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

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Message-ID: <20181126012310.GA26621@andestech.com>
Date:   Mon, 26 Nov 2018 09:23:10 +0800
From:   Vincent Chen <vincentc@...estech.com>
To:     David Laight <David.Laight@...LAB.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>
Subject: Re: [PATCH v4 2/5] nds32: Support FP emulation

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.


> If it doesn't generate denormalised results (but does detect them)
> then I assume it never generates FP values with exponent zero.
> (So the gap around zero is even larger than it would be if exponent
> zero values had a hidden bit.)
> So is it really worth doing anything other than treating
> denormalised values as zero?
> 
> 	David
>

Sorry, I can't fully understand what you mean, but we really have
encountered the above two cases in some testsuites such as glibc testsuite.
Hence we implement a FPU emulator in this commit which will recalculate the
instructions again by software to enhance the precision.
> -
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