target/arm: Explicitly set 2-NaN propagation rule

Set the 2-NaN propagation rule explicitly in the float_status words
we use.  We wrap this plus the pre-existing setting of the
tininess-before-rounding flag in a new function
arm_set_default_fp_behaviours() to avoid repetition, since we have a
lot of float_status words at this point.

The situation with FPA11 emulation in linux-user is a little odd, and
arguably "correct" behaviour there would be to exactly match a real
Linux kernel's FPA11 emulation.  However FPA11 emulation is
essentially dead at this point and so it seems better to continue
with QEMU's current behaviour and leave a comment describing the
situation.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241025141254.2141506-4-peter.maydell@linaro.org

1 files changed, 17 insertions, 8 deletions

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 5b751439bd..6938161b95 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -168,6 +168,18 @@ void arm_register_el_change_hook(ARMCPU *cpu, ARMELChangeHookFn *hook,
     QLIST_INSERT_HEAD(&cpu->el_change_hooks, entry, node);
 }
 
+/*
+ * Set the float_status behaviour to match the Arm defaults:
+ *  * tininess-before-rounding
+ *  * 2-input NaN propagation prefers SNaN over QNaN, and then
+ *    operand A over operand B (see FPProcessNaNs() pseudocode)
+ */
+static void arm_set_default_fp_behaviours(float_status *s)
+{
+    set_float_detect_tininess(float_tininess_before_rounding, s);
+    set_float_2nan_prop_rule(float_2nan_prop_s_ab, s);
+}
+
 static void cp_reg_reset(gpointer key, gpointer value, gpointer opaque)
 {
     /* Reset a single ARMCPRegInfo register */
@@ -549,14 +561,11 @@ static void arm_cpu_reset_hold(Object *obj, ResetType type)
     set_flush_inputs_to_zero(1, &env->vfp.standard_fp_status);
     set_default_nan_mode(1, &env->vfp.standard_fp_status);
     set_default_nan_mode(1, &env->vfp.standard_fp_status_f16);
-    set_float_detect_tininess(float_tininess_before_rounding,
-                              &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);
-    set_float_detect_tininess(float_tininess_before_rounding,
-                              &env->vfp.standard_fp_status_f16);
+    arm_set_default_fp_behaviours(&env->vfp.fp_status);
+    arm_set_default_fp_behaviours(&env->vfp.standard_fp_status);
+    arm_set_default_fp_behaviours(&env->vfp.fp_status_f16);
+    arm_set_default_fp_behaviours(&env->vfp.standard_fp_status_f16);
+
 #ifndef CONFIG_USER_ONLY
     if (kvm_enabled()) {
         kvm_arm_reset_vcpu(cpu);