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Date: Mon, 5 Jan 2009 10:29:56 -0800
From: Nathan Froyd <froydnj@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH 20/40] Add vs{l,r} instructions.
Message-ID: <20090105182956.GA28711@codesourcery.com>
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To: qemu-devel@nongnu.org

On Sat, Jan 03, 2009 at 10:26:55PM +0100, Aurelien Jarno wrote:
> On Tue, Dec 30, 2008 at 07:10:02PM -0800, Nathan Froyd wrote:
> > +#define VSHIFT(suffix, leftp)                                           \
> > +    void helper_vs##suffix (ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b)   \
> > +    {                                                                   \
> > +        int shift = b->u8[LO_IDX*0x15] & 0x7;                           \
> > +        int doit = 1;                                                   \
> > +        int i;                                                          \
> > +        for (i = 0; i < ARRAY_SIZE(r->u8); i++) {                       \
> > +            doit = doit && ((b->u8[i] & 0x7) == shift);                 \
> > +        }                                                               \
> 
> According to the specification, the result is undefined in that case. I
> think that always doing the computation is fine.

FWIW, doing the check has the nice property of delivering the same
results as real hardware.  If you're using a comparison program like
ppctester, reducing those spurious failures is a win.  (There's already
spurious failures for div instructions in corner cases.)

-Nathan