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authorAleksandar Markovic <aleksandar.markovic@imgtec.com>2016-06-10 11:57:28 +0200
committerLeon Alrae <leon.alrae@imgtec.com>2016-06-24 13:40:37 +0100
commitaf39bc8c49224771ec0d38f1b693ea78e221d7bc (patch)
treee169b8cd0a4fa36228406260b30ad8aa1e90a1fb /target-m68k
parentc7288767523f6510cf557707d3eb5e78e519b90d (diff)
downloadqemu-arm-af39bc8c49224771ec0d38f1b693ea78e221d7bc.tar.gz
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>
Diffstat (limited to 'target-m68k')
-rw-r--r--target-m68k/helper.c6
1 files changed, 3 insertions, 3 deletions
diff --git a/target-m68k/helper.c b/target-m68k/helper.c
index 427cbedfd5..f52d0e3036 100644
--- a/target-m68k/helper.c
+++ b/target-m68k/helper.c
@@ -558,10 +558,10 @@ float64 HELPER(sub_cmp_f64)(CPUM68KState *env, float64 a, float64 b)
/* ??? Should flush denormals to zero. */
float64 res;
res = float64_sub(a, b, &env->fp_status);
- if (float64_is_quiet_nan(res)) {
+ if (float64_is_quiet_nan(res, &env->fp_status)) {
/* +/-inf compares equal against itself, but sub returns nan. */
- if (!float64_is_quiet_nan(a)
- && !float64_is_quiet_nan(b)) {
+ if (!float64_is_quiet_nan(a, &env->fp_status)
+ && !float64_is_quiet_nan(b, &env->fp_status)) {
res = float64_zero;
if (float64_lt_quiet(a, res, &env->fp_status))
res = float64_chs(res);