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target/i386: fix fscale handling of rounding precision

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The fscale implementation uses floatx80_scalbn for the final scaling
operation. floatx80_scalbn ends up rounding the result using the
dynamic rounding precision configured for the FPU. But only a limited
set of x87 floating-point instructions are supposed to respect the
dynamic rounding precision, and fscale is not in that set. Fix the
implementation to save and restore the rounding precision around the
call to floatx80_scalbn.

Backports commit c535d68755576bfa33be7aef7bd294a601f776e0 from qemu
This commit is contained in:
Joseph Myers 2020-06-15 13:05:29 -04:00 committed by Lioncash
parent ad83656acc
commit 95368d250b
1 changed files with 3 additions and 0 deletions
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3
qemu/target/i386/fpu_helper.c
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@ -967,7 +967,10 @@ void helper_fscale(CPUX86State *env)
}
} else {
int n = floatx80_to_int32_round_to_zero(ST1, &env->fp_status);
signed char save = env->fp_status.floatx80_rounding_precision;
env->fp_status.floatx80_rounding_precision = 80;
ST0 = floatx80_scalbn(ST0, n, &env->fp_status);
env->fp_status.floatx80_rounding_precision = save;
}
}