From: David Edelsohn <dje@watson.ibm.com>
To: Holger Bettag <hobold@Informatik.Uni-Bremen.DE>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
Kaoru Fukui <k_fukui@highway.ne.jp>,
Jean-Jacques.Levy@inria.fr,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
linuxppc-dev@lists.linuxppc.org
Subject: Re: __divdi3 unresolved + X pointer blocks
Date: Thu, 05 Jul 2001 11:29:01 -0400 [thread overview]
Message-ID: <200107051529.LAA29124@makai.watson.ibm.com> (raw)
In-Reply-To: Message from Holger Bettag <hobold@Informatik.Uni-Bremen.DE> of "05 Jul 2001 13:48:41 +0200." <8xr8vv61gm.fsf@s62.informatik.uni-bremen.de>
>>>>> Holger Bettag writes:
Holger> And: 'arithmetic shift right' is _not_ the same as signed divide, because the
Holger> rounding of negative operands or results may differ.
Exactly! For a 32-bit operand, a signed divide by an exact power
of two is equivalent to:
srawi
addze
For synthesized 64-bit operand, this is more complicated, which is
why GCC calls __divdi3 instead of open-coding it.
Will all of the people posting INCORRECT information about logical
versus arithmetic shift and converting signed divide into shift, please
stop. You are propagating bad information.
If one converts a divide into a shift, one is assuming more
information than one has informed the compiler.
This whole discussion thread is confusing two issues. If one
wants GCC to produce a shift, one needs to write the code so that GCC
safely can convert the divide into a shift. If the shift is not safe,
then the Linux kernel needs to provide __divdi3.
The two suggestions of writing the divide as a shift and linking
with libgcc.a BOTH ARE WRONG.
David
** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/
next prev parent reply other threads:[~2001-07-05 15:29 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-07-02 20:31 __divdi3 unresolved + X pointer blocks Jean-Jacques Levy
2001-07-02 14:51 ` Kaoru Fukui
2001-07-05 1:06 ` Jean-Jacques Levy
2001-07-04 20:05 ` Benjamin Herrenschmidt
2001-07-05 5:50 ` Kernel crash with BenH' tree (was: Re: __divdi3...) Michel Lanners
2001-07-08 22:34 ` __divdi3 unresolved + X pointer blocks Jean-Jacques Levy
2001-07-04 22:22 ` David Edelsohn
2001-07-04 22:24 ` Kaoru Fukui
2001-07-04 22:45 ` Kaoru Fukui
2001-07-04 23:18 ` David Edelsohn
2001-07-05 8:13 ` Geert Uytterhoeven
2001-07-05 8:44 ` Kaoru Fukui
2001-07-05 11:48 ` Holger Bettag
2001-07-05 15:29 ` David Edelsohn [this message]
2001-07-05 16:55 ` Michael Schmitz
[not found] <20010711010133.Postino-018213@smtp01.highway.ne.jp>
2001-07-10 16:41 ` Kaoru Fukui
-- strict thread matches above, loose matches on Subject: below --
2001-07-02 14:55 Kaoru Fukui
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