Re: [Xen-devel] fsincos emulation on AMD CPUs

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

On 12/15/2011 11:15 AM, Jan Beulich wrote:
> > >  If you really cared, perhaps fsincos can be replaced by this sequence in
> > >  the emulator:
> > >
> > >                        ; x
> > >        fld   %st       ; x x
> > >        fsin            ; x sin(x)
> > >        fxch  %st(1)    ; sin(x) x
> > >        fcos            ; sin(x) cos(x)
> I had thought of this at first too, but this is problematic in terms of
> exception handling: fpu_handle_exception() expects to see an
> exception only on the very first instruction (as it's assumed to be
> the only one), and aborts the rest of the sequence if the exception
> doesn't happen on the last instruction.
Can it just be (%0 is fic.insn_bytes):

            movb $4f-1f,%0  ; do nothing on exception here
1:          fld  %st        ; x x
            movb $3f-1f,%0  ; pop on exception here
1:          fsin            ; x sin(x)
            fxch %st(1)     ; sin(x) x
            movb $2f-1f,%0  ; xch+pop on exception here
1:          fcos            ; sin(x) cos(x)
            jmp  2f
4:          fxch %st(1)     ; x sin(x)
3:          fstp %st        ; x
2:

Paolo

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