17 lines
487 B
C
17 lines
487 B
C
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#include "libc/math/libm.h"
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/* k is such that k*ln2 has minimal relative error and x - kln2 > log(FLT_MIN) */
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static const int k = 235;
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static const float kln2 = 0x1.45c778p+7f;
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/* expf(x)/2 for x >= log(FLT_MAX), slightly better than 0.5f*expf(x/2)*expf(x/2) */
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float __expo2f(float x)
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{
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float scale;
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/* note that k is odd and scale*scale overflows */
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SET_FLOAT_WORD(scale, (uint32_t)(0x7f + k/2) << 23);
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/* exp(x - k ln2) * 2**(k-1) */
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return expf(x - kln2) * scale * scale;
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}
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