Re: [Qemu-ppc] [PATCH] fpu_helper.c: fix setting FPSCR[FI] bit

[David Gibson]

On Sun, Jun 24, 2018 at 07:12:48PM -0400, John Arbuckle wrote:
> The FPSCR[FI] bit indicates if the last floating point instruction had a 
> result that was rounded. Each consecutive floating point instruction is 
> suppose to set this bit to the correct value. What currently happens is this 
> bit is not set as often as it should be. I have verified that this is the 
> behavior of a real PowerPC 950. This patch fixes that problem by deciding to 
> set this bit after each floating point instruction.
> 
> https://www.pdfdrive.net/powerpc-microprocessor-family-the-programming-environments-for-32-e3087633.html
> Page 63 in table 2-4 is where the description of this bit can be found.
> 
> Signed-off-by: John Arbuckle <address@hidden>

I'm not sure it's the nicest way to fix it, but the existing code is
pretty horrible, so, whatever.

It looks correct, so, applied to ppc-for-3.0.

> ---
>  target/ppc/fpu_helper.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/target/ppc/fpu_helper.c b/target/ppc/fpu_helper.c
> index d31a933cbb..9c841864c8 100644
> --- a/target/ppc/fpu_helper.c
> +++ b/target/ppc/fpu_helper.c
> @@ -274,6 +274,7 @@ static inline void float_inexact_excp(CPUPPCState *env)
>  {
>      CPUState *cs = CPU(ppc_env_get_cpu(env));
>  
> +    env->fpscr |= 1 << FPSCR_FI;
>      env->fpscr |= 1 << FPSCR_XX;
>      /* Update the floating-point exception summary */
>      env->fpscr |= FP_FX;
> @@ -505,6 +506,7 @@ static void do_float_check_status(CPUPPCState *env, 
> uintptr_t raddr)
>  {
>      CPUState *cs = CPU(ppc_env_get_cpu(env));
>      int status = get_float_exception_flags(&env->fp_status);
> +    bool inexact_happened = false;
>  
>      if (status & float_flag_divbyzero) {
>          float_zero_divide_excp(env, raddr);
> @@ -514,6 +516,12 @@ static void do_float_check_status(CPUPPCState *env, 
> uintptr_t raddr)
>          float_underflow_excp(env);
>      } else if (status & float_flag_inexact) {
>          float_inexact_excp(env);
> +        inexact_happened = true;
> +    }
> +
> +    /* if the inexact flag was not set */
> +    if (inexact_happened == false) {
> +        env->fpscr &= ~(1 << FPSCR_FI); /* clear the FPSCR[FI] bit */
>      }
>  
>      if (cs->exception_index == POWERPC_EXCP_PROGRAM &&

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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