mirrors/qemu
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

target/i386: Set 2-NaN propagation rule explicitly

Browse Source 
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 <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241025141254.2141506-16-peter.maydell@linaro.org
This commit is contained in:
Peter Maydell 2024-11-05 10:09:56 +00:00
parent 8d988eb44c
commit 62d39b28ef
4 changed files with 49 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
fpu/softfloat-specialize.c.inc
View File

@ -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;

4
target/i386/cpu.c
View File

@ -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 */

3
target/i386/cpu.h
View File

@ -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);

40
target/i386/tcg/fpu_helper.c
View File

@ -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);