[Qemu-devel] [PATCH] softfloat: Fix exception flag handling for float32_to_float16()

[Peter Maydell <peter.maydell@linaro.org>]

Our float32 to float16 conversion routine was generating the correct
numerical answers, but not always setting the right set of exception
flags. Fix this, mostly by rearranging the code to more closely
resemble RoundAndPackFloat*, and in particular:
 * non-IEEE halfprec always raises Invalid for input NaNs
 * we need to check for the overflow case before underflow
 * we weren't getting the tininess-detected-after-rounding
   case correct (somewhat academic since only ARM uses halfprec
   and it is always tininess-detected-before-rounding)
 * non-IEEE halfprec overflow raises only Invalid, not
   Invalid + Inexact
 * we weren't setting Inexact when we should

Also add some clarifying comments about what the code is doing.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
I've had the "fpscr exception flags wrong for halfprec VCVT
for 32-bit ARM" bug on my list for years, and finally got round
to tackling it, since I'd rather not copy a buggy function when
implementing float64_to_float16 for A64...


 fpu/softfloat.c | 105 +++++++++++++++++++++++++++++++++++---------------------
 1 file changed, 66 insertions(+), 39 deletions(-)

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index dbda61b..6a6b656 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -3046,6 +3046,10 @@ float16 float32_to_float16(float32 a, flag ieee STATUS_PARAM)
     uint32_t mask;
     uint32_t increment;
     int8 roundingMode;
+    int maxexp = ieee ? 15 : 16;
+    bool rounding_bumps_exp;
+    bool is_tiny = false;
+
     a = float32_squash_input_denormal(a STATUS_VAR);
 
     aSig = extractFloat32Frac( a );
@@ -3054,11 +3058,12 @@ float16 float32_to_float16(float32 a, flag ieee STATUS_PARAM)
     if ( aExp == 0xFF ) {
         if (aSig) {
             /* Input is a NaN */
-            float16 r = commonNaNToFloat16( float32ToCommonNaN( a STATUS_VAR ) STATUS_VAR );
             if (!ieee) {
+                float_raise(float_flag_invalid STATUS_VAR);
                 return packFloat16(aSign, 0, 0);
             }
-            return r;
+            return commonNaNToFloat16(
+                float32ToCommonNaN(a STATUS_VAR) STATUS_VAR);
         }
         /* Infinity */
         if (!ieee) {
@@ -3070,58 +3075,80 @@ float16 float32_to_float16(float32 a, flag ieee STATUS_PARAM)
     if (aExp == 0 && aSig == 0) {
         return packFloat16(aSign, 0, 0);
     }
-    /* Decimal point between bits 22 and 23.  */
+    /* Decimal point between bits 22 and 23. Note that we add the 1 bit
+     * even if the input is denormal; however this is harmless because
+     * the largest possible single-precision denormal is still smaller
+     * than the smallest representable half-precision denormal, and so we
+     * will end up ignoring aSig and returning via the "always return zero"
+     * codepath.
+     */
     aSig |= 0x00800000;
     aExp -= 0x7f;
+    /* Calculate the mask of bits of the mantissa which are not
+     * representable in half-precision and will be lost.
+     */
     if (aExp < -14) {
+        /* Will be denormal in halfprec */
         mask = 0x00ffffff;
         if (aExp >= -24) {
             mask >>= 25 + aExp;
         }
     } else {
+        /* Normal number in halfprec */
         mask = 0x00001fff;
     }
-    if (aSig & mask) {
-        float_raise( float_flag_underflow STATUS_VAR );
-        roundingMode = STATUS(float_rounding_mode);
-        switch (roundingMode) {
-        case float_round_nearest_even:
-            increment = (mask + 1) >> 1;
-            if ((aSig & mask) == increment) {
-                increment = aSig & (increment << 1);
-            }
-            break;
-        case float_round_up:
-            increment = aSign ? 0 : mask;
-            break;
-        case float_round_down:
-            increment = aSign ? mask : 0;
-            break;
-        default: /* round_to_zero */
-            increment = 0;
-            break;
-        }
-        aSig += increment;
-        if (aSig >= 0x01000000) {
-            aSig >>= 1;
-            aExp++;
-        }
-    } else if (aExp < -14
-          && STATUS(float_detect_tininess) == float_tininess_before_rounding) {
-        float_raise( float_flag_underflow STATUS_VAR);
-    }
 
-    if (ieee) {
-        if (aExp > 15) {
-            float_raise( float_flag_overflow | float_flag_inexact STATUS_VAR);
+    roundingMode = STATUS(float_rounding_mode);
+    switch (roundingMode) {
+    case float_round_nearest_even:
+        increment = (mask + 1) >> 1;
+        if ((aSig & mask) == increment) {
+            increment = aSig & (increment << 1);
+        }
+        break;
+    case float_round_up:
+        increment = aSign ? 0 : mask;
+        break;
+    case float_round_down:
+        increment = aSign ? mask : 0;
+        break;
+    default: /* round_to_zero */
+        increment = 0;
+        break;
+    }
+
+    rounding_bumps_exp = (aSig + increment >= 0x01000000);
+
+    if (aExp > maxexp || (aExp == maxexp && rounding_bumps_exp)) {
+        if (ieee) {
+            float_raise(float_flag_overflow | float_flag_inexact STATUS_VAR);
             return packFloat16(aSign, 0x1f, 0);
-        }
-    } else {
-        if (aExp > 16) {
-            float_raise(float_flag_invalid | float_flag_inexact STATUS_VAR);
+        } else {
+            float_raise(float_flag_invalid STATUS_VAR);
             return packFloat16(aSign, 0x1f, 0x3ff);
         }
     }
+
+    if (aExp < -14) {
+        /* Note that flush-to-zero does not affect half-precision results */
+        is_tiny =
+            (STATUS(float_detect_tininess) == float_tininess_before_rounding)
+            || (aExp < -15)
+            || (!rounding_bumps_exp);
+    }
+    if (aSig & mask) {
+        float_raise(float_flag_inexact STATUS_VAR);
+        if (is_tiny) {
+            float_raise(float_flag_underflow STATUS_VAR);
+        }
+    }
+
+    aSig += increment;
+    if (rounding_bumps_exp) {
+        aSig >>= 1;
+        aExp++;
+    }
+
     if (aExp < -24) {
         return packFloat16(aSign, 0, 0);
     }
-- 
1.8.5