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

fpu/softfloat-specialize.c.inc

@@ -402,19 +402,10 @@ static int pickNaN(FloatClass a_cls, FloatClass b_cls,
         /* target didn't set the rule: fall back to old ifdef choices */
 #if defined(TARGET_AVR) || defined(TARGET_HEXAGON) \
     || defined(TARGET_RISCV) || defined(TARGET_SH4) \
-    || defined(TARGET_TRICORE)
+    || defined(TARGET_TRICORE) || defined(TARGET_ARM)
         g_assert_not_reached();
-#elif defined(TARGET_ARM) || defined(TARGET_MIPS) || defined(TARGET_HPPA) || \
+#elif defined(TARGET_MIPS) || defined(TARGET_HPPA) || \
     defined(TARGET_LOONGARCH64) || defined(TARGET_S390X)
-        /*
-         * ARM mandated NaN propagation rules (see FPProcessNaNs()), take
-         * the first of:
-         *  1. A if it is signaling
-         *  2. B if it is signaling
-         *  3. A (quiet)
-         *  4. B (quiet)
-         * A signaling NaN is always quietened before returning it.
-         */
         /*
          * According to MIPS specifications, if one of the two operands is
          * a sNaN, a new qNaN has to be generated. This is done in

linux-user/arm/nwfpe/fpa11.c

@@ -51,6 +51,24 @@ void resetFPA11(void)
 #ifdef MAINTAIN_FPCR
   fpa11->fpcr = MASK_RESET;
 #endif
+
+  /*
+   * Real FPA11 hardware does not handle NaNs, but always takes an
+   * exception for them to be software-emulated (ARM7500FE datasheet
+   * section 10.4). There is no documented architectural requirement
+   * for NaN propagation rules and it will depend on how the OS
+   * level software emulation opted to do it. We here use prop_s_ab
+   * which matches the later VFP hardware choice and how QEMU's
+   * fpa11 emulation has worked in the past. The real Linux kernel
+   * does something slightly different: arch/arm/nwfpe/softfloat-specialize
+   * propagateFloat64NaN() has the curious behaviour that it prefers
+   * the QNaN over the SNaN, but if both are QNaN it picks A and
+   * if both are SNaN it picks B. In theory we could add this as
+   * a NaN propagation rule, but in practice FPA11 emulation is so
+   * close to totally dead that it's not worth trying to match it at
+   * this late date.
+   */
+  set_float_2nan_prop_rule(float_2nan_prop_s_ab, &fpa11->fp_status);
 }
 
 void SetRoundingMode(const unsigned int opcode)

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,
+    arm_set_default_fp_behaviours(&env->vfp.fp_status);
-                              &env->vfp.fp_status);
+    arm_set_default_fp_behaviours(&env->vfp.standard_fp_status);
-    set_float_detect_tininess(float_tininess_before_rounding,
+    arm_set_default_fp_behaviours(&env->vfp.fp_status_f16);
-                              &env->vfp.standard_fp_status);
+    arm_set_default_fp_behaviours(&env->vfp.standard_fp_status_f16);
-    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);
 #ifndef CONFIG_USER_ONLY
     if (kvm_enabled()) {
         kvm_arm_reset_vcpu(cpu);