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authorPeter Maydell <peter.maydell@linaro.org>2010-12-16 11:51:17 +0000
committerAurelien Jarno <aurelien@aurel32.net>2011-01-02 23:58:57 +0100
commit354f211b1a49a7387929e22d6e63849fcba48f8a (patch)
tree7aebf4c50edb02147a2164ae038659f17c65cf28 /fpu/softfloat-specialize.h
parent185698715dfb18c82ad2a5dbc169908602d43e81 (diff)
downloadqemu-arm-354f211b1a49a7387929e22d6e63849fcba48f8a.tar.gz
softfloat: abstract out target-specific NaN propagation rules
IEEE754 doesn't specify precisely what NaN should be returned as the result of an operation on two input NaNs. This is therefore target-specific. Abstract out the code in propagateFloat*NaN() which was implementing the x87 propagation rules, so that it can be easily replaced on a per-target basis. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Diffstat (limited to 'fpu/softfloat-specialize.h')
-rw-r--r--fpu/softfloat-specialize.h160
1 files changed, 100 insertions, 60 deletions
diff --git a/fpu/softfloat-specialize.h b/fpu/softfloat-specialize.h
index f382f7a00c..8dcf469ee6 100644
--- a/fpu/softfloat-specialize.h
+++ b/fpu/softfloat-specialize.h
@@ -153,6 +153,52 @@ static float32 commonNaNToFloat32( commonNaNT a )
}
/*----------------------------------------------------------------------------
+| Select which NaN to propagate for a two-input operation.
+| IEEE754 doesn't specify all the details of this, so the
+| algorithm is target-specific.
+| The routine is passed various bits of information about the
+| two NaNs and should return 0 to select NaN a and 1 for NaN b.
+| Note that signalling NaNs are always squashed to quiet NaNs
+| by the caller, by flipping the SNaN bit before returning them.
+|
+| aIsLargerSignificand is only valid if both a and b are NaNs
+| of some kind, and is true if a has the larger significand,
+| or if both a and b have the same significand but a is
+| positive but b is negative. It is only needed for the x87
+| tie-break rule.
+*----------------------------------------------------------------------------*/
+
+static int pickNaN(flag aIsQNaN, flag aIsSNaN, flag bIsQNaN, flag bIsSNaN,
+ flag aIsLargerSignificand)
+{
+ /* This implements x87 NaN propagation rules:
+ * SNaN + QNaN => return the QNaN
+ * two SNaNs => return the one with the larger significand, silenced
+ * two QNaNs => return the one with the larger significand
+ * SNaN and a non-NaN => return the SNaN, silenced
+ * QNaN and a non-NaN => return the QNaN
+ *
+ * If we get down to comparing significands and they are the same,
+ * return the NaN with the positive sign bit (if any).
+ */
+ if (aIsSNaN) {
+ if (bIsSNaN) {
+ return aIsLargerSignificand ? 0 : 1;
+ }
+ return bIsQNaN ? 1 : 0;
+ }
+ else if (aIsQNaN) {
+ if (bIsSNaN || !bIsQNaN)
+ return 0;
+ else {
+ return aIsLargerSignificand ? 0 : 1;
+ }
+ } else {
+ return 1;
+ }
+}
+
+/*----------------------------------------------------------------------------
| Takes two single-precision floating-point values `a' and `b', one of which
| is a NaN, and returns the appropriate NaN result. If either `a' or `b' is a
| signaling NaN, the invalid exception is raised.
