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softfloat: fix floatx80 pseudo-denormal round to integer

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The softfloat function floatx80_round_to_int incorrectly handles the
case of a pseudo-denormal where only the high bit of the significand
is set, ignoring that bit (treating the number as an exact zero)
rather than treating the number as an alternative representation of
+/- 2^-16382 (which may round to +/- 1 depending on the rounding mode)
as hardware does.  Fix this check (simplifying the code in the
process).

Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2005042339420.22972@digraph.polyomino.org.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Joseph Myers 2020-05-04 23:40:20 +00:00 committed by Richard Henderson
parent be53fa785a
commit 9ecaf5ccec
2 changed files with 11 additions and 1 deletions
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2
fpu/softfloat.c
View file

@ -5741,7 +5741,7 @@ floatx80 floatx80_round_to_int(floatx80 a, float_status *status)
}
if ( aExp < 0x3FFF ) {
if ( ( aExp == 0 )
&& ( (uint64_t) ( extractFloatx80Frac( a )<<1 ) == 0 ) ) {
&& ( (uint64_t) ( extractFloatx80Frac( a ) ) == 0 ) ) {
return a;
}
status->float_exception_flags |= float_flag_inexact;

10
tests/tcg/i386/test-i386-pseudo-denormal.c
View file

@ -14,6 +14,7 @@ volatile long double ld_res;
int main(void)
{
short cw;
int ret = 0;
ld_res = ld_pseudo_m16382.ld + ld_pseudo_m16382.ld;
if (ld_res != 0x1p-16381L) {
@ -24,5 +25,14 @@ int main(void)
printf("FAIL: pseudo-denormal compare\n");
ret = 1;
}
/* Set round-upward. */
__asm__ volatile ("fnstcw %0" : "=m" (cw));
cw = (cw & ~0xc00) | 0x800;
__asm__ volatile ("fldcw %0" : : "m" (cw));
__asm__ ("frndint" : "=t" (ld_res) : "0" (ld_pseudo_m16382.ld));
if (ld_res != 1.0L) {
printf("FAIL: pseudo-denormal round-to-integer\n");
ret = 1;
}
return ret;
}