[Qemu-devel] [PATCH v2 16/24] softfloat: Add support for ties-away rounding

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

IEEE754-2008 specifies a new rounding mode:

"roundTiesToAway: the floating-point number nearest to the infinitely
precise result shall be delivered; if the two nearest floating-point
numbers bracketing an unrepresentable infinitely precise result are
equally near, the one with larger magnitude shall be delivered."

Implement this new mode (it is needed for ARM). The general principle
is that the required code is exactly like the ties-to-even code,
except that we do not need to do the "in case of exact tie clear LSB
to round-to-even", because the rounding operation naturally causes
the exact tie to round up in magnitude.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 fpu/softfloat.c         | 54 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/fpu/softfloat.h |  3 ++-
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index 42cb705..49d90ef 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -111,6 +111,7 @@ static int32 roundAndPackInt32( flag zSign, uint64_t absZ STATUS_PARAM)
     roundNearestEven = ( roundingMode == float_round_nearest_even );
     switch (roundingMode) {
     case float_round_nearest_even:
+    case float_round_ties_away:
         roundIncrement = 0x40;
         break;
     case float_round_to_zero:
@@ -161,6 +162,7 @@ static int64 roundAndPackInt64( flag zSign, uint64_t absZ0, uint64_t absZ1 STATU
     roundNearestEven = ( roundingMode == float_round_nearest_even );
     switch (roundingMode) {
     case float_round_nearest_even:
+    case float_round_ties_away:
         increment = ((int64_t) absZ1 < 0);
         break;
     case float_round_to_zero:
@@ -214,6 +216,7 @@ static int64 roundAndPackUint64(flag zSign, uint64_t absZ0,
     roundNearestEven = (roundingMode == float_round_nearest_even);
     switch (roundingMode) {
     case float_round_nearest_even:
+    case float_round_ties_away:
         increment = ((int64_t)absZ1 < 0);
         break;
     case float_round_to_zero:
@@ -366,6 +369,7 @@ static float32 roundAndPackFloat32(flag zSign, int_fast16_t zExp, uint32_t zSig
     roundNearestEven = ( roundingMode == float_round_nearest_even );
     switch (roundingMode) {
     case float_round_nearest_even:
+    case float_round_ties_away:
         roundIncrement = 0x40;
         break;
     case float_round_to_zero:
@@ -550,6 +554,7 @@ static float64 roundAndPackFloat64(flag zSign, int_fast16_t zExp, uint64_t zSig
     roundNearestEven = ( roundingMode == float_round_nearest_even );
     switch (roundingMode) {
     case float_round_nearest_even:
+    case float_round_ties_away:
         roundIncrement = 0x200;
         break;
     case float_round_to_zero:
@@ -734,6 +739,7 @@ static floatx80
     zSig0 |= ( zSig1 != 0 );
     switch (roundingMode) {
     case float_round_nearest_even:
+    case float_round_ties_away:
         break;
     case float_round_to_zero:
         roundIncrement = 0;
@@ -794,6 +800,7 @@ static floatx80
  precision80:
     switch (roundingMode) {
     case float_round_nearest_even:
+    case float_round_ties_away:
         increment = ((int64_t)zSig1 < 0);
         break;
     case float_round_to_zero:
@@ -838,6 +845,7 @@ static floatx80
             if ( zSig1 ) STATUS(float_exception_flags) |= float_flag_inexact;
             switch (roundingMode) {
             case float_round_nearest_even:
+            case float_round_ties_away:
                 increment = ((int64_t)zSig1 < 0);
                 break;
             case float_round_to_zero:
@@ -1052,6 +1060,7 @@ static float128
     roundNearestEven = ( roundingMode == float_round_nearest_even );
     switch (roundingMode) {
     case float_round_nearest_even:
+    case float_round_ties_away:
         increment = ((int64_t)zSig2 < 0);
         break;
     case float_round_to_zero:
@@ -1114,6 +1123,7 @@ static float128
             if ( isTiny && zSig2 ) float_raise( float_flag_underflow STATUS_VAR);
             switch (roundingMode) {
             case float_round_nearest_even:
+            case float_round_ties_away:
                 increment = ((int64_t)zSig2 < 0);
                 break;
             case float_round_to_zero:
@@ -1785,6 +1795,11 @@ float32 float32_round_to_int( float32 a STATUS_PARAM)
