linux-kernel - Re: [PATCH v3 11/16] MIPS: math-emu: <MADDF|MSUBF>.<D|S>: Fix NaN propagation

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Date:   Mon, 24 Jul 2017 11:24:12 +0100
From:   James Hogan <james.hogan@...tec.com>
To:     Aleksandar Markovic <aleksandar.markovic@...rk.com>
CC:     <linux-mips@...ux-mips.org>,
        Aleksandar Markovic <aleksandar.markovic@...tec.com>,
        Miodrag Dinic <miodrag.dinic@...tec.com>,
        Goran Ferenc <goran.ferenc@...tec.com>,
        Douglas Leung <douglas.leung@...tec.com>,
        <linux-kernel@...r.kernel.org>,
        Paul Burton <paul.burton@...tec.com>,
        Petar Jovanovic <petar.jovanovic@...tec.com>,
        Raghu Gandham <raghu.gandham@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH v3 11/16] MIPS: math-emu: <MADDF|MSUBF>.<D|S>: Fix NaN
 propagation

On Fri, Jul 21, 2017 at 04:09:09PM +0200, Aleksandar Markovic wrote:
> From: Aleksandar Markovic <aleksandar.markovic@...tec.com>
> 
> Fix the cases of <MADDF|MSUBF>.<D|S> when any of three inputs is any
> NaN. Correct behavior of <MADDF|MSUBF>.<D|S> fd, fs, ft is following:
> 
>   - if any of inputs is sNaN, return a sNaN using following rules: if
>     only one input is sNaN, return that one; if more than one input is
>     sNaN, order of precedence for return value is fd, fs, ft
>   - if no input is sNaN, but at least one of inputs is qNaN, return a
>     qNaN using following rules: if only one input is qNaN, return that
>     one; if more than one input is qNaN, order of precedence for
>     return value is fd, fs, ft
> 
> The previous code contained handling of some above cases, but not all.
> Also, such handling was scattered into various cases of
> "switch (CLPAIR(xc, yc))" statement and elsewhere. With this patch,
> this logic is placed in one place, and "switch (CLPAIR(xc, yc))" is
> significantly simplified.
> 
> The relevant example:
> 
> MADDF.S fd,fs,ft:
>   If fs contains qNaN1, ft contains qNaN2, and fd contains qNaN3, fd
>   is going to contain qNaN3 (without this patch, it used to contain
>   qNaN1).
> 

Fixes: e24c3bec3e8e ("MIPS: math-emu: Add support for the MIPS R6 MADDF FPU instruction")
Fixes: 83d43305a1df ("MIPS: math-emu: Add support for the MIPS R6 MSUBF FPU instruction")

> Signed-off-by: Miodrag Dinic <miodrag.dinic@...tec.com>
> Signed-off-by: Goran Ferenc <goran.ferenc@...tec.com>
> Signed-off-by: Aleksandar Markovic <aleksandar.markovic@...tec.com>

If backported, I suspect commits:
6162051e87f6 ("MIPS: math-emu: Unify ieee754sp_m{add,sub}f")
and
d728f6709bcc ("MIPS: math-emu: Unify ieee754dp_m{add,sub}f")
in 4.7 will require manual backporting between 4.3 and 4.7 (due to
separation of maddf/msubf before that point), so I suppose tagging
stable 4.7+ and backporting is best (assuming you consider this fix
worth backporting).

