Re: [RFC PATCH] target/i386: for maximum rounding precision for fildll

["Alex Bennée" <alex.bennee@linaro.org>]

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