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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
155 lines
4.8 KiB
C
155 lines
4.8 KiB
C
/*
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* QEMU float support - standalone helpers
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*
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* This is provided for files that don't need the access to the full
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* set of softfloat functions. Typically this is cpu initialisation
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* code which wants to set default rounding and exceptions modes.
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*
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* The code in this source file is derived from release 2a of the SoftFloat
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* IEC/IEEE Floating-point Arithmetic Package. Those parts of the code (and
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* some later contributions) are provided under that license, as detailed below.
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* It has subsequently been modified by contributors to the QEMU Project,
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* so some portions are provided under:
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* the SoftFloat-2a license
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* the BSD license
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* GPL-v2-or-later
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*
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* Any future contributions to this file after December 1st 2014 will be
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* taken to be licensed under the Softfloat-2a license unless specifically
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* indicated otherwise.
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*/
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/*
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===============================================================================
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This C header file is part of the SoftFloat IEC/IEEE Floating-point
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Arithmetic Package, Release 2a.
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Written by John R. Hauser. This work was made possible in part by the
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International Computer Science Institute, located at Suite 600, 1947 Center
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Street, Berkeley, California 94704. Funding was partially provided by the
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National Science Foundation under grant MIP-9311980. The original version
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of this code was written as part of a project to build a fixed-point vector
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processor in collaboration with the University of California at Berkeley,
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overseen by Profs. Nelson Morgan and John Wawrzynek. More information
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is available through the Web page `http://HTTP.CS.Berkeley.EDU/~jhauser/
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arithmetic/SoftFloat.html'.
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THIS SOFTWARE IS DISTRIBUTED AS IS, FOR FREE. Although reasonable effort
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has been made to avoid it, THIS SOFTWARE MAY CONTAIN FAULTS THAT WILL AT
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TIMES RESULT IN INCORRECT BEHAVIOR. USE OF THIS SOFTWARE IS RESTRICTED TO
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PERSONS AND ORGANIZATIONS WHO CAN AND WILL TAKE FULL RESPONSIBILITY FOR ANY
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AND ALL LOSSES, COSTS, OR OTHER PROBLEMS ARISING FROM ITS USE.
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Derivative works are acceptable, even for commercial purposes, so long as
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(1) they include prominent notice that the work is derivative, and (2) they
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include prominent notice akin to these four paragraphs for those parts of
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this code that are retained.
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===============================================================================
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*/
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#ifndef SOFTFLOAT_HELPERS_H
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#define SOFTFLOAT_HELPERS_H
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#include "fpu/softfloat-types.h"
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static inline void set_float_detect_tininess(bool val, float_status *status)
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{
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status->tininess_before_rounding = val;
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}
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static inline void set_float_rounding_mode(FloatRoundMode val,
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float_status *status)
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{
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status->float_rounding_mode = val;
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}
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static inline void set_float_exception_flags(int val, float_status *status)
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{
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status->float_exception_flags = val;
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}
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static inline void set_floatx80_rounding_precision(FloatX80RoundPrec val,
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float_status *status)
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{
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status->floatx80_rounding_precision = val;
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}
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static inline void set_float_2nan_prop_rule(Float2NaNPropRule rule,
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float_status *status)
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{
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status->float_2nan_prop_rule = rule;
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}
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static inline void set_flush_to_zero(bool val, float_status *status)
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{
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status->flush_to_zero = val;
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}
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static inline void set_flush_inputs_to_zero(bool val, float_status *status)
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{
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status->flush_inputs_to_zero = val;
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}
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static inline void set_default_nan_mode(bool val, float_status *status)
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{
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status->default_nan_mode = val;
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}
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static inline void set_snan_bit_is_one(bool val, float_status *status)
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{
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status->snan_bit_is_one = val;
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}
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static inline void set_use_first_nan(bool val, float_status *status)
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{
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status->use_first_nan = val;
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}
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static inline void set_no_signaling_nans(bool val, float_status *status)
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{
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status->no_signaling_nans = val;
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}
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static inline bool get_float_detect_tininess(float_status *status)
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{
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return status->tininess_before_rounding;
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}
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static inline FloatRoundMode get_float_rounding_mode(float_status *status)
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{
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return status->float_rounding_mode;
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}
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static inline int get_float_exception_flags(float_status *status)
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{
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return status->float_exception_flags;
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}
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static inline FloatX80RoundPrec
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get_floatx80_rounding_precision(float_status *status)
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{
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return status->floatx80_rounding_precision;
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}
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static inline Float2NaNPropRule get_float_2nan_prop_rule(float_status *status)
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{
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return status->float_2nan_prop_rule;
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}
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static inline bool get_flush_to_zero(float_status *status)
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{
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return status->flush_to_zero;
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}
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static inline bool get_flush_inputs_to_zero(float_status *status)
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{
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return status->flush_inputs_to_zero;
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}
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static inline bool get_default_nan_mode(float_status *status)
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{
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return status->default_nan_mode;
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}
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#endif /* SOFTFLOAT_HELPERS_H */
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