> ---
>  arch/mips/math-emu/dp_maddf.c | 71 ++++++++++++++-----------------------------
>  arch/mips/math-emu/sp_maddf.c | 69 ++++++++++++++---------------------------
>  2 files changed, 46 insertions(+), 94 deletions(-)
> 
> diff --git a/arch/mips/math-emu/dp_maddf.c b/arch/mips/math-emu/dp_maddf.c
> index caa62f2..4f2e783 100644
> --- a/arch/mips/math-emu/dp_maddf.c
> +++ b/arch/mips/math-emu/dp_maddf.c
> @@ -48,52 +48,35 @@ static union ieee754dp _dp_maddf(union ieee754dp z, union ieee754dp x,
>  
>  	ieee754_clearcx();
>  
> -	switch (zc) {
> -	case IEEE754_CLASS_SNAN:
> -		ieee754_setcx(IEEE754_INVALID_OPERATION);
> -		return ieee754dp_nanxcpt(z);
> -	case IEEE754_CLASS_DNORM:
> -		DPDNORMZ;
> -	/* QNAN and ZERO cases are handled separately below */
> -	}
> -
> -	switch (CLPAIR(xc, yc)) {
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_NORM, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_SNAN):
> -		return ieee754dp_nanxcpt(y);
> -
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_ZERO):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_NORM):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_DNORM):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_INF):
> -		return ieee754dp_nanxcpt(x);
> -
> -	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_NORM, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_QNAN):
> +	/* handle the cases when at least one of x, y or z is a NaN */
> +	if (((xc == IEEE754_CLASS_SNAN) || (xc == IEEE754_CLASS_QNAN)) ||
> +	    ((yc == IEEE754_CLASS_SNAN) || (yc == IEEE754_CLASS_QNAN)) ||
> +	    ((zc == IEEE754_CLASS_SNAN) || (zc == IEEE754_CLASS_QNAN))) {

This condition basically covers all of the cases below. Any particular
reason not to skip it ...

> +		/* order of precedence is z, x, y */
> +		if (zc == IEEE754_CLASS_SNAN)
> +			return ieee754dp_nanxcpt(z);
> +		if (xc == IEEE754_CLASS_SNAN)
> +			return ieee754dp_nanxcpt(x);
> +		if (yc == IEEE754_CLASS_SNAN)
> +			return ieee754dp_nanxcpt(y);
> +		if (zc == IEEE754_CLASS_QNAN)
> +			return z;
> +		if (xc == IEEE754_CLASS_QNAN)
> +			return x;
>  		return y;

... and make this return conditional on (yc == IEEE754_CLASS_QNAN)?

Same for sp_maddf.c too.

