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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.

Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2005070045430.18350@digraph.polyomino.org.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Joseph Myers 2020-05-07 00:46:28 +00:00 committed by Paolo Bonzini
parent c1c5fb8f90
commit c535d68755
2 changed files with 16 additions and 0 deletions
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3
target/i386/fpu_helper.c
View File

@ -1001,7 +1001,10 @@ 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;
}
}

13
tests/tcg/i386/test-i386-fscale.c
View File

@ -8,6 +8,8 @@ union u {
long double ld;
};
volatile long double ld_third = 1.0L / 3.0L;
volatile long double ld_four_thirds = 4.0L / 3.0L;
volatile union u ld_invalid_1 = { .s = { 1, 1234 } };
volatile union u ld_invalid_2 = { .s = { 0, 1234 } };
volatile union u ld_invalid_3 = { .s = { 0, 0x7fff } };
@ -91,5 +93,16 @@ int main(void)
printf("FAIL: fscale finite down inf\n");
ret = 1;
}
/* Set round-to-nearest with single-precision rounding. */
cw = cw & ~0xf00;
__asm__ volatile ("fldcw %0" : : "m" (cw));
__asm__ volatile ("fscale" : "=t" (ld_res) :
"0" (ld_third), "u" (2.0L));
cw = cw | 0x300;
__asm__ volatile ("fldcw %0" : : "m" (cw));
if (ld_res != ld_four_thirds) {
printf("FAIL: fscale single-precision\n");
ret = 1;
}
return ret;
}