Re: [PATCH] target/s390x: Fix CC set by CONVERT TO FIXED/LOGICAL

[Richard Henderson]

On 6/23/21 7:50 AM, Ulrich Weigand wrote:
> @@ -506,6 +534,7 @@ uint64_t HELPER(cgeb)(CPUS390XState *env, uint64_t v2, 
> uint32_t m34)
>   {
>       int old_mode = s390_swap_bfp_rounding_mode(env, round_from_m34(m34));
>       int64_t ret = float32_to_int64(v2, &env->fpu_status);
> +    env->cc_op = set_cc_conv_f32(v2, &env->fpu_status);
...

> @@ -1875,7 +1860,7 @@ static DisasJumpType op_cgeb(DisasContext *s, DisasOps *o)
>      }
>      gen_helper_cgeb(o->out, cpu_env, o->in2, m34);
>      tcg_temp_free_i32(m34);
> -    gen_set_cc_nz_f32(s, o->in2);
> +    set_cc_static(s);
>      return DISAS_NEXT;

...

> helper.h:DEF_HELPER_FLAGS_3(clgdb, TCG_CALL_NO_WG, i64, env, i64, i32)

This won't work reliably.  You're writing to a tcg global inside of a function that says 
that it won't.

It's probably time to take care of

>     /*
>      * FIXME:
>      * 1. Right now, all inexact conditions are inidicated as
>      *    "truncated" (0) and never as "incremented" (1) in the DXC.
>      * 2. Only traps due to invalid/divbyzero are suppressing. Other traps
>      *    are completing, meaning the target register has to be written!
>      *    This, however will mean that we have to write the register before
>      *    triggering the trap - impossible right now.
>      */

point 2, by splitting the fpu helpers.  In the first part, take care of the optimization 
and suppressed traps, and return the register value.  In the second part, take care of FPC 
write-back, completing traps, and return any cc value.  Which you can then assign, 
properly, to the cc_op tcg global.

BTW, we can now improve fpu performance by keeping masked exceptions in the float_status. 
 Or at least inexact.  Once that's set, we allow softfloat.c to use host floating-point 
insns under certain conditions.  I'm sure this code pre-dates that, so it made sense to 
clear it all out at the time.


r~