Szabolcs NagyandRich Felker
79ca86094d
The old code used the rounding idiom incorrectly: y = (double)(x + 0x1p52) - 0x1p52; the cast is useless if FLT_EVAL_METHOD==0 and causes a second rounding if FLT_EVAL_METHOD==2 which can give incorrect result in nearest rounding mode, so the correct idiom is to add/sub a power-of-2 according to the characteristics of double_t. This did not cause actual bug because only i386 is affected where rint is implemented in asm. Other rounding functions use a similar idiom, but they give correct results because they only rely on getting a neighboring integer result and the rounding direction is fixed up separately independently of the current rounding mode. However they should be fixed to use the idiom correctly too.
29 lines
489 B
C
29 lines
489 B
C
#include <float.h>
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#include <math.h>
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#include <stdint.h>
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#if FLT_EVAL_METHOD==0 || FLT_EVAL_METHOD==1
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#define EPS DBL_EPSILON
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#elif FLT_EVAL_METHOD==2
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#define EPS LDBL_EPSILON
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#endif
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static const double_t toint = 1/EPS;
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double rint(double x)
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{
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union {double f; uint64_t i;} u = {x};
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int e = u.i>>52 & 0x7ff;
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int s = u.i>>63;
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double_t y;
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if (e >= 0x3ff+52)
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return x;
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if (s)
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y = x - toint + toint;
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else
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y = x + toint - toint;
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if (y == 0)
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return s ? -0.0 : 0;
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return y;
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}
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