                 return packFloat32( aSign, 0x7F, 0 );
             }
             break;
+        case float_round_ties_away:
+            if (aExp == 0x7E) {
+                return packFloat32(aSign, 0x7F, 0);
+            }
+            break;
          case float_round_down:
             return make_float32(aSign ? 0xBF800000 : 0);
          case float_round_up:
@@ -1803,6 +1818,9 @@ float32 float32_round_to_int( float32 a STATUS_PARAM)
             z &= ~lastBitMask;
         }
         break;
+    case float_round_ties_away:
+        z += lastBitMask >> 1;
+        break;
     case float_round_to_zero:
         break;
     case float_round_up:
@@ -3183,6 +3201,9 @@ static float32 roundAndPackFloat16(flag zSign, int_fast16_t zExp,
             increment = zSig & (increment << 1);
         }
         break;
+    case float_round_ties_away:
+        increment = (mask + 1) >> 1;
+        break;
     case float_round_up:
         increment = zSign ? 0 : mask;
         break;
@@ -3487,6 +3508,11 @@ float64 float64_round_to_int( float64 a STATUS_PARAM )
                 return packFloat64( aSign, 0x3FF, 0 );
             }
             break;
+        case float_round_ties_away:
+            if (aExp == 0x3FE) {
+                return packFloat64(aSign, 0x3ff, 0);
+            }
+            break;
          case float_round_down:
             return make_float64(aSign ? LIT64( 0xBFF0000000000000 ) : 0);
          case float_round_up:
@@ -3506,6 +3532,9 @@ float64 float64_round_to_int( float64 a STATUS_PARAM )
             z &= ~lastBitMask;
         }
         break;
+    case float_round_ties_away:
+        z += lastBitMask >> 1;
+        break;
     case float_round_to_zero:
         break;
     case float_round_up:
@@ -4771,6 +4800,11 @@ floatx80 floatx80_round_to_int( floatx80 a STATUS_PARAM )
                     packFloatx80( aSign, 0x3FFF, LIT64( 0x8000000000000000 ) );
             }
             break;
+        case float_round_ties_away:
+            if (aExp == 0x3FFE) {
+                return packFloatx80(aSign, 0x3FFF, LIT64(0x8000000000000000));
+            }
+            break;
          case float_round_down:
             return
                   aSign ?
@@ -4794,6 +4828,9 @@ floatx80 floatx80_round_to_int( floatx80 a STATUS_PARAM )
             z.low &= ~lastBitMask;
         }
         break;
+    case float_round_ties_away:
+        z.low += lastBitMask >> 1;
+        break;
     case float_round_to_zero:
         break;
     case float_round_up:
@@ -5862,6 +5899,15 @@ float128 float128_round_to_int( float128 a STATUS_PARAM )
                 }
             }
             break;
+        case float_round_ties_away:
+            if (lastBitMask) {
+                add128(z.high, z.low, 0, lastBitMask >> 1, &z.high, &z.low);
+            } else {
+                if ((int64_t) z.low < 0) {
+                    ++z.high;
+                }
+            }
+            break;
         case float_round_to_zero:
             break;
         case float_round_up:
@@ -5893,6 +5939,11 @@ float128 float128_round_to_int( float128 a STATUS_PARAM )
                     return packFloat128( aSign, 0x3FFF, 0, 0 );
                 }
                 break;
+            case float_round_ties_away:
+                if (aExp == 0x3FFE) {
+                    return packFloat128(aSign, 0x3FFF, 0, 0);
+                }
+                break;
              case float_round_down:
                 return
                       aSign ? packFloat128( 1, 0x3FFF, 0, 0 )
@@ -5916,6 +5967,9 @@ float128 float128_round_to_int( float128 a STATUS_PARAM )
                 z.high &= ~ lastBitMask;
             }
             break;
+        case float_round_ties_away:
+            z.high += lastBitMask>>1;
+            break;
         case float_round_to_zero:
             break;
         case float_round_up:
diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
index 83d324a..806ae13 100644
--- a/include/fpu/softfloat.h
+++ b/include/fpu/softfloat.h
@@ -152,7 +152,8 @@ enum {
     float_round_nearest_even = 0,
     float_round_down         = 1,
     float_round_up           = 2,
-    float_round_to_zero      = 3
+    float_round_to_zero      = 3,
+    float_round_ties_away    = 4,
 };
 
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-- 
1.8.5