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target/arm: Set up float_status to use for FPCR.AH=1 behaviour

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When FPCR.AH is 1, the behaviour of some instructions changes:
 * AdvSIMD BFCVT, BFCVTN, BFCVTN2, BFMLALB, BFMLALT
 * SVE BFCVT, BFCVTNT, BFMLALB, BFMLALT, BFMLSLB, BFMLSLT
 * SME BFCVT, BFCVTN, BFMLAL, BFMLSL (these are all in SME2 which
   QEMU does not yet implement)
 * FRECPE, FRECPS, FRECPX, FRSQRTE, FRSQRTS

The behaviour change is:
 * the instructions do not update the FPSR cumulative exception flags
 * trapped floating point exceptions are disabled (a no-op for QEMU,
   which doesn't implement FPCR.{IDE,IXE,UFE,OFE,DZE,IOE})
 * rounding is always round-to-nearest-even regardless of FPCR.RMode
 * denormalized inputs and outputs are always flushed to zero, as if
   FPCR.{FZ,FIZ} is {1,1}
 * FPCR.FZ16 is still honoured for half-precision inputs

(See the Arm ARM DDI0487L.a section A1.5.9.)

We can provide all these behaviours with another pair of float_status fields
which we use only for these insns, when FPCR.AH is 1. These float_status
fields will always have:
 * flush_to_zero and flush_inputs_to_zero set for the non-F16 field
 * rounding mode set to round-to-nearest-even
and so the only FPCR fields they need to honour are DN and FZ16.

In this commit we only define the new fp_status fields and give them
the required behaviour when FPSR is updated.  In subsequent commits
we will arrange to use this new fp_status field for the instructions
that should be affected by FPCR.AH in this way.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Peter Maydell 2025-02-01 16:39:14 +00:00
parent 731528d35e
commit 1828053990
5 changed files with 47 additions and 1 deletions
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4
target/arm/cpu.c
View file

@ -556,6 +556,10 @@ static void arm_cpu_reset_hold(Object *obj, ResetType type)
arm_set_default_fp_behaviours(&env->vfp.fp_status_f16_a32);
arm_set_default_fp_behaviours(&env->vfp.fp_status_f16_a64);
arm_set_default_fp_behaviours(&env->vfp.standard_fp_status_f16);
arm_set_ah_fp_behaviours(&env->vfp.ah_fp_status);
set_flush_to_zero(1, &env->vfp.ah_fp_status);
set_flush_inputs_to_zero(1, &env->vfp.ah_fp_status);
arm_set_ah_fp_behaviours(&env->vfp.ah_fp_status_f16);
#ifndef CONFIG_USER_ONLY
if (kvm_enabled()) {