[Qemu-devel] [PATCH 09/18] softfloat: Fix Handling of Small Negatives in float64_to_uint64

[Tom Musta <tommusta@gmail.com>]

The float64_to_uint64 routine exits early for all negative numbers.
While the integer result is always correctly returned as 0, the
exception flags are also always set to float_flag_invalid.  This
is incorrect for those cases where a small negative number (-1 < x < 0)
rounds to zero.  In such a case, the flag should be reported as
inexact.

The following patch allows these small numbers to flow through the
rounding and packing code.

Some interesting test patterns are:

(1) BC6AEEBA7F390215 / -0x1.aeeba7f390215p-57, round to nearest
    even should round up to zero (inexact)

(2) A66A44F252C9AAAC /-0x1.a44f252c9aaacp-409 round up should round
    up to zero (inexact)

(3) B4692F3AFFAF2716 / -0x1.92f3affaf2716p-185 round down should
    be invalid.

Signed-off-by: Tom Musta <tommusta@gmail.com>
---
 fpu/softfloat.c |   41 +++++++++++++++++++++--------------------
 1 files changed, 21 insertions(+), 20 deletions(-)

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index cb03dca..3d7a8ff 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -161,7 +161,6 @@ static int32 roundAndPackInt32( flag zSign, uint64_t absZ STATUS_PARAM)
 | exception is raised and the largest positive or negative integer is
 | returned.
 *----------------------------------------------------------------------------*/
-
 static int64 roundAndPackInt64( flag zSign, uint64_t absZ0, uint64_t absZ1 STATUS_PARAM)
 {
     int8 roundingMode;
@@ -213,20 +212,24 @@ static int64 roundAndPackInt64( flag zSign, uint64_t absZ0, uint64_t absZ1 STATU
 | exception is raised and the largest unsigned integer is returned.
 *----------------------------------------------------------------------------*/
 
-static int64 roundAndPackUint64(uint64_t absZ0, uint64_t absZ1 STATUS_PARAM)
+static int64 roundAndPackUint64(flag zSign, uint64_t absZ0,
+                                uint64_t absZ1 STATUS_PARAM)
 {
     int8 roundingMode;
     flag roundNearestEven, increment;
-    int64_t z;
 
     roundingMode = STATUS(float_rounding_mode);
     roundNearestEven = (roundingMode == float_round_nearest_even);
-    increment = ((int64_t) absZ1 < 0);
+    increment = ((int64_t)absZ1 < 0);
     if (!roundNearestEven) {
         if (roundingMode == float_round_to_zero) {
             increment = 0;
-        } else {
-            increment = (roundingMode == float_round_up) && absZ1;
+        } else if (absZ1) {
+            if (zSign) {
+                increment = (roundingMode == float_round_down) && absZ1;
+            } else {
+                increment = (roundingMode == float_round_up) && absZ1;
+            }
         }
     }
     if (increment) {
@@ -237,11 +240,16 @@ static int64 roundAndPackUint64(uint64_t absZ0, uint64_t absZ1 STATUS_PARAM)
         }
         absZ0 &= ~(((uint64_t)(absZ1<<1) == 0) & roundNearestEven);
     }
-    z = absZ0;
+
+    if (zSign && absZ0) {
+        float_raise(float_flag_invalid STATUS_VAR);
+        return 0;
+    }
+
     if (absZ1) {
         STATUS(float_exception_flags) |= float_flag_inexact;
     }
-    return z;
+    return absZ0;
 }
 
 /*----------------------------------------------------------------------------
@@ -1590,7 +1598,7 @@ uint64 float32_to_uint64(float32 a STATUS_PARAM)
     aSig64 = aSig;
     aSig64 <<= 40;
     shift64ExtraRightJamming(aSig64, 0, shiftCount, &aSig64, &aSigExtra);
-    return roundAndPackUint64(aSig64, aSigExtra STATUS_VAR);
+    return roundAndPackUint64(aSign, aSig64, aSigExtra STATUS_VAR);
 }
 
 /*----------------------------------------------------------------------------
@@ -6643,12 +6651,8 @@ uint64_t float64_to_uint64(float64 a STATUS_PARAM)
     aSig = extractFloat64Frac(a);
     aExp = extractFloat64Exp(a);
     aSign = extractFloat64Sign(a);
-    if (aSign) {
-        if (aExp) {
-            float_raise(float_flag_invalid STATUS_VAR);
-        } else if (aSig) { /* negative denormalized */
-            float_raise(float_flag_inexact STATUS_VAR);
-        }
+    if (aSign && (aExp > 1022)) {
+        float_raise(float_flag_invalid STATUS_VAR);
         return 0;
     }
     if (aExp) {
@@ -6657,10 +6661,7 @@ uint64_t float64_to_uint64(float64 a STATUS_PARAM)
     shiftCount = 0x433 - aExp;
     if (shiftCount <= 0) {
         if (0x43E < aExp) {
-            if ((aSig != LIT64(0x0010000000000000)) ||
-                 (aExp == 0x7FF)) {
-                float_raise(float_flag_invalid STATUS_VAR);
-            }
+            float_raise(float_flag_invalid STATUS_VAR);
             return LIT64(0xFFFFFFFFFFFFFFFF);
         }
         aSigExtra = 0;
@@ -6668,7 +6669,7 @@ uint64_t float64_to_uint64(float64 a STATUS_PARAM)
     } else {
         shift64ExtraRightJamming(aSig, 0, shiftCount, &aSig, &aSigExtra);
     }
-    return roundAndPackUint64(aSig, aSigExtra STATUS_VAR);
+    return roundAndPackUint64(aSign, aSig, aSigExtra STATUS_VAR);
 }
 
 
-- 
1.7.1