17 lines
495 B
C
17 lines
495 B
C
#include "libc/math/libm.h"
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/* k is such that k*ln2 has minimal relative error and x - kln2 > log(DBL_MIN) */
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static const int k = 2043;
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static const double kln2 = 0x1.62066151add8bp+10;
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/* exp(x)/2 for x >= log(DBL_MAX), slightly better than 0.5*exp(x/2)*exp(x/2) */
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double __expo2(double x)
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{
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double scale;
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/* note that k is odd and scale*scale overflows */
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INSERT_WORDS(scale, (uint32_t)(0x3ff + k/2) << 20, 0);
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/* exp(x - k ln2) * 2**(k-1) */
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return exp(x - kln2) * scale * scale;
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}
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