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softfloat: Support halving the result of muladd operation

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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>
This commit is contained in:
Peter Maydell
2014-02-20 10:35:50 +00:00
parent bc242f9bb6
commit 67d43538ae
2 changed files with 41 additions and 0 deletions
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3
include/fpu/softfloat.h
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@@ -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,
};
/*----------------------------------------------------------------------------