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Date: Fri, 17 Jun 2022 13:08:21 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: HAO CHEN GUI <guihaoc@linux.ibm.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>, David <dje.gcc@gmail.com>,
 "Kewen.Lin" <linkw@linux.ibm.com>, Peter Bergner <bergner@linux.ibm.com>
Subject: Re: [PATCH, rs6000] Use CC for BCD operations [PR100736]
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Hi!

On Fri, Jun 17, 2022 at 04:19:37PM +0800, HAO CHEN GUI wrote:
> On 16/6/2022 下午 7:24, Segher Boessenkool wrote:
> > You shouldn't need anything like this, bcdinvalid will work just fine if
> > written as bcdadd_ov (with vector of 0 as the second op)?
> 
> The vector of 0 is not equal to BCD 0, I think. The BCD number contains
> preferred sign (PS) bit. So all zeros itself is an invalid encoding. We may
> use bcdsub_ov with duplicated op to implement bcdinvalid.

For the machine, you should use 0x0c or 0x0f, sure.  But in RTL we can
do whatever we want.

But bcdsub is easier indeed, and we don't need to construct a 0 first
then.


Segher