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softfloat: Support halving the result of muladd operation 

The ARMv8 instruction set includes a fused floating point
reciprocal square root step instruction which demands an
"(x * y + z) / 2" fused operation. Support this by adding
a flag to the softfloat muladd operations which requests
that the result is halved before rounding.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
master
Peter Maydell 2014-02-20 10:35:50 +00:00
parent bc242f9bb6
commit 67d43538ae
2 changed files with 41 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

38
fpu/softfloat.c
View File

@ -2372,6 +2372,17 @@ float32 float32_muladd(float32 a, float32 b, float32 c, int flags STATUS_PARAM)
}
}
/* Zero plus something non-zero : just return the something */
if (flags & float_muladd_halve_result) {
if (cExp == 0) {
normalizeFloat32Subnormal(cSig, &cExp, &cSig);
}
/* Subtract one to halve, and one again because roundAndPackFloat32
* wants one less than the true exponent.
*/
cExp -= 2;
cSig = (cSig | 0x00800000) << 7;
return roundAndPackFloat32(cSign ^ signflip, cExp, cSig STATUS_VAR);
}
return packFloat32(cSign ^ signflip, cExp, cSig);
}
@ -2408,6 +2419,9 @@ float32 float32_muladd(float32 a, float32 b, float32 c, int flags STATUS_PARAM)
/* Throw out the special case of c being an exact zero now */
shift64RightJamming(pSig64, 32, &pSig64);
pSig = pSig64;
if (flags & float_muladd_halve_result) {
pExp--;
}
return roundAndPackFloat32(zSign, pExp - 1,
pSig STATUS_VAR);
}
@ -2472,6 +2486,10 @@ float32 float32_muladd(float32 a, float32 b, float32 c, int flags STATUS_PARAM)
zSig64 <<= shiftcount;
zExp -= shiftcount;
}
if (flags & float_muladd_halve_result) {
zExp--;
}
shift64RightJamming(zSig64, 32, &zSig64);
return roundAndPackFloat32(zSign, zExp, zSig64 STATUS_VAR);
}
@ -4088,6 +4106,17 @@ float64 float64_muladd(float64 a, float64 b, float64 c, int flags STATUS_PARAM)
}
}
/* Zero plus something non-zero : just return the something */
if (flags & float_muladd_halve_result) {
if (cExp == 0) {
normalizeFloat64Subnormal(cSig, &cExp, &cSig);
}
/* Subtract one to halve, and one again because roundAndPackFloat64
* wants one less than the true exponent.
*/
cExp -= 2;
cSig = (cSig | 0x0010000000000000ULL) << 10;
return roundAndPackFloat64(cSign ^ signflip, cExp, cSig STATUS_VAR);
}
return packFloat64(cSign ^ signflip, cExp, cSig);
}
@ -4123,6 +4152,9 @@ float64 float64_muladd(float64 a, float64 b, float64 c, int flags STATUS_PARAM)
if (!cSig) {
/* Throw out the special case of c being an exact zero now */
shift128RightJamming(pSig0, pSig1, 64, &pSig0, &pSig1);
if (flags & float_muladd_halve_result) {
pExp--;
}
return roundAndPackFloat64(zSign, pExp - 1,
pSig1 STATUS_VAR);
}
@ -4159,6 +4191,9 @@ float64 float64_muladd(float64 a, float64 b, float64 c, int flags STATUS_PARAM)
zExp--;
}
shift128RightJamming(zSig0, zSig1, 64, &zSig0, &zSig1);
if (flags & float_muladd_halve_result) {
zExp--;
}
return roundAndPackFloat64(zSign, zExp, zSig1 STATUS_VAR);
} else {
/* Subtraction */
@ -4209,6 +4244,9 @@ float64 float64_muladd(float64 a, float64 b, float64 c, int flags STATUS_PARAM)
zExp -= (shiftcount + 64);
}
}
if (flags & float_muladd_halve_result) {
zExp--;
}
return roundAndPackFloat64(zSign, zExp, zSig0 STATUS_VAR);
}
}

3
include/fpu/softfloat.h
View File

@ -249,11 +249,14 @@ void float_raise( int8 flags STATUS_PARAM);
| Using these differs from negating an input or output before calling
| the muladd function in that this means that a NaN doesn't have its
| sign bit inverted before it is propagated.
| We also support halving the result before rounding, as a special
| case to support the ARM fused-sqrt-step instruction FRSQRTS.
*----------------------------------------------------------------------------*/
enum {
float_muladd_negate_c = 1,
float_muladd_negate_product = 2,
float_muladd_negate_result = 4,
float_muladd_halve_result = 8,
};
/*----------------------------------------------------------------------------