I've been playing with the double precision facilities in IL and testing out how the _neg() operator affects them.
The IL doc suggests _neg() is different for ints and floats but doesn't say what it does on doubles. Usage in lushaders.h (e.g. dmad r1.zw, r0.zw_neg(xyzw), r1.xy, r1.zw) seems to indicate that _neg(xyzw) will negate the double in xy and the one in zw. However, if _neg() acts on a register containing two doubles just as it might on one containing four floats you think you might only need _neg(yw) to flip the sign bits.
On testing it seems that either work for dadd but not for dmul or dmad: only _neg(yw) consistently works. If you use _neg(xyzw) for dmul or dmad you will get almost the right answer but not quite: one of the bits of the mantissa, corresponding to the high bit of x or z, of the operand will be wrong.
So it seems LUShaders and code that similarly negates doubles will suffer from a loss of precision.
Will _neg(yw) remain the correct way to negate doubles or should _neg(xyzw) work for dmul and dmad etc.?