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Re: [RFC PATCH] target/i386: for maximum rounding precision for fildll
From: |
Alex Bennée |
Subject: |
Re: [RFC PATCH] target/i386: for maximum rounding precision for fildll |
Date: |
Mon, 07 Mar 2022 20:48:26 +0000 |
User-agent: |
mu4e 1.7.9; emacs 28.0.91 |
Richard Henderson <richard.henderson@linaro.org> writes:
> On 3/7/22 08:44, Alex Bennée wrote:
>> The instruction description says "It is loaded without rounding
>> errors." which implies we should have the widest rounding mode
>> possible.
>> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/888
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> ---
>> target/i386/tcg/fpu_helper.c | 4 ++++
>> 1 file changed, 4 insertions(+)
>> diff --git a/target/i386/tcg/fpu_helper.c
>> b/target/i386/tcg/fpu_helper.c
>> index cdd8e9f947..d986fd5792 100644
>> --- a/target/i386/tcg/fpu_helper.c
>> +++ b/target/i386/tcg/fpu_helper.c
>> @@ -250,11 +250,15 @@ void helper_fildl_ST0(CPUX86State *env, int32_t val)
>> void helper_fildll_ST0(CPUX86State *env, int64_t val)
>> {
>> int new_fpstt;
>> + FloatX80RoundPrec old =
>> get_floatx80_rounding_precision(&env->fp_status);
>> + set_floatx80_rounding_precision(floatx80_precision_x, &env->fp_status);
>> new_fpstt = (env->fpstt - 1) & 7;
>> env->fpregs[new_fpstt].d = int64_to_floatx80(val, &env->fp_status);
>> env->fpstt = new_fpstt;
>> env->fptags[new_fpstt] = 0; /* validate stack entry */
>> +
>> + set_floatx80_rounding_precision(old, &env->fp_status);
>> }
>
> Yep.
>
> Need a similar fix for fildl_ST0, for the case floatx80_precision_s is
> currently set (int32_t has more than the 23 bits of single-precision).
It can't hurt to convert with:
set_floatx80_rounding_precision(floatx80_precision_x, &env->fp_status);
in that case as well right?
--8<---------------cut here---------------start------------->8---
target/i386: for maximum rounding precision for fildll
The instruction description says "It is loaded without rounding
errors." which implies we should have the widest rounding mode
possible.
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/888
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
1 file changed, 13 insertions(+)
target/i386/tcg/fpu_helper.c | 13 +++++++++++++
modified target/i386/tcg/fpu_helper.c
@@ -237,24 +237,37 @@ void helper_fldl_ST0(CPUX86State *env, uint64_t val)
merge_exception_flags(env, old_flags);
}
+static FloatX80RoundPrec tmp_maximise_precision(float_status *st)
+{
+ FloatX80RoundPrec old = get_floatx80_rounding_precision(st);
+ set_floatx80_rounding_precision(floatx80_precision_x, st);
+ return old;
+}
+
void helper_fildl_ST0(CPUX86State *env, int32_t val)
{
int new_fpstt;
+ FloatX80RoundPrec old = tmp_maximise_precision(&env->fp_status);
new_fpstt = (env->fpstt - 1) & 7;
env->fpregs[new_fpstt].d = int32_to_floatx80(val, &env->fp_status);
env->fpstt = new_fpstt;
env->fptags[new_fpstt] = 0; /* validate stack entry */
+
+ set_floatx80_rounding_precision(old, &env->fp_status);
}
void helper_fildll_ST0(CPUX86State *env, int64_t val)
{
int new_fpstt;
+ FloatX80RoundPrec old = tmp_maximise_precision(&env->fp_status);
new_fpstt = (env->fpstt - 1) & 7;
env->fpregs[new_fpstt].d = int64_to_floatx80(val, &env->fp_status);
env->fpstt = new_fpstt;
env->fptags[new_fpstt] = 0; /* validate stack entry */
+
+ set_floatx80_rounding_precision(old, &env->fp_status);
}
--8<---------------cut here---------------end--------------->8---
>
>
> r~
--
Alex Bennée