Otherwise:
Reviewed-by: James Hogan <james.hogan@...tec.com>

Cheers
James

> +	}
>  
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_ZERO):
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_NORM):
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_DNORM):
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_INF):
> -		return x;
> +	if (zc == IEEE754_CLASS_DNORM)
> +		DPDNORMZ;
> +	/* ZERO z cases are handled separately below */
>  
> +	switch (CLPAIR(xc, yc)) {
>  
>  	/*
>  	 * Infinity handling
>  	 */
>  	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_ZERO):
>  	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_INF):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
>  		ieee754_setcx(IEEE754_INVALID_OPERATION);
>  		return ieee754dp_indef();
>  
> @@ -102,8 +85,6 @@ static union ieee754dp _dp_maddf(union ieee754dp z, union ieee754dp x,
>  	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_NORM):
>  	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_DNORM):
>  	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_INF):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
>  		return ieee754dp_inf(xs ^ ys);
>  
>  	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_ZERO):
> @@ -120,25 +101,19 @@ static union ieee754dp _dp_maddf(union ieee754dp z, union ieee754dp x,
>  		DPDNORMX;
>  
>  	case CLPAIR(IEEE754_CLASS_NORM, IEEE754_CLASS_DNORM):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
> -		else if (zc == IEEE754_CLASS_INF)
> +		if (zc == IEEE754_CLASS_INF)
>  			return ieee754dp_inf(zs);
>  		DPDNORMY;
>  		break;
>  
>  	case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_NORM):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
> -		else if (zc == IEEE754_CLASS_INF)
> +		if (zc == IEEE754_CLASS_INF)
>  			return ieee754dp_inf(zs);
>  		DPDNORMX;
>  		break;
>  
>  	case CLPAIR(IEEE754_CLASS_NORM, IEEE754_CLASS_NORM):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
> -		else if (zc == IEEE754_CLASS_INF)
> +		if (zc == IEEE754_CLASS_INF)
>  			return ieee754dp_inf(zs);
>  		/* fall through to real computations */
>  	}
> diff --git a/arch/mips/math-emu/sp_maddf.c b/arch/mips/math-emu/sp_maddf.c
> index c91d5e5..9fd2035 100644
> --- a/arch/mips/math-emu/sp_maddf.c
> +++ b/arch/mips/math-emu/sp_maddf.c
> @@ -48,51 +48,36 @@ static union ieee754sp _sp_maddf(union ieee754sp z, union ieee754sp x,
>  
>  	ieee754_clearcx();
>  
> -	switch (zc) {
> -	case IEEE754_CLASS_SNAN:
> -		ieee754_setcx(IEEE754_INVALID_OPERATION);
> -		return ieee754sp_nanxcpt(z);
> -	case IEEE754_CLASS_DNORM:
> -		SPDNORMZ;
> -	/* QNAN and ZERO cases are handled separately below */
> +	/* handle the cases when at least one of x, y or z is a NaN */
> +	if (((xc == IEEE754_CLASS_SNAN) || (xc == IEEE754_CLASS_QNAN)) ||
> +	    ((yc == IEEE754_CLASS_SNAN) || (yc == IEEE754_CLASS_QNAN)) ||
> +	    ((zc == IEEE754_CLASS_SNAN) || (zc == IEEE754_CLASS_QNAN))) {
> +		/* order of precedence is z, x, y */
> +		if (zc == IEEE754_CLASS_SNAN)
> +			return ieee754sp_nanxcpt(z);
> +		if (xc == IEEE754_CLASS_SNAN)
> +			return ieee754sp_nanxcpt(x);
> +		if (yc == IEEE754_CLASS_SNAN)
> +			return ieee754sp_nanxcpt(y);
> +		if (zc == IEEE754_CLASS_QNAN)
> +			return z;
> +		if (xc == IEEE754_CLASS_QNAN)
> +			return x;
> +		return y;
>  	}
>  
> +	if (zc == IEEE754_CLASS_DNORM)
> +		SPDNORMZ;
> +	/* ZERO z cases are handled separately below */
> +
>  	switch (CLPAIR(xc, yc)) {
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_NORM, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_SNAN):
> -		return ieee754sp_nanxcpt(y);
> -
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_SNAN):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_ZERO):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_NORM):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_DNORM):
> -	case CLPAIR(IEEE754_CLASS_SNAN, IEEE754_CLASS_INF):
> -		return ieee754sp_nanxcpt(x);
> -
> -	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_NORM, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_QNAN):
> -		return y;
>  
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_QNAN):
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_ZERO):
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_NORM):
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_DNORM):
> -	case CLPAIR(IEEE754_CLASS_QNAN, IEEE754_CLASS_INF):
> -		return x;
>  
>  	/*
>  	 * Infinity handling
>  	 */
>  	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_ZERO):
>  	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_INF):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
>  		ieee754_setcx(IEEE754_INVALID_OPERATION);
>  		return ieee754sp_indef();
>  
> @@ -101,8 +86,6 @@ static union ieee754sp _sp_maddf(union ieee754sp z, union ieee754sp x,
>  	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_NORM):
>  	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_DNORM):
>  	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_INF):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
>  		return ieee754sp_inf(xs ^ ys);
>  
>  	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_ZERO):
> @@ -119,25 +102,19 @@ static union ieee754sp _sp_maddf(union ieee754sp z, union ieee754sp x,
>  		SPDNORMX;
>  
>  	case CLPAIR(IEEE754_CLASS_NORM, IEEE754_CLASS_DNORM):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
> -		else if (zc == IEEE754_CLASS_INF)
> +		if (zc == IEEE754_CLASS_INF)
>  			return ieee754sp_inf(zs);
>  		SPDNORMY;
>  		break;
>  
>  	case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_NORM):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
> -		else if (zc == IEEE754_CLASS_INF)
> +		if (zc == IEEE754_CLASS_INF)
>  			return ieee754sp_inf(zs);
>  		SPDNORMX;
>  		break;
>  
>  	case CLPAIR(IEEE754_CLASS_NORM, IEEE754_CLASS_NORM):
> -		if (zc == IEEE754_CLASS_QNAN)
> -			return z;
> -		else if (zc == IEEE754_CLASS_INF)
> +		if (zc == IEEE754_CLASS_INF)
>  			return ieee754sp_inf(zs);
>  		/* fall through to real computations */
>  	}
> -- 
> 2.7.4
> 

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