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From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] hw/audio/fmopl.c: Avoid clang warning
 about shifting negative number
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To: Peter Maydell <peter.maydell@linaro.org>, qemu-devel@nongnu.org
Cc: Gerd Hoffmann <kraxel@redhat.com>, patches@linaro.org



On 16/11/2015 16:16, Peter Maydell wrote:
> Newer versions of clang warn:
> 
> hw/audio/fmopl.c:1085:39: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
>                 data = Limit( outd[0] , OPL_MAXOUT, OPL_MINOUT );
>                                                     ^~~~~~~~~~
> hw/audio/fmopl.c:75:28: note: expanded from macro 'OPL_MINOUT'
> #define OPL_MINOUT (-0x8000<<OPL_OUTSB)
>                     ~~~~~~~^
> 
> Rephrase the definition of OPL_MINOUT to avoid the undefined behaviour.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>  hw/audio/fmopl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/audio/fmopl.c b/hw/audio/fmopl.c
> index 81c0c1b..807b29c 100644
> --- a/hw/audio/fmopl.c
> +++ b/hw/audio/fmopl.c
> @@ -72,7 +72,7 @@ static int opl_dbg_maxchip,opl_dbg_chip;
>  /* final output shift , limit minimum and maximum */
>  #define OPL_OUTSB   (TL_BITS+3-16)		/* OPL output final shift 16bit */
>  #define OPL_MAXOUT (0x7fff<<OPL_OUTSB)
> -#define OPL_MINOUT (-0x8000<<OPL_OUTSB)
> +#define OPL_MINOUT (-(0x8000<<OPL_OUTSB))

Again: let's stop this madness!!!!!!!!!!!!!!!!!!!!!!

(Yes, so many exclamation marks).

This is clearly computing -32768 * 2^N, not -(32768 * 2^N).  The latter
is totally, utterly wrong, because 32768 is _not even expressible_ as a
16-bit fixed point value, which OPL_MINOUT/OPL_MAXOUT obviously are.

I'll shortly post a patch to disable this obnoxious warning.

Paolo