Re: About hardfloat in ppc

["罗勇刚(Yonggang Luo)" <luoyonggang@gmail.com>]

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On Sat, May 2, 2020 at 12:51 AM Richard Henderson <
richard.henderson@linaro.org> wrote:

> On 5/1/20 9:29 AM, 罗勇刚(Yonggang Luo) wrote:
> > On Fri, May 1, 2020 at 10:18 PM Richard Henderson <
> richard.henderson@linaro.org
> >     Step 1 is to rearrange the fp helpers to eliminate
> helper_reset_fpstatus().
> >     I've mentioned this before, that it's possible to leave the
> steady-state of
> >     env->fp_status.exception_flags == 0, so there's no need for a
> separate function
> >     call.  I suspect this is worth a decent speedup by itself.
> >
> > Hi Richard, what kinds of rearrange the fp need to be done? Can you give
> me a
> > more detailed example? I am still not get the idea.
>
> See target/openrisc, helper_update_fpcsr.
>
> This is like target/ppc helper_float_check_status, in that it is called
> after
> the primary fpu helper, after the fpu result is written back to the
> architectural register, to process fpu exceptions.
>
> Note that if get_float_exception_flags returns non-zero, we immediately
> reset
> them to zero.  Thus the exception flags are only ever non-zero in between
> the
> primary fpu operation and the update of the fpscr.
>
According to
```
void HELPER(update_fpcsr)(CPUOpenRISCState *env)
{
    int tmp = get_float_exception_flags(&env->fp_status);

    if (tmp) {
        set_float_exception_flags(0, &env->fp_status);
        tmp = ieee_ex_to_openrisc(tmp);
        if (tmp) {
            env->fpcsr |= tmp;
            if (env->fpcsr & FPCSR_FPEE) {
                helper_exception(env, EXCP_FPE);
            }
        }
    }
}
```
The openrisc also clearing the flags before each fp operation?

>
> Thus, no need for a separate helper_reset_fpstatus.
>
>
> r~
>


-- 
         此致
礼
罗勇刚
Yours
    sincerely,
Yonggang Luo

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