[Qemu-devel] [RFC PATCH v1] softfloat: Add round-to-odd rounding mode

[Bharata B Rao]

Power ISA 3.0 introduces a few quadruple precision floating point
instructions that support round-to-odd rounding mode. The
round-to-odd mode is explained as under:

Let Z be the intermediate arithmetic result or the operand of a convert
operation. If Z can be represented exactly in the target format, the
result is Z. Otherwise the result is either Z1 or Z2 whichever is odd.
Here Z1 and Z2 are the next larger and smaller numbers representable
in the target format respectively.

Signed-off-by: Bharata B Rao <address@hidden>
---
Changes in v1:
- Addressed rounding to infinity in roundAndPackFloat128().
- Added 64bit round to odd implementation(roundAndPackFloat64()) as
  it is needed by xscvqpdp instruction of Power ISA 3.0.

v0: http://patchwork.ozlabs.org/patch/716956/

 fpu/softfloat.c         | 10 ++++++++++
 include/fpu/softfloat.h |  2 ++
 2 files changed, 12 insertions(+)

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index c295f31..b9b36ad 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -623,6 +623,9 @@ static float64 roundAndPackFloat64(flag zSign, int zExp, 
uint64_t zSig,
     case float_round_down:
         roundIncrement = zSign ? 0x3ff : 0;
         break;
+    case float_round_to_odd:
+        roundIncrement = (zSig & 0x200) ? 0 : 0x3ff;
+        break;
     default:
         abort();
     }
@@ -1149,6 +1152,9 @@ static float128 roundAndPackFloat128(flag zSign, int32_t 
zExp,
     case float_round_down:
         increment = zSign && zSig2;
         break;
+    case float_round_to_odd:
+        increment = !(zSig1 & 0x1) && zSig2;
+        break;
     default:
         abort();
     }
@@ -1168,6 +1174,7 @@ static float128 roundAndPackFloat128(flag zSign, int32_t 
zExp,
             if (    ( roundingMode == float_round_to_zero )
                  || ( zSign && ( roundingMode == float_round_up ) )
                  || ( ! zSign && ( roundingMode == float_round_down ) )
+                 || ( roundingMode == float_round_to_odd )
                ) {
                 return
                     packFloat128(
@@ -1215,6 +1222,9 @@ static float128 roundAndPackFloat128(flag zSign, int32_t 
zExp,
             case float_round_down:
                 increment = zSign && zSig2;
                 break;
+            case float_round_to_odd:
+                increment = !(zSig1 & 0x1) && zSig2;
+                break;
             default:
                 abort();
             }
diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
index 842ec6b..8a39028 100644
--- a/include/fpu/softfloat.h
+++ b/include/fpu/softfloat.h
@@ -180,6 +180,8 @@ enum {
     float_round_up           = 2,
     float_round_to_zero      = 3,
     float_round_ties_away    = 4,
+    /* Not an IEEE rounding mode: round to the closest odd mantissa value */
+    float_round_to_odd       = 5,
 };
 
 /*----------------------------------------------------------------------------
-- 
2.7.4