linux-kernel - RE: [PATCH v4 2/5] nds32: Support FP emulation

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Message-ID: <3dcd9764570e4565911b74b08513e2d3@AcuMS.aculab.com>
Date:   Mon, 26 Nov 2018 10:18:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Vincent Chen' <vincentc@...estech.com>
CC:     "green.hu@...il.com" <green.hu@...il.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Nick Chun-Ming Hu(?????????)" <nickhu@...estech.com>,
        "deanbo422@...il.com" <deanbo422@...il.com>
Subject: RE: [PATCH v4 2/5] nds32: Support FP emulation

From: Vincent Chen
> Sent: 26 November 2018 01:23
> On Fri, Nov 23, 2018 at 06:53:37PM +0800, David Laight wrote:
> > From: Vincent Chen
> > > Sent: 22 November 2018 03:15
> > >
> > > The Andes FPU coprocessor does not support denormalized number handling.
> > > According to the specification, FPU generates a denorm input exception
> > > that requires the kernel to deal with this instrution operation when it
> > > encounters denormalized operands. Hence an nds32 FPU ISA emulator in the
> > > kernel is required to meet requirement.
> >
> > What does the FPU generate for results near zero?
> 
> 1. The calculation result is a denormalized number
> Depending on the state of underflow trap, the FPU will raise an underflow
> exception or flash the result to zero.
> 
> 2. One of the operands is a denormalized number
> Depending on the state of the flash-to-zero mode, the FPU will raise a
> denormalized input exception, which is a specific exception of the nds FPU,
> or directly treats the operand as 0.

You didn't mention (1) in the commit message.

It ought to be possibly for the exception routine to rescale the values,
re-issue the same FP instruction, then scale the result.

	David

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