softfloat: Implement run-time-configurable meaning of signaling NaN bit

This patch modifies SoftFloat library so that it can be configured in
run-time in relation to the meaning of signaling NaN bit, while, at the
same time, strictly preserving its behavior on all existing platforms.

Background:

In floating-point calculations, there is a need for denoting undefined or
unrepresentable values. This is achieved by defining certain floating-point
numerical values to be NaNs (which stands for "not a number"). For additional
reasons, virtually all modern floating-point unit implementations use two
kinds of NaNs: quiet and signaling. The binary representations of these two
kinds of NaNs, as a rule, differ only in one bit (that bit is, traditionally,
the first bit of mantissa).

Up to 2008, standards for floating-point did not specify all details about
binary representation of NaNs. More specifically, the meaning of the bit
that is used for distinguishing between signaling and quiet NaNs was not
strictly prescribed. (IEEE 754-2008 was the first floating-point standard
that defined that meaning clearly, see [1], p. 35) As a result, different
platforms took different approaches, and that presented considerable
challenge for multi-platform emulators like QEMU.

Mips platform represents the most complex case among QEMU-supported
platforms regarding signaling NaN bit. Up to the Release 6 of Mips
architecture, "1" in signaling NaN bit denoted signaling NaN, which is
opposite to IEEE 754-2008 standard. From Release 6 on, Mips architecture
adopted IEEE standard prescription, and "0" denotes signaling NaN. On top of
that, Mips architecture for SIMD (also known as MSA, or vector instructions)
also specifies signaling bit in accordance to IEEE standard. MSA unit can be
implemented with both pre-Release 6 and Release 6 main processor units.

QEMU uses SoftFloat library to implement various floating-point-related
instructions on all platforms. The current QEMU implementation allows for
defining meaning of signaling NaN bit during build time, and is implemented
via preprocessor macro called SNAN_BIT_IS_ONE.

On the other hand, the change in this patch enables SoftFloat library to be
configured in run-time. This configuration is meant to occur during CPU
initialization, at the moment when it is definitely known what desired
behavior for particular CPU (or any additional FPUs) is.

The change is implemented so that it is consistent with existing
implementation of similar cases. This means that structure float_status is
used for passing the information about desired signaling NaN bit on each
invocation of SoftFloat functions. The additional field in float_status is
called snan_bit_is_one, which supersedes macro SNAN_BIT_IS_ONE.

IMPORTANT:

This change is not meant to create any change in emulator behavior or
functionality on any platform. It just provides the means for SoftFloat
library to be used in a more flexible way - in other words, it will just
prepare SoftFloat library for usage related to Mips platform and its
specifics regarding signaling bit meaning, which is done in some of
subsequent patches from this series.

Further break down of changes:

  1) Added field snan_bit_is_one to the structure float_status, and
     correspondent setter function set_snan_bit_is_one().

  2) Constants <float16|float32|float64|floatx80|float128>_default_nan
     (used both internally and externally) converted to functions
     <float16|float32|float64|floatx80|float128>_default_nan(float_status*).
     This is necessary since they are dependent on signaling bit meaning.
     At the same time, for the sake of code cleanup and simplicity, constants
     <floatx80|float128>_default_nan_<low|high> (used only internally within
     SoftFloat library) are removed, as not needed.

  3) Added a float_status* argument to SoftFloat library functions
     XXX_is_quiet_nan(XXX a_), XXX_is_signaling_nan(XXX a_),
     XXX_maybe_silence_nan(XXX a_). This argument must be present in
     order to enable correct invocation of new version of functions
     XXX_default_nan(). (XXX is <float16|float32|float64|floatx80|float128>
     here)

  4) Updated code for all platforms to reflect changes in SoftFloat library.
     This change is twofolds: it includes modifications of SoftFloat library
     functions invocations, and an addition of invocation of function
     set_snan_bit_is_one() during CPU initialization, with arguments that
     are appropriate for each particular platform. It was established that
     all platforms zero their main CPU data structures, so snan_bit_is_one(0)
     in appropriate places is not added, as it is not needed.

[1] "IEEE Standard for Floating-Point Arithmetic",
    IEEE Computer Society, August 29, 2008.

Signed-off-by: Thomas Schwinge <thomas@codesourcery.com>
Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Tested-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Tested-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[leon.alrae@imgtec.com:
 * cherry-picked 2 chunks from patch #2 to fix compilation warnings]
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>

1 files changed, 25 insertions, 20 deletions

diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
index c937062..95a1103 100644
--- a/include/fpu/softfloat.h
+++ b/include/fpu/softfloat.h
@@ -205,6 +205,7 @@ typedef struct float_status {
     /* should denormalised inputs go to zero and set the input_denormal flag? */
     flag flush_inputs_to_zero;
     flag default_nan_mode;
+    flag snan_bit_is_one;
 } float_status;
 
 static inline void set_float_detect_tininess(int val, float_status *status)
@@ -236,6 +237,10 @@ static inline void set_default_nan_mode(flag val, float_status *status)
 {
     status->default_nan_mode = val;
 }
+static inline void set_snan_bit_is_one(flag val, float_status *status)
+{
+    status->snan_bit_is_one = val;
+}
 static inline int get_float_detect_tininess(float_status *status)
 {
     return status->float_detect_tininess;
@@ -342,9 +347,9 @@ float64 float16_to_float64(float16 a, flag ieee, float_status *status);
 /*----------------------------------------------------------------------------
 | Software half-precision operations.
 *----------------------------------------------------------------------------*/
-int float16_is_quiet_nan( float16 );
-int float16_is_signaling_nan( float16 );
-float16 float16_maybe_silence_nan( float16 );
+int float16_is_quiet_nan(float16, float_status *status);
+int float16_is_signaling_nan(float16, float_status *status);
+float16 float16_maybe_silence_nan(float16, float_status *status);
 
 static inline int float16_is_any_nan(float16 a)
 {
@@ -354,7 +359,7 @@ static inline int float16_is_any_nan(float16 a)
 /*----------------------------------------------------------------------------
 | The pattern for a default generated half-precision NaN.
 *----------------------------------------------------------------------------*/
-extern const float16 float16_default_nan;
+float16 float16_default_nan(float_status *status);
 
