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target/i386: Detect flush-to-zero after rounding

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The Intel SDM section 10.2.3.3 on the MXCSR.FTZ bit says that we
flush outputs to zero when we detect underflow, which is after
rounding.  Set the detect_ftz flag accordingly.

This allows us to enable the test in fma.c which checks this
behaviour.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250519145114.2786534-2-peter.maydell@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Peter Maydell 2025-05-19 15:51:11 +01:00 • committed by Paolo Bonzini
parent 23aec0d0e8
commit bc40e4fe62
2 changed files with 4 additions and 9 deletions
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8
target/i386/tcg/fpu_helper.c
View file

@ -189,13 +189,13 @@ void cpu_init_fp_statuses(CPUX86State *env)
set_float_default_nan_pattern(0b11000000, &env->mmx_status);
set_float_default_nan_pattern(0b11000000, &env->sse_status);
/*
* TODO: x86 does flush-to-zero detection after rounding (the SDM
* x86 does flush-to-zero detection after rounding (the SDM
* section 10.2.3.3 on the FTZ bit of MXCSR says that we flush
* when we detect underflow, which x86 does after rounding).
*/
set_float_ftz_detection(float_ftz_before_rounding, &env->fp_status);
set_float_ftz_detection(float_ftz_before_rounding, &env->mmx_status);
set_float_ftz_detection(float_ftz_before_rounding, &env->sse_status);
set_float_ftz_detection(float_ftz_after_rounding, &env->fp_status);
set_float_ftz_detection(float_ftz_after_rounding, &env->mmx_status);
set_float_ftz_detection(float_ftz_after_rounding, &env->sse_status);
}
static inline uint8_t save_exception_flags(CPUX86State *env)

5
tests/tcg/x86_64/fma.c
View file

@ -79,14 +79,9 @@ static testdata tests[] = {
/*
* Flushing of denormal outputs to zero should also happen after
* rounding, so setting FTZ should not affect the result or the flags.
* QEMU currently does not emulate this correctly because we do the
* flush-to-zero check before rounding, so we incorrectly produce a
* zero result and set Underflow as well as Precision.
*/
#ifdef ENABLE_FAILING_TESTS
{ 0x3fdfffffffffffff, 0x001fffffffffffff, 0x801fffffffffffff, true,
0x8010000000000000, 0x20 }, /* Enabling FTZ shouldn't change flags */
#endif
};
int main(void)