Re: [Qemu-devel] [PATCH v3 2/5] fpu/softfloat: re-factor float to float conversions

[Peter Maydell]

On 10 May 2018 at 10:42, Alex Bennée <address@hidden> wrote:
> This allows us to delete a lot of additional boilerplate code which is
> no longer needed. Currently the ieee flag is ignored (everything is
> assumed to be ieee). Handling for ARM AHP will be in the next patch.
>
> Signed-off-by: Alex Bennée <address@hidden>
> Reviewed-by: Richard Henderson <address@hidden>
>
> ---
> v2
>   - pass FloatFmt to float_to_float instead of sizes
>   - split AHP handling to another patch
>   - use rth's suggested re-packing (+ setting .exp)
> v3
>   - also rm extractFloat16Sign
> ---
>  fpu/softfloat-specialize.h |  40 ----
>  fpu/softfloat.c            | 452 +++++++------------------------------
>  include/fpu/softfloat.h    |   8 +-
>  3 files changed, 88 insertions(+), 412 deletions(-)

This introduces a regression where we don't get tininess-before-rounding
for double/single to halfprec conversions. This is because we're
now using the fp_status_f16 status field, and it has not had
the detect_tininess setting initialized. This fixes it:

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index d175c5e94f..7939c6b8ae 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -324,6 +324,8 @@ static void arm_cpu_reset(CPUState *s)
                               &env->vfp.fp_status);
     set_float_detect_tininess(float_tininess_before_rounding,
                               &env->vfp.standard_fp_status);
+    set_float_detect_tininess(float_tininess_before_rounding,
+                              &env->vfp.fp_status_f16);
 #ifndef CONFIG_USER_ONLY
     if (kvm_enabled()) {
         kvm_arm_reset_vcpu(cpu);

(You can see this if you try something like fcvt h1, d0 where
d0 == 0x3f0f_ffff_ffff_ffff -- we get the right answer of 0x0400
but fail to set Underflow as well as Inexact.)

thanks
-- PMM