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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>
This commit is contained in:
Peter Maydell 2010-12-16 11:51:17 +00:00 committed by Aurelien Jarno
parent 185698715d
commit 354f211b1a
1 changed files with 100 additions and 60 deletions
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160
fpu/softfloat-specialize.h
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@ -152,6 +152,52 @@ static float32 commonNaNToFloat32( commonNaNT a )
return float32_default_nan; return float32_default_nan;
} }
/*----------------------------------------------------------------------------
| 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 | 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 | is a NaN, and returns the appropriate NaN result. If either `a' or `b' is a
@ -160,7 +206,7 @@ static float32 commonNaNToFloat32( commonNaNT a )
static float32 propagateFloat32NaN( float32 a, float32 b STATUS_PARAM) static float32 propagateFloat32NaN( float32 a, float32 b STATUS_PARAM)
{ {
flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN; flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN, aIsLargerSignificand;
bits32 av, bv, res; bits32 av, bv, res;
if ( STATUS(default_nan_mode) ) if ( STATUS(default_nan_mode) )
@ -180,26 +226,22 @@ static float32 propagateFloat32NaN( float32 a, float32 b STATUS_PARAM)
bv |= 0x00400000; bv |= 0x00400000;
#endif #endif
if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR); if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR);
if ( aIsSignalingNaN ) {
if ( bIsSignalingNaN ) goto returnLargerSignificand; if ((bits32)(av<<1) < (bits32)(bv<<1)) {
res = bIsNaN ? bv : av; aIsLargerSignificand = 0;
} else if ((bits32)(bv<<1) < (bits32)(av<<1)) {
aIsLargerSignificand = 1;
} else {
aIsLargerSignificand = (av < bv) ? 1 : 0;
} }
else if ( aIsNaN ) {
if ( bIsSignalingNaN || ! bIsNaN ) if (pickNaN(aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN,
res = av; aIsLargerSignificand)) {
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;
}
}
else {
res = bv; res = bv;
} else {
res = av;
} }
return make_float32(res); return make_float32(res);
} }
@ -314,7 +356,7 @@ static float64 commonNaNToFloat64( commonNaNT a )
static float64 propagateFloat64NaN( float64 a, float64 b STATUS_PARAM) static float64 propagateFloat64NaN( float64 a, float64 b STATUS_PARAM)
{ {
flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN; flag aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN, aIsLargerSignificand;
bits64 av, bv, res; bits64 av, bv, res;
if ( STATUS(default_nan_mode) ) if ( STATUS(default_nan_mode) )
@ -334,26 +376,22 @@ static float64 propagateFloat64NaN( float64 a, float64 b STATUS_PARAM)
bv |= LIT64( 0x0008000000000000 ); bv |= LIT64( 0x0008000000000000 );
#endif #endif
if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR); if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR);
if ( aIsSignalingNaN ) {
if ( bIsSignalingNaN ) goto returnLargerSignificand; if ((bits64)(av<<1) < (bits64)(bv<<1)) {
res = bIsNaN ? bv : av; aIsLargerSignificand = 0;
} else if ((bits64)(bv<<1) < (bits64)(av<<1)) {
aIsLargerSignificand = 1;
} else {
aIsLargerSignificand = (av < bv) ? 1 : 0;
} }
else if ( aIsNaN ) {
if ( bIsSignalingNaN || ! bIsNaN ) if (pickNaN(aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN,
res = av; aIsLargerSignificand)) {
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;
}
}
else {
res = bv; res = bv;
} else {
res = av;
} }
return make_float64(res); return make_float64(res);
} }
@ -454,7 +492,7 @@ static floatx80 commonNaNToFloatx80( commonNaNT a )
static floatx80 propagateFloatx80NaN( floatx80 a, floatx80 b STATUS_PARAM) 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) ) { if ( STATUS(default_nan_mode) ) {
a.low = floatx80_default_nan_low; a.low = floatx80_default_nan_low;
@ -474,19 +512,20 @@ static floatx80 propagateFloatx80NaN( floatx80 a, floatx80 b STATUS_PARAM)
b.low |= LIT64( 0xC000000000000000 ); b.low |= LIT64( 0xC000000000000000 );
#endif #endif
if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR); if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR);
if ( aIsSignalingNaN ) {
if ( bIsSignalingNaN ) goto returnLargerSignificand; if (a.low < b.low) {
return bIsNaN ? b : a; aIsLargerSignificand = 0;
} else if (b.low < a.low) {
aIsLargerSignificand = 1;
} else {
aIsLargerSignificand = (a.high < b.high) ? 1 : 0;
} }
else if ( aIsNaN ) {
if ( bIsSignalingNaN || ! bIsNaN ) return a; if (pickNaN(aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN,
returnLargerSignificand: aIsLargerSignificand)) {
if ( a.low < b.low ) return b;
if ( b.low < a.low ) return a;
return ( a.high < b.high ) ? a : b;
}
else {
return b; return b;
} else {
return a;
} }
} }
@ -580,7 +619,7 @@ static float128 commonNaNToFloat128( commonNaNT a )
static float128 propagateFloat128NaN( float128 a, float128 b STATUS_PARAM) 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) ) { if ( STATUS(default_nan_mode) ) {
a.low = float128_default_nan_low; a.low = float128_default_nan_low;
@ -600,19 +639,20 @@ static float128 propagateFloat128NaN( float128 a, float128 b STATUS_PARAM)
b.high |= LIT64( 0x0000800000000000 ); b.high |= LIT64( 0x0000800000000000 );
#endif #endif
if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR); if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid STATUS_VAR);
if ( aIsSignalingNaN ) {
if ( bIsSignalingNaN ) goto returnLargerSignificand; if (lt128(a.high<<1, a.low, b.high<<1, b.low)) {
return bIsNaN ? b : a; 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 ( aIsNaN ) {
if ( bIsSignalingNaN || ! bIsNaN ) return a; if (pickNaN(aIsNaN, aIsSignalingNaN, bIsNaN, bIsSignalingNaN,
returnLargerSignificand: aIsLargerSignificand)) {
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;
}
else {
return b; return b;
} else {
return a;
} }
} }