 /*----------------------------------------------------------------------------
 | Software IEC/IEEE single-precision conversion routines.
@@ -404,9 +409,9 @@ float32 float32_minnum(float32, float32, float_status *status);
 float32 float32_maxnum(float32, float32, float_status *status);
 float32 float32_minnummag(float32, float32, float_status *status);
 float32 float32_maxnummag(float32, float32, float_status *status);
-int float32_is_quiet_nan( float32 );
-int float32_is_signaling_nan( float32 );
-float32 float32_maybe_silence_nan( float32 );
+int float32_is_quiet_nan(float32, float_status *status);
+int float32_is_signaling_nan(float32, float_status *status);
+float32 float32_maybe_silence_nan(float32, float_status *status);
 float32 float32_scalbn(float32, int, float_status *status);
 
 static inline float32 float32_abs(float32 a)
@@ -466,7 +471,7 @@ static inline float32 float32_set_sign(float32 a, int sign)
 /*----------------------------------------------------------------------------
 | The pattern for a default generated single-precision NaN.
 *----------------------------------------------------------------------------*/
-extern const float32 float32_default_nan;
+float32 float32_default_nan(float_status *status);
 
 /*----------------------------------------------------------------------------
 | Software IEC/IEEE double-precision conversion routines.
@@ -516,9 +521,9 @@ float64 float64_minnum(float64, float64, float_status *status);
 float64 float64_maxnum(float64, float64, float_status *status);
 float64 float64_minnummag(float64, float64, float_status *status);
 float64 float64_maxnummag(float64, float64, float_status *status);
-int float64_is_quiet_nan( float64 a );
-int float64_is_signaling_nan( float64 );
-float64 float64_maybe_silence_nan( float64 );
+int float64_is_quiet_nan(float64 a, float_status *status);
+int float64_is_signaling_nan(float64, float_status *status);
+float64 float64_maybe_silence_nan(float64, float_status *status);
 float64 float64_scalbn(float64, int, float_status *status);
 
 static inline float64 float64_abs(float64 a)
@@ -578,7 +583,7 @@ static inline float64 float64_set_sign(float64 a, int sign)
 /*----------------------------------------------------------------------------
 | The pattern for a default generated double-precision NaN.
 *----------------------------------------------------------------------------*/
-extern const float64 float64_default_nan;
+float64 float64_default_nan(float_status *status);
 
 /*----------------------------------------------------------------------------
 | Software IEC/IEEE extended double-precision conversion routines.
@@ -611,9 +616,9 @@ int floatx80_lt_quiet(floatx80, floatx80, float_status *status);
 int floatx80_unordered_quiet(floatx80, floatx80, float_status *status);
 int floatx80_compare(floatx80, floatx80, float_status *status);
 int floatx80_compare_quiet(floatx80, floatx80, float_status *status);
-int floatx80_is_quiet_nan( floatx80 );
-int floatx80_is_signaling_nan( floatx80 );
-floatx80 floatx80_maybe_silence_nan( floatx80 );
+int floatx80_is_quiet_nan(floatx80, float_status *status);
+int floatx80_is_signaling_nan(floatx80, float_status *status);
+floatx80 floatx80_maybe_silence_nan(floatx80, float_status *status);
 floatx80 floatx80_scalbn(floatx80, int, float_status *status);
 
 static inline floatx80 floatx80_abs(floatx80 a)
@@ -663,7 +668,7 @@ static inline int floatx80_is_any_nan(floatx80 a)
 /*----------------------------------------------------------------------------
 | The pattern for a default generated extended double-precision NaN.
 *----------------------------------------------------------------------------*/
-extern const floatx80 floatx80_default_nan;
+floatx80 floatx80_default_nan(float_status *status);
 
 /*----------------------------------------------------------------------------
 | Software IEC/IEEE quadruple-precision conversion routines.
@@ -696,9 +701,9 @@ int float128_lt_quiet(float128, float128, float_status *status);
 int float128_unordered_quiet(float128, float128, float_status *status);
 int float128_compare(float128, float128, float_status *status);
 int float128_compare_quiet(float128, float128, float_status *status);
-int float128_is_quiet_nan( float128 );
-int float128_is_signaling_nan( float128 );
-float128 float128_maybe_silence_nan( float128 );
+int float128_is_quiet_nan(float128, float_status *status);
+int float128_is_signaling_nan(float128, float_status *status);
+float128 float128_maybe_silence_nan(float128, float_status *status);
 float128 float128_scalbn(float128, int, float_status *status);
 
 static inline float128 float128_abs(float128 a)
@@ -744,6 +749,6 @@ static inline int float128_is_any_nan(float128 a)
 /*----------------------------------------------------------------------------
 | The pattern for a default generated quadruple-precision NaN.
 *----------------------------------------------------------------------------*/
-extern const float128 float128_default_nan;
+float128 float128_default_nan(float_status *status);
 
 #endif /* !SOFTFLOAT_H */