Re: [Qemu-devel] [PATCH] Fix overflow conditions for MIPS add/subtract

[Fabrice Bellard]

OK. I hope this is correct now :-)

Just a note : there is already a lot of code in QEMU to compute 
correctly the overflow and carry flags (for example in the i386 
target)... don't spend your time on reinventing them !

Fabrice.

Dirk Behme wrote:
> Fix overflow conditions for MIPS add/subtract as proposed by
> Daniel Jacobowitz.
>
> http://lists.gnu.org/archive/html/qemu-devel/2006-04/msg00538.html
>
> Regards
>
> Dirk
>
>
> ------------------------------------------------------------------------
>
> --- target-mips/op.c_orig       2006-04-30 09:40:46.000000000 +0200
> +++ target-mips/op.c    2006-04-30 09:41:52.000000000 +0200
> @@ -206,7 +206,7 @@ void op_addo (void)
>  
>      tmp = T0;
>      T0 += T1;
> -    if (((tmp ^ T1 ^ (-1)) & (T0 ^ T1)) >> 31) {
> +    if (~(T0 ^ T1) & (T0 ^ tmp) & 0x80000000) {
>         /* operands of same sign, result different sign */
>          CALL_FROM_TB1(do_raise_exception_direct, EXCP_OVERFLOW);
>      }
> @@ -225,7 +225,7 @@ void op_subo (void)
>  
>      tmp = T0;
>      T0 = (int32_t)T0 - (int32_t)T1;
> -    if (((tmp ^ T1) & (tmp ^ T0)) >> 31) {
> +    if ((T0 ^ T1) & (T0 ^ tmp) & 0x80000000) {
>         /* operands of different sign, first operand and result different sign 
> */
>          CALL_FROM_TB1(do_raise_exception_direct, EXCP_OVERFLOW);
>      }
>
>
>
>
> ------------------------------------------------------------------------
>
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