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>
diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
index 806ae13..4b4df88 100644
--- a/include/fpu/softfloat.h
+++ b/include/fpu/softfloat.h
@@ -249,11 +249,14 @@
| 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,
};
/*----------------------------------------------------------------------------