@@ -160,7 +206,7 @@ static float32 commonNaNToFloat32( commonNaNT a )
static float32 propagateFloat32NaN( float32 a, float32 b STATUS_PARAM)
{
- flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN;
+ flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN, aIsLargerSignificand;
bits32 av, bv, res;
if ( STATUS(default_nan_mode) )
@@ -180,26 +226,22 @@ static float32 propagateFloat32NaN( float32 a, float32 b STATUS_PARAM)
bv |= 0x00400000;
#endif
if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR);
- if ( aIsSignalingNaN ) {
- if ( bIsSignalingNaN ) goto returnLargerSignificand;
- res = bIsNaN ? bv : av;
- }
- else if ( aIsNaN ) {
- if ( bIsSignalingNaN || ! bIsNaN )
- res = av;
- else {
- returnLargerSignificand:
- if ( (bits32) ( av<<1 ) < (bits32) ( bv<<1 ) )
- res = bv;
- else if ( (bits32) ( bv<<1 ) < (bits32) ( av<<1 ) )
- res = av;
- else
- res = ( av < bv ) ? av : bv;
- }
+
+ if ((bits32)(av<<1) < (bits32)(bv<<1)) {
+ aIsLargerSignificand = 0;
+ } else if ((bits32)(bv<<1) < (bits32)(av<<1)) {
+ aIsLargerSignificand = 1;
+ } else {
+ aIsLargerSignificand = (av < bv) ? 1 : 0;
}
- else {
+
+ if (pickNaN(aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN,
+ aIsLargerSignificand)) {
res = bv;
+ } else {
+ res = av;
}
+
return make_float32(res);
}
@@ -314,7 +356,7 @@ static float64 commonNaNToFloat64( commonNaNT a )
static float64 propagateFloat64NaN( float64 a, float64 b STATUS_PARAM)
{
- flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN;
+ flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN, aIsLargerSignificand;
bits64 av, bv, res;
if ( STATUS(default_nan_mode) )
@@ -334,26 +376,22 @@ static float64 propagateFloat64NaN( float64 a, float64 b STATUS_PARAM)
bv |= LIT64( 0x0008000000000000 );
#endif
if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR);
- if ( aIsSignalingNaN ) {
- if ( bIsSignalingNaN ) goto returnLargerSignificand;
- res = bIsNaN ? bv : av;
- }
- else if ( aIsNaN ) {
- if ( bIsSignalingNaN || ! bIsNaN )
- res = av;
- else {
- returnLargerSignificand:
- if ( (bits64) ( av<<1 ) < (bits64) ( bv<<1 ) )
- res = bv;
- else if ( (bits64) ( bv<<1 ) < (bits64) ( av<<1 ) )
- res = av;
- else
- res = ( av < bv ) ? av : bv;
- }
+
+ if ((bits64)(av<<1) < (bits64)(bv<<1)) {
+ aIsLargerSignificand = 0;
+ } else if ((bits64)(bv<<1) < (bits64)(av<<1)) {
+ aIsLargerSignificand = 1;
+ } else {
+ aIsLargerSignificand = (av < bv) ? 1 : 0;
}
- else {
+
+ if (pickNaN(aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN,
+ aIsLargerSignificand)) {
res = bv;
+ } else {
+ res = av;
}
+
return make_float64(res);
}
@@ -454,7 +492,7 @@ static floatx80 commonNaNToFloatx80( commonNaNT a )
static floatx80 propagateFloatx80NaN( floatx80 a, floatx80 b STATUS_PARAM)
{
- flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN;
+ flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN, aIsLargerSignificand;
if ( STATUS(default_nan_mode) ) {
a.low = floatx80_default_nan_low;
@@ -474,19 +512,20 @@ static floatx80 propagateFloatx80NaN( floatx80 a, floatx80 b STATUS_PARAM)
b.low |= LIT64( 0xC000000000000000 );
#endif
if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR);
- if ( aIsSignalingNaN ) {
- if ( bIsSignalingNaN ) goto returnLargerSignificand;
- return bIsNaN ? b : a;
- }
- else if ( aIsNaN ) {
- if ( bIsSignalingNaN || ! bIsNaN ) return a;
- returnLargerSignificand:
- if ( a.low < b.low ) return b;
- if ( b.low < a.low ) return a;
- return ( a.high < b.high ) ? a : b;
+
+ if (a.low < b.low) {
+ aIsLargerSignificand = 0;
+ } else if (b.low < a.low) {
+ aIsLargerSignificand = 1;
+ } else {
+ aIsLargerSignificand = (a.high < b.high) ? 1 : 0;
}
- else {
+
+ if (pickNaN(aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN,
+ aIsLargerSignificand)) {
return b;
+ } else {
+ return a;
}
}
@@ -580,7 +619,7 @@ static float128 commonNaNToFloat128( commonNaNT a )
static float128 propagateFloat128NaN( float128 a, float128 b STATUS_PARAM)
{
- flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN;
+ flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN, aIsLargerSignificand;
if ( STATUS(default_nan_mode) ) {
a.low = float128_default_nan_low;
@@ -600,19 +639,20 @@ static float128 propagateFloat128NaN( float128 a, float128 b STATUS_PARAM)
b.high |= LIT64( 0x0000800000000000 );
#endif
if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR);
- if ( aIsSignalingNaN ) {
- if ( bIsSignalingNaN ) goto returnLargerSignificand;
- return bIsNaN ? b : a;
- }
- else if ( aIsNaN ) {
- if ( bIsSignalingNaN || ! bIsNaN ) return a;
- returnLargerSignificand:
- if ( lt128( a.high<<1, a.low, b.high<<1, b.low ) ) return b;
- if ( lt128( b.high<<1, b.low, a.high<<1, a.low ) ) return a;
- return ( a.high < b.high ) ? a : b;
+
+ if (lt128(a.high<<1, a.low, b.high<<1, b.low)) {
+ aIsLargerSignificand = 0;
+ } else if (lt128(b.high<<1, b.low, a.high<<1, a.low)) {
+ aIsLargerSignificand = 1;
+ } else {
+ aIsLargerSignificand = (a.high < b.high) ? 1 : 0;
}
- else {
+
+ if (pickNaN(aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN,
+ aIsLargerSignificand)) {
return b;
+ } else {
+ return a;
}
}