softfloat: Implement flushing input denormals to zero

Add support to softfloat for flushing input denormal float32 and float64
to zero. softfloat's existing 'flush_to_zero' flag only flushes denormals
to zero on output. Some CPUs need input denormals to be flushed before
processing as well. Implement this, using a new status flag to enable it
and a new exception status bit to indicate when it has happened. Existing
CPUs should be unaffected as there is no behaviour change unless the
mode is enabled.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

1 files changed, 21 insertions, 1 deletions

diff --git a/fpu/softfloat.h b/fpu/softfloat.h
index f2104c6..15052cc 100644
--- a/fpu/softfloat.h
+++ b/fpu/softfloat.h
@@ -180,7 +180,8 @@ enum {
     float_flag_divbyzero =  4,
     float_flag_overflow  =  8,
     float_flag_underflow = 16,
-    float_flag_inexact   = 32
+    float_flag_inexact   = 32,
+    float_flag_input_denormal = 64
 };
 
 typedef struct float_status {
@@ -190,7 +191,10 @@ typedef struct float_status {
 #ifdef FLOATX80
     signed char floatx80_rounding_precision;
 #endif
+    /* should denormalised results go to zero and set the inexact flag? */
     flag flush_to_zero;
+    /* should denormalised inputs go to zero and set the input_denormal flag? */
+    flag flush_inputs_to_zero;
     flag default_nan_mode;
 } float_status;
 
@@ -200,6 +204,10 @@ INLINE void set_flush_to_zero(flag val STATUS_PARAM)
 {
     STATUS(flush_to_zero) = val;
 }
+INLINE void set_flush_inputs_to_zero(flag val STATUS_PARAM)
+{
+    STATUS(flush_inputs_to_zero) = val;
+}
 INLINE void set_default_nan_mode(flag val STATUS_PARAM)
 {
     STATUS(default_nan_mode) = val;
@@ -294,11 +302,17 @@ float32 float32_scalbn( float32, int STATUS_PARAM );
 
 INLINE float32 float32_abs(float32 a)
 {
+    /* Note that abs does *not* handle NaN specially, nor does
+     * it flush denormal inputs to zero.
+     */
     return make_float32(float32_val(a) & 0x7fffffff);
 }
 
 INLINE float32 float32_chs(float32 a)
 {
+    /* Note that chs does *not* handle NaN specially, nor does
+     * it flush denormal inputs to zero.
+     */
     return make_float32(float32_val(a) ^ 0x80000000);
 }
 
@@ -374,11 +388,17 @@ float64 float64_scalbn( float64, int STATUS_PARAM );
 
 INLINE float64 float64_abs(float64 a)
 {
+    /* Note that abs does *not* handle NaN specially, nor does
+     * it flush denormal inputs to zero.
+     */
     return make_float64(float64_val(a) & 0x7fffffffffffffffLL);
 }
 
 INLINE float64 float64_chs(float64 a)
 {
+    /* Note that chs does *not* handle NaN specially, nor does
+     * it flush denormal inputs to zero.
+     */
     return make_float64(float64_val(a) ^ 0x8000000000000000LL);
 }