From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ralf Baechle <ralf@linux-mips.org>
Cc: David Howells <dhowells@redhat.com>,
mingo@elte.hu, tglx@linutronix.de, linux-arch@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()
Date: Fri, 26 Mar 2010 11:03:09 -0700 (PDT) [thread overview]
Message-ID: <alpine.LFD.2.00.1003261100440.3721@i5.linux-foundation.org> (raw)
In-Reply-To: <20100326175827.GD20055@linux-mips.org>
On Fri, 26 Mar 2010, Ralf Baechle wrote:
>
> My trusty old 486 book [1] in the remarks about the BSF instruction:
>
> "The documentation on the 80386 and 80486 states that op1 is undefined if
> op2 is 0. In reality the 80386 will leave the value in op1 unchanged.
> The first versions of the 80486 will change op1 to an undefined value.
> Later version again will leave it unchanged."
>
> [1] Die Intel Familie in German language, by Robert Hummel, 1992
Ok, that explains my memory of us having tried this, at least.
But I do wonder if any of the people working for Intel could ask the CPU
architects whether we could depend on the "don't write" for 64-bit mode.
If AMD already documents the don't-touch semantics, and if Intel were to
be ok with documenting it for their 64-bit capable CPU's, we wouldn't then
need to rely on undefined behavior.
Linus
next prev parent reply other threads:[~2010-03-26 18:08 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <4BACCB4E.7010108@draigBrady.com>
2010-03-26 14:42 ` [PATCH 1/3] X86: Optimise fls(), ffs() and fls64() David Howells
2010-03-26 14:42 ` [PATCH 2/3] Adjust the comment on get_order() to describe the size==0 case David Howells
2010-03-26 14:42 ` [PATCH 3/3] Optimise get_order() David Howells
2010-03-26 14:42 ` David Howells
2010-03-26 17:23 ` [PATCH 1/3] X86: Optimise fls(), ffs() and fls64() Linus Torvalds
2010-03-26 17:37 ` Scott Lurndal
2010-03-26 17:42 ` Linus Torvalds
2010-04-06 13:57 ` Jamie Lokier
2010-04-06 14:40 ` Linus Torvalds
2010-03-26 17:42 ` David Howells
2010-03-26 17:45 ` Linus Torvalds
2010-03-26 17:58 ` Ralf Baechle
2010-03-26 18:03 ` Linus Torvalds [this message]
2010-03-26 18:16 ` Matthew Wilcox
2010-04-06 13:30 ` Matthew Wilcox
2010-04-14 11:49 ` David Howells
2010-04-14 14:30 ` Avi Kivity
2010-04-15 8:48 ` David Howells
2010-04-15 8:49 ` Avi Kivity
2010-04-15 11:41 ` Jamie Lokier
2010-03-26 17:52 ` Matthew Wilcox
2010-04-14 13:13 ` David Howells
2010-01-13 19:39 David Howells
2010-01-13 20:15 ` Geert Uytterhoeven
2010-01-13 21:59 ` David Howells
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