IEEE 758 does not define a fixed rule for which NaN to pick as the
result if both operands of a 2-operand operation are NaNs. As a
result different architectures have ended up with different rules for
propagating NaNs.
QEMU currently hardcodes the NaN propagation logic into the binary
because pickNaN() has an ifdef ladder for different targets. We want
to make the propagation rule instead be selectable at runtime,
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
* x86 specifies different propagation rules for x87 FPU ops
and for SSE ops, and specifying the rule in the float_status
would let us emulate this, instead of wrongly using the
x87 rules everywhere
In this commit we add an enum for the propagation rule, the field in
float_status, and the corresponding getters and setters. We change
pickNaN 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.
It's valid not to set a propagation rule if default_nan_mode is
enabled, because in that case there's no need to pick a NaN; all the
callers of pickNaN() catch this case and skip calling it. So we can
already assert that we don't get into the "no rule defined" codepath
for our four targets which always set default_nan_mode: Hexagon,
RiscV, SH4 and Tricore, and for the one target which does not have FP
at all: avr. These targets will not need to be updated to call
set_float_2nan_prop_rule().
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241025141254.2141506-2-peter.maydell@linaro.org
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