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[2001:8b0:1d0::2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-434aa7d25d1sm56193595e9.31.2024.11.28.06.54.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 Nov 2024 06:54:57 -0800 (PST) From: Peter Maydell To: qemu-devel@nongnu.org Cc: Richard Henderson Subject: [RFC PATCH] softfloat: Allow runtime choice of default NaN value Date: Thu, 28 Nov 2024 14:54:55 +0000 Message-Id: <20241128145455.3742294-1-peter.maydell@linaro.org> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 Received-SPF: pass client-ip=2a00:1450:4864:20::334; envelope-from=peter.maydell@linaro.org; helo=mail-wm1-x334.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Currently we hardcode the default NaN value in parts64_default_nan() using a compile-time ifdef ladder. This is awkward for two cases: * for single-QEMU-binary we can't hard-code target-specifics like this * for Arm FEAT_AFP the default NaN value depends on FPCR.AH (specifically the sign bit is different) Add a field to float_status to specify the default NaN value; fall back to the old ifdef behaviour if these are not set. The default NaN value is specified by setting a uint8_t to a pattern corresponding to the sign and upper fraction parts of the NaN; the lower bits of the fraction are set from bit 0 of the pattern. This is an RFC to ask for opinions on whether this is the right way to let the target set its default NaN. I can't decide whether I think encoding it into a uint8_t like that is clever, or merely too clever :-) The other options would be e.g. separate bool default_nan_sign; uint8_t default_nan_frac; or an enum for the frac possibilities ("default_nan_frac_01__1", "default_nan_frac_010__0", etc ???). The ones we currently need are: frac 1....1 sign 0 (m68k, sparc), sign 1 (hexagon) (with sign 0 for m68k, sparc; sign 1 for hexagon) frac 10...0 (with sign 0 for many targets, sign 1 for i386, microblaze) frac 01...1 (always with sign 0; sh4, some MIPS configs) frac 010..0 (always with sign 0; hppa) Signed-off-by: Peter Maydell Reviewed-by: Richard Henderson --- include/fpu/softfloat-types.h | 10 ++++++ fpu/softfloat-specialize.c.inc | 61 ++++++++++++++++++++++------------ 2 files changed, 49 insertions(+), 22 deletions(-) diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h index 6e237fb697d..0a0508fe2d8 100644 --- a/include/fpu/softfloat-types.h +++ b/include/fpu/softfloat-types.h @@ -285,6 +285,16 @@ typedef struct float_status { /* should denormalised inputs go to zero and set the input_denormal flag? */ bool flush_inputs_to_zero; bool default_nan_mode; + /* + * The pattern to use for the default NaN. Here the high bit specifies + * the default NaN's sign bit, and bits 6..0 specify the high bits of the + * fractional part. The low bits of the fractional part are copies of bit 0. + * The exponent of the default NaN is (as for any NaN) always all 1s. + * Note that a value of 0 here is not a valid NaN. The target must set + * this to the correct non-zero value, or we will assert when trying to + * create a default NaN. + */ + uint8_t default_nan_pattern; /* * The flags below are not used on all specializations and may * constant fold away (see snan_bit_is_one()/no_signalling_nans() in diff --git a/fpu/softfloat-specialize.c.inc b/fpu/softfloat-specialize.c.inc index 353b524d2de..90b6c18a733 100644 --- a/fpu/softfloat-specialize.c.inc +++ b/fpu/softfloat-specialize.c.inc @@ -133,35 +133,52 @@ static void parts64_default_nan(FloatParts64 *p, float_status *status) { bool sign = 0; uint64_t frac; + uint8_t dnan_pattern = status->default_nan_pattern; + if (dnan_pattern == 0) { #if defined(TARGET_SPARC) || defined(TARGET_M68K) - /* !snan_bit_is_one, set all bits */ - frac = (1ULL << DECOMPOSED_BINARY_POINT) - 1; -#elif defined(TARGET_I386) || defined(TARGET_X86_64) \ + /* Sign bit clear, all frac bits set */ + dnan_pattern = 0b01111111; +#elif defined(TARGET_I386) || defined(TARGET_X86_64) \ || defined(TARGET_MICROBLAZE) - /* !snan_bit_is_one, set sign and msb */ - frac = 1ULL << (DECOMPOSED_BINARY_POINT - 1); - sign = 1; + /* Sign bit set, most significant frac bit set */ + dnan_pattern = 0b11000000; #elif defined(TARGET_HPPA) - /* snan_bit_is_one, set msb-1. */ - frac = 1ULL << (DECOMPOSED_BINARY_POINT - 2); + /* Sign bit clear, msb-1 frac bit set */ + dnan_pattern = 0b00100000; #elif defined(TARGET_HEXAGON) - sign = 1; - frac = ~0ULL; + /* + * Sign bit set, all frac bits set. This is an odd special case, + * where our value doesn't match up with the snan_bit_is_one setting. + * This is because for Hexagon the returned value is always -1, + * not a real NaN value. + */ + dnan_pattern = 0b11111111; #else - /* - * This case is true for Alpha, ARM, MIPS, OpenRISC, PPC, RISC-V, - * S390, SH4, TriCore, and Xtensa. Our other supported targets - * do not have floating-point. - */ - if (snan_bit_is_one(status)) { - /* set all bits other than msb */ - frac = (1ULL << (DECOMPOSED_BINARY_POINT - 1)) - 1; - } else { - /* set msb */ - frac = 1ULL << (DECOMPOSED_BINARY_POINT - 1); - } + /* + * This case is true for Alpha, ARM, MIPS, OpenRISC, PPC, RISC-V, + * S390, SH4, TriCore, and Xtensa. Our other supported targets + * do not have floating-point. + */ + if (snan_bit_is_one(status)) { + /* sign bit clear, set all frac bits other than msb */ + dnan_pattern = 0b00111111; + } else { + /* sign bit clear, set frac msb */ + dnan_pattern = 0b01000000; + } #endif + } + + sign = status->default_nan_pattern >> 7; + /* + * Place default_nan_pattern [6:0] into bits [62:56], + * and replicate bit [0] down into [55:0] + */ + frac = deposit64(0, DECOMPOSED_BINARY_POINT - 7, 7, + status->default_nan_pattern); + frac = deposit64(frac, 0, DECOMPOSED_BINARY_POINT - 7, + -(status->default_nan_pattern & 1)); *p = (FloatParts64) { .cls = float_class_qnan,