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softfloat: Fix factor 2 error for scalbn on denormal inputs 

If the input to float*_scalbn() is denormal then it represents
a number 0.[mantissabits] * 2^(1-exponentbias) (and the actual
exponent field is all zeroes). This means that when we convert
it to our unpacked encoding the unpacked exponent must be one
greater than for a normal number, which represents
1.[mantissabits] * 2^(e-exponentbias) for an exponent field e.

This meant we were giving answers too small by a factor of 2 for
all denormal inputs.

Note that the float-to-int routines also have this behaviour
of not adjusting the exponent for denormals; however there it is
harmless because denormals will all convert to integer zero anyway.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
This commit is contained in:
Peter Maydell 2014-01-07 17:17:50 +00:00
parent 34e1c27bc3
commit 3c85c37f25
1 changed files with 21 additions and 8 deletions
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29
fpu/softfloat.c
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@ -6991,10 +6991,13 @@ float32 float32_scalbn( float32 a, int n STATUS_PARAM )
}
return a;
}
if ( aExp != 0 )
if (aExp != 0) {
aSig |= 0x00800000;
else if ( aSig == 0 )
} else if (aSig == 0) {
return a;
} else {
aExp++;
}
if (n > 0x200) {
n = 0x200;
@ -7024,10 +7027,13 @@ float64 float64_scalbn( float64 a, int n STATUS_PARAM )
}
return a;
}
if ( aExp != 0 )
if (aExp != 0) {
aSig |= LIT64( 0x0010000000000000 );
else if ( aSig == 0 )
} else if (aSig == 0) {
return a;
} else {
aExp++;
}
if (n > 0x1000) {
n = 0x1000;
@ -7057,8 +7063,12 @@ floatx80 floatx80_scalbn( floatx80 a, int n STATUS_PARAM )
return a;
}
if (aExp == 0 && aSig == 0)
return a;
if (aExp == 0) {
if (aSig == 0) {
return a;
}
aExp++;
}
if (n > 0x10000) {
n = 0x10000;
@ -7087,10 +7097,13 @@ float128 float128_scalbn( float128 a, int n STATUS_PARAM )
}
return a;
}
if ( aExp != 0 )
if (aExp != 0) {
aSig0 |= LIT64( 0x0001000000000000 );
else if ( aSig0 == 0 && aSig1 == 0 )
} else if (aSig0 == 0 && aSig1 == 0) {
return a;
} else {
aExp++;
}
if (n > 0x10000) {
n = 0x10000;