linux-kernel - RE: [PATCH 2/2] MIPS: math-emu: Declare ys variable as possibly unused

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Date:   Tue, 26 Dec 2017 15:12:06 +0000
From:   Aleksandar Markovic <Aleksandar.Markovic@...s.com>
To:     Mathieu Malaterre <malat@...ian.org>
CC:     Ralf Baechle <ralf@...ux-mips.org>,
        Miodrag Dinic <Miodrag.Dinic@...s.com>,
        Goran Ferenc <goran.ferenc@...tec.com>,
        James Hogan <jhogan@...nel.org>,
        Douglas Leung <douglas.leung@...tec.com>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/2] MIPS: math-emu: Declare ys variable as possibly
 unused

> Fix non-fatal warning:
>
> arch/mips/math-emu/sp_fdp.c: In function ‘ieee754sp_fdp’:
> arch/mips/math-emu/ieee754int.h:60:31: warning: variable ‘ys’ set but not used [-Wunused-but-set-variable]
>   unsigned int ym; int ye; int ys; int yc
>                                ^
> arch/mips/math-emu/sp_fdp.c:37:2: note: in expansion of macro ‘COMPYSP’
>   COMPYSP;
>   ^~~~~~~
> 
> Signed-off-by: Mathieu Malaterre <malat@...ian.org>
> ---
>  arch/mips/math-emu/ieee754int.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/mips/math-emu/ieee754int.h b/arch/mips/math-emu/ieee754int.h
> index 06ac0e2ac7ac..cb8f04cd24bf 100644
> --- a/arch/mips/math-emu/ieee754int.h
> +++ b/arch/mips/math-emu/ieee754int.h
> @@ -57,7 +57,7 @@ static inline int ieee754_class_nan(int xc)
>          unsigned int xm; int xe; int xs __maybe_unused; int xc
>  
>  #define COMPYSP \
> -       unsigned int ym; int ye; int ys; int yc
> +       unsigned int ym; int ye; int ys __maybe_unused; int yc
>  
>  #define COMPZSP \
>          unsigned int zm; int ze; int zs; int zc

This will silence the warning, but will do it for all future cases of unused
ys too - in other words, it may well silence even useful, valid warnings.
Also, this introduces an inconsistency among COMPXSP, COMPYSP, and COMPZSP
macros.

A better solution would be to reduce the scope of ys, so that it is always
used, if declared. Instead of this code segment (in arch/mips/math-emu/sp_fdp.c):

union ieee754sp ieee754sp_fdp(union ieee754dp x)
{
	union ieee754sp y;
	u32 rm;

	COMPXDP;
	COMPYSP;

	EXPLODEXDP;

	ieee754_clearcx();

	FLUSHXDP;

	switch (xc) {
	case IEEE754_CLASS_SNAN:
		x = ieee754dp_nanxcpt(x);
		EXPLODEXDP;
		/* Fall through.  */
	case IEEE754_CLASS_QNAN:
		y = ieee754sp_nan_fdp(xs, xm);
		if (!ieee754_csr.nan2008) {
			EXPLODEYSP;
			if (!ieee754_class_nan(yc))
				y = ieee754sp_indef();
		}
		return y;


.. should be the following: (COMPYSP is moved to a smaller code block)

union ieee754sp ieee754sp_fdp(union ieee754dp x)
{
	union ieee754sp y;
	u32 rm;

	COMPXDP;	

	EXPLODEXDP;

	ieee754_clearcx();

	FLUSHXDP;

	switch (xc) {
	case IEEE754_CLASS_SNAN:
		x = ieee754dp_nanxcpt(x);
		EXPLODEXDP;
		/* Fall through.  */
	case IEEE754_CLASS_QNAN:
		{
			COMPYSP;

			y = ieee754sp_nan_fdp(xs, xm);
			if (!ieee754_csr.nan2008) {
				EXPLODEYSP;
				if (!ieee754_class_nan(yc))
					y = ieee754sp_indef();
			}
			return y;
		}

Regards,
Aleksandar