target/arm: Adjust FP behaviour for FPCR.AH = 1

When FPCR.AH is set, various behaviours of AArch64 floating point
operations which are controlled by softfloat config settings change:
 * tininess and ftz detection before/after rounding
 * NaN propagation order
 * result of 0 * Inf + NaN
 * default NaN value

When the guest changes the value of the AH bit, switch these config
settings on the fp_status_a64 and fp_status_f16_a64 float_status
fields.

This requires us to make the arm_set_default_fp_behaviours() function
global, since we now need to call it from cpu.c and vfp_helper.c; we
move it to vfp_helper.c so it can be next to the new
arm_set_ah_fp_behaviours().

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

1 files changed, 57 insertions, 1 deletions

diff --git a/target/arm/vfp_helper.c b/target/arm/vfp_helper.c
index 30c170ecee..ef85c186f1 100644
--- a/target/arm/vfp_helper.c
+++ b/target/arm/vfp_helper.c
@@ -22,15 +22,59 @@
 #include "exec/helper-proto.h"
 #include "internals.h"
 #include "cpu-features.h"
+#include "fpu/softfloat.h"
 #ifdef CONFIG_TCG
 #include "qemu/log.h"
-#include "fpu/softfloat.h"
 #endif
 
 /* VFP support.  We follow the convention used for VFP instructions:
    Single precision routines have a "s" suffix, double precision a
    "d" suffix.  */
 
+/*
+ * 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)
+ *  * 3-input NaN propagation prefers SNaN over QNaN, and then
+ *    operand C over A over B (see FPProcessNaNs3() pseudocode,
+ *    but note that for QEMU muladd is a * b + c, whereas for
+ *    the pseudocode function the arguments are in the order c, a, b.
+ *  * 0 * Inf + NaN returns the default NaN if the input NaN is quiet,
+ *    and the input NaN if it is signalling
+ *  * Default NaN has sign bit clear, msb frac bit set
+ */
+void arm_set_default_fp_behaviours(float_status *s)
+{
+    set_float_detect_tininess(float_tininess_before_rounding, s);
+    set_float_ftz_detection(float_ftz_before_rounding, s);
+    set_float_2nan_prop_rule(float_2nan_prop_s_ab, s);
+    set_float_3nan_prop_rule(float_3nan_prop_s_cab, s);
+    set_float_infzeronan_rule(float_infzeronan_dnan_if_qnan, s);
+    set_float_default_nan_pattern(0b01000000, s);
+}
+
+/*
+ * Set the float_status behaviour to match the FEAT_AFP
+ * FPCR.AH=1 requirements:
+ *  * tininess-after-rounding
+ *  * 2-input NaN propagation prefers the first NaN
+ *  * 3-input NaN propagation prefers a over b over c
+ *  * 0 * Inf + NaN always returns the input NaN and doesn't
+ *    set Invalid for a QNaN
+ *  * default NaN has sign bit set, msb frac bit set
+ */
+static void arm_set_ah_fp_behaviours(float_status *s)
+{
+    set_float_detect_tininess(float_tininess_after_rounding, s);
+    set_float_ftz_detection(float_ftz_after_rounding, s);
+    set_float_2nan_prop_rule(float_2nan_prop_ab, s);
+    set_float_3nan_prop_rule(float_3nan_prop_abc, s);
+    set_float_infzeronan_rule(float_infzeronan_dnan_never |
+                              float_infzeronan_suppress_invalid, s);
+    set_float_default_nan_pattern(0b11000000, s);
+}
+
 #ifdef CONFIG_TCG
 
 /* Convert host exception flags to vfp form.  */
@@ -173,6 +217,18 @@ static void vfp_set_fpcr_to_host(CPUARMState *env, uint32_t val, uint32_t mask)
         set_default_nan_mode(dnan_enabled, &env->vfp.fp_status_f16_a32);
         set_default_nan_mode(dnan_enabled, &env->vfp.fp_status_f16_a64);
     }
+    if (changed & FPCR_AH) {
+        bool ah_enabled = val & FPCR_AH;
+
+        if (ah_enabled) {
+            /* Change behaviours for A64 FP operations */
+            arm_set_ah_fp_behaviours(&env->vfp.fp_status_a64);
+            arm_set_ah_fp_behaviours(&env->vfp.fp_status_f16_a64);
+        } else {
+            arm_set_default_fp_behaviours(&env->vfp.fp_status_a64);
+            arm_set_default_fp_behaviours(&env->vfp.fp_status_f16_a64);
+        }
+    }
     /*
      * If any bits changed that we look at in vfp_get_fpsr_from_host(),
      * we must sync the float_status flags into vfp.fpsr now (under the