softfloat: fix floatx80 pseudo-denormal addition / subtraction

The softfloat function addFloatx80Sigs, used for addition of values
with the same sign and subtraction of values with opposite sign, fails
to handle the case where the two values both have biased exponent zero
and there is a carry resulting from adding the significands, which can
occur if one or both values are pseudo-denormals (biased exponent
zero, explicit integer bit 1).  Add a check for that case, so making
the results match those seen on x86 hardware for pseudo-denormals.

Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2005042337570.22972@digraph.polyomino.org.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index ac116c7..6094d26 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -5866,6 +5866,12 @@ static floatx80 addFloatx80Sigs(floatx80 a, floatx80 b, flag zSign,
         zSig1 = 0;
         zSig0 = aSig + bSig;
         if ( aExp == 0 ) {
+            if ((aSig | bSig) & UINT64_C(0x8000000000000000) && zSig0 < aSig) {
+                /* At least one of the values is a pseudo-denormal,
+                 * and there is a carry out of the result.  */
+                zExp = 1;
+                goto shiftRight1;
+            }
             if (zSig0 == 0) {
                 return packFloatx80(zSign, 0, 0);
             }