softfloat: Allow runtime choice of inf * 0 + NaN result

softfloat: Allow runtime choice of inf * 0 + NaN result

IEEE 758 does not define a fixed rule for what NaN to return in
the case of a fused multiply-add of inf * 0 + NaN. Different
architectures thus do different things:
 * some return the default NaN
 * some return the input NaN
 * Arm returns the default NaN if the input NaN is quiet,
   and the input NaN if it is signalling

We want to make this logic be runtime selected rather than
hardcoded into the binary, because:
 * this will let us have multiple targets in one QEMU binary
 * the Arm FEAT_AFP architectural feature includes letting
   the guest select a NaN propagation rule at runtime

In this commit we add an enum for the propagation rule, the field in
float_status, and the corresponding getters and setters.  We change
pickNaNMulAdd to honour this, but because all targets still leave
this field at its default 0 value, the fallback logic will pick the
rule type with the old ifdef ladder.

Note that four architectures both use the muladd softfloat functions
and did not have a branch of the ifdef ladder to specify their
behaviour (and so were ending up with the "default" case, probably
wrongly): i386, HPPA, SH4 and Tricore.  SH4 and Tricore both set
default_nan_mode, and so will never get into pickNaNMulAdd().  For
HPPA and i386 we retain the same behaviour as the old default-case,
which is to not ever return the default NaN.  This might not be
correct but it is not a behaviour change.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241202131347.498124-4-peter.maydell@linaro.org