From 62d39b28ef4e7ad41a3ef3e37037615de6dfc194 Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Tue, 5 Nov 2024 10:09:56 +0000 Subject: [PATCH] target/i386: Set 2-NaN propagation rule explicitly Set the NaN propagation rule explicitly for the float_status words used in the x86 target. This is a no-behaviour-change commit, so we retain the existing behaviour of using the x87-style "prefer QNaN over SNaN, then prefer the NaN with the larger significand" for MMX and SSE. This is however not the documented hardware behaviour, so we leave a TODO note about what we should be doing instead. Signed-off-by: Peter Maydell Reviewed-by: Richard Henderson Message-id: 20241025141254.2141506-16-peter.maydell@linaro.org --- fpu/softfloat-specialize.c.inc | 3 ++- target/i386/cpu.c | 4 ++++ target/i386/cpu.h | 3 +++ target/i386/tcg/fpu_helper.c | 40 ++++++++++++++++++++++++++++++++++ 4 files changed, 49 insertions(+), 1 deletion(-) diff --git a/fpu/softfloat-specialize.c.inc b/fpu/softfloat-specialize.c.inc index b050c5eb04..77ebc8216f 100644 --- a/fpu/softfloat-specialize.c.inc +++ b/fpu/softfloat-specialize.c.inc @@ -405,7 +405,8 @@ static int pickNaN(FloatClass a_cls, FloatClass b_cls, || defined(TARGET_TRICORE) || defined(TARGET_ARM) || defined(TARGET_MIPS) \ || defined(TARGET_LOONGARCH64) || defined(TARGET_HPPA) \ || defined(TARGET_S390X) || defined(TARGET_PPC) || defined(TARGET_M68K) \ - || defined(TARGET_SPARC) || defined(TARGET_XTENSA) + || defined(TARGET_SPARC) || defined(TARGET_XTENSA) \ + || defined(TARGET_I386) g_assert_not_reached(); #else rule = float_2nan_prop_x87; diff --git a/target/i386/cpu.c b/target/i386/cpu.c index 3baa95481f..3d2874cf78 100644 --- a/target/i386/cpu.c +++ b/target/i386/cpu.c @@ -7200,6 +7200,10 @@ static void x86_cpu_reset_hold(Object *obj, ResetType type) memset(env, 0, offsetof(CPUX86State, end_reset_fields)); + if (tcg_enabled()) { + cpu_init_fp_statuses(env); + } + env->old_exception = -1; /* init to reset state */ diff --git a/target/i386/cpu.h b/target/i386/cpu.h index 59959b8b7a..c24d81bf31 100644 --- a/target/i386/cpu.h +++ b/target/i386/cpu.h @@ -2614,6 +2614,9 @@ static inline bool cpu_vmx_maybe_enabled(CPUX86State *env) int get_pg_mode(CPUX86State *env); /* fpu_helper.c */ + +/* Set all non-runtime-variable float_status fields to x86 handling */ +void cpu_init_fp_statuses(CPUX86State *env); void update_fp_status(CPUX86State *env); void update_mxcsr_status(CPUX86State *env); void update_mxcsr_from_sse_status(CPUX86State *env); diff --git a/target/i386/tcg/fpu_helper.c b/target/i386/tcg/fpu_helper.c index e1b850f3fc..53b49bb297 100644 --- a/target/i386/tcg/fpu_helper.c +++ b/target/i386/tcg/fpu_helper.c @@ -135,6 +135,46 @@ static void fpu_set_exception(CPUX86State *env, int mask) } } +void cpu_init_fp_statuses(CPUX86State *env) +{ + /* + * Initialise the non-runtime-varying fields of the various + * float_status words to x86 behaviour. This must be called at + * CPU reset because the float_status words are in the + * "zeroed on reset" portion of the CPU state struct. + * Fields in float_status that vary under guest control are set + * via the codepath for setting that register, eg cpu_set_fpuc(). + */ + /* + * Use x87 NaN propagation rules: + * SNaN + QNaN => return the QNaN + * two SNaNs => return the one with the larger significand, silenced + * two QNaNs => return the one with the larger significand + * SNaN and a non-NaN => return the SNaN, silenced + * QNaN and a non-NaN => return the QNaN + * + * If we get down to comparing significands and they are the same, + * return the NaN with the positive sign bit (if any). + */ + set_float_2nan_prop_rule(float_2nan_prop_x87, &env->fp_status); + /* + * TODO: These are incorrect: the x86 Software Developer's Manual vol 1 + * section 4.8.3.5 "Operating on SNaNs and QNaNs" says that the + * "larger significand" behaviour is only used for x87 FPU operations. + * For SSE the required behaviour is to always return the first NaN, + * which is float_2nan_prop_ab. + * + * mmx_status is used only for the AMD 3DNow! instructions, which + * are documented in the "3DNow! Technology Manual" as not supporting + * NaNs or infinities as inputs. The result of passing two NaNs is + * documented as "undefined", so we can do what we choose. + * (Strictly there is some behaviour we don't implement correctly + * for these "unsupported" NaN and Inf values, like "NaN * 0 == 0".) + */ + set_float_2nan_prop_rule(float_2nan_prop_x87, &env->mmx_status); + set_float_2nan_prop_rule(float_2nan_prop_x87, &env->sse_status); +} + static inline uint8_t save_exception_flags(CPUX86State *env) { uint8_t old_flags = get_float_exception_flags(&env->fp_status);