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target/i386: fix fscale handling of infinite exponents

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The fscale implementation passes infinite exponents through to generic
code that rounds the exponent to a 32-bit integer before using
floatx80_scalbn. In round-to-nearest mode, and ignoring exceptions,
this works in many cases. But it fails to handle the special cases of
scaling 0 by a +Inf exponent or an infinity by a -Inf exponent, which
should produce a NaN, and because it produces an inexact result for
finite nonzero numbers being scaled, the result is sometimes incorrect
in other rounding modes. Add appropriate handling of infinite
exponents to produce a NaN or an appropriately signed exact zero or
infinity as a result

Backports commit c1c5fb8f9067c830e36830c2b82c0ec146c03d7b from qemu
This commit is contained in:
Joseph Myers 2020-06-15 13:04:43 -04:00 committed by Lioncash
parent bbbf25fdd9
commit ad83656acc
1 changed files with 22 additions and 0 deletions
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22
qemu/target/i386/fpu_helper.c
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@ -943,6 +943,28 @@ void helper_fscale(CPUX86State *env)
float_raise(float_flag_invalid, &env->fp_status);
ST0 = floatx80_silence_nan(ST0, &env->fp_status);
}
} else if (floatx80_is_infinity(ST1) &&
!floatx80_invalid_encoding(ST0) &&
!floatx80_is_any_nan(ST0)) {
if (floatx80_is_neg(ST1)) {
if (floatx80_is_infinity(ST0)) {
float_raise(float_flag_invalid, &env->fp_status);
ST0 = floatx80_default_nan(&env->fp_status);
} else {
ST0 = (floatx80_is_neg(ST0) ?
floatx80_chs(floatx80_zero) :
floatx80_zero);
}
} else {
if (floatx80_is_zero(ST0)) {
float_raise(float_flag_invalid, &env->fp_status);
ST0 = floatx80_default_nan(&env->fp_status);
} else {
ST0 = (floatx80_is_neg(ST0) ?
floatx80_chs(floatx80_infinity) :
floatx80_infinity);
}
}
} else {
int n = floatx80_to_int32_round_to_zero(ST1, &env->fp_status);
ST0 = floatx80_scalbn(ST0, n, &env->fp_status);