
2015-01-22 14:05     ` Jan Beulich
2015-01-22 14:29       ` Tim Deegan [this message]
2015-01-22 14:59 ` [PATCH 0/2] fls() / ffs() adjustments Ian Campbell

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