RFR: JDK-8301202: Port fdlibm log to Java

Joe Darcy darcy at openjdk.org
Wed Feb 8 00:38:46 UTC 2023


Next up on the FDLIBM porting countdown, the log method.

Original C vs transliteration port:


$ diff -w Log.c Log.translit.java 
1c1
< /* __ieee754_log(x)
---
>     /**
51,58c51,52
< 
< #include "fdlibm.h"
< 
< #ifdef __STDC__
< static const double
< #else
< static double
< #endif
---
>     static class Log {
>         private static final  double
70c64
< static double zero   =  0.0;
---
>         private static double zero   =  0.0;
72,78c66
< #ifdef __STDC__
<         double __ieee754_log(double x)
< #else
<         double __ieee754_log(x)
<         double x;
< #endif
< {
---
>         static double compute(double x) {
81c69
<         unsigned lx;
---
>             /*unsigned*/ int lx;
98c86,87
<         __HI(x) = hx|(i^0x3ff00000);    /* normalize x or x/2 */
---
>             // __HI(x) = hx|(i^0x3ff00000);    /* normalize x or x/2 */
>             x =__HI(x, hx|(i^0x3ff00000));    /* normalize x or x/2 */
128a118
>     }


Transliteration port vs more idiomatic port:


$ diff -w  Log.translit.java Log.fdlibm.java 
2c2
<      * Return the logarithm of x
---
>      * Return the (natural) logarithm of x
53,62c53,54
<             ln2_hi  =  6.93147180369123816490e-01,  /* 3fe62e42 fee00000 */
<             ln2_lo  =  1.90821492927058770002e-10,  /* 3dea39ef 35793c76 */
<             two54   =  1.80143985094819840000e+16,  /* 43500000 00000000 */
<             Lg1 = 6.666666666666735130e-01,  /* 3FE55555 55555593 */
<             Lg2 = 3.999999999940941908e-01,  /* 3FD99999 9997FA04 */
<             Lg3 = 2.857142874366239149e-01,  /* 3FD24924 94229359 */
<             Lg4 = 2.222219843214978396e-01,  /* 3FCC71C5 1D8E78AF */
<             Lg5 = 1.818357216161805012e-01,  /* 3FC74664 96CB03DE */
<             Lg6 = 1.531383769920937332e-01,  /* 3FC39A09 D078C69F */
<             Lg7 = 1.479819860511658591e-01;  /* 3FC2F112 DF3E5244 */
---
>             ln2_hi = 0x1.62e42feep-1,       // 6.93147180369123816490e-01
>             ln2_lo = 0x1.a39ef35793c76p-33, // 1.90821492927058770002e-10
64c56,64
<         private static double zero   =  0.0;
---
>             Lg1    = 0x1.5555555555593p-1,  // 6.666666666666735130e-01
>             Lg2    = 0x1.999999997fa04p-2,  // 3.999999999940941908e-01
>             Lg3    = 0x1.2492494229359p-2,  // 2.857142874366239149e-01
>             Lg4    = 0x1.c71c51d8e78afp-3,  // 2.222219843214978396e-01
>             Lg5    = 0x1.7466496cb03dep-3,  // 1.818357216161805012e-01
>             Lg6    = 0x1.39a09d078c69fp-3,  // 1.531383769920937332e-01
>             Lg7    = 0x1.2f112df3e5244p-3;  // 1.479819860511658591e-01
> 
>         private static final double zero = 0.0;
71,72c71,72
<             hx = __HI(x);           /* high word of x */
<             lx = __LO(x);           /* low  word of x */
---
>             hx = __HI(x);           // high word of x
>             lx = __LO(x);           // low  word of x
75,80c75,87
<             if (hx < 0x00100000) {                  /* x < 2**-1022  */
<                 if (((hx&0x7fffffff)|lx)==0)
<                     return -two54/zero;             /* log(+-0)=-inf */
<                 if (hx<0) return (x-x)/zero;        /* log(-#) = NaN */
<                 k -= 54; x *= two54; /* subnormal number, scale up x */
<                 hx = __HI(x);               /* high word of x */
---
>             if (hx < 0x0010_0000) {                  // x < 2**-1022
>                 if (((hx & 0x7fff_ffff) | lx) ==0) { // log(+-0) = -inf
>                     return -TWO54/zero;
>                 }
>                 if (hx < 0) {                        // log(-#) = NaN
>                     return (x - x)/zero;
>                 }
>                 k -= 54;
>                 x *= TWO54;    // subnormal number, scale up x
>                 hx = __HI(x);  // high word of x
>             }
>             if (hx >= 0x7ff0_0000) {
>                 return x+x;
82d88
<             if (hx >= 0x7ff00000) return x+x;
84,87c90,92
<             hx &= 0x000fffff;
<             i = (hx+0x95f64)&0x100000;
<             // __HI(x) = hx|(i^0x3ff00000);    /* normalize x or x/2 */
<             x =__HI(x, hx|(i^0x3ff00000));    /* normalize x or x/2 */
---
>             hx &= 0x000f_ffff;
>             i = (hx + 0x9_5f64) & 0x10_0000;
>             x =__HI(x, hx|(i ^ 0x3ff0_0000));  // normalize x or x/2
90c95
<             if((0x000fffff&(2+hx))<3) {     /* |f| < 2**-20 */
---
>             if ((0x000f_ffff & (2 + hx)) < 3) {// |f| < 2**-20
92,93c97,102
<                     if (k==0) return zero;
<                     else {dk=(double)k; return dk*ln2_hi+dk*ln2_lo;}
---
>                     if (k == 0) {
>                         return zero;
>                     } else {
>                         dk = (double)k;
>                         return dk*ln2_hi + dk*ln2_lo;
>                     }
96,97c105,110
<                 if(k==0) return f-R; else {dk=(double)k;
<                     return dk*ln2_hi-((R-dk*ln2_lo)-f);}
---
>                 if (k == 0) {
>                     return f - R;
>                 } else {
>                     dk = (double)k;
>                     return dk*ln2_hi - ((R - dk*ln2_lo) - f);
>                 }
102c115
<             i = hx-0x6147a;
---
>             i = hx - 0x6_147a;
111c124,126
<                 if(k==0) return f-(hfsq-s*(hfsq+R)); else
---
>                 if (k == 0) {
>                     return f-(hfsq-s*(hfsq+R));
>                 } else {
112a128,131
>                 }
>             } else {
>                 if (k == 0) {
>                     return f - s*(f - R);
114d132
<                 if(k==0) return f-s*(f-R); else
115a134
>                 }


The transliteration port passes the "exhausting" test of all float arguments when run against a JDK 20 build. The idiomatic port matches the transliteration port on the same test.

-------------

Commit messages:
 - Appease jcheck.
 - Preliminary version of regression test.
 - Switch to hex literals.
 - JDK-8301202: Port fdlibm log to Java

Changes: https://git.openjdk.org/jdk/pull/12465/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12465&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301202
  Stats: 392 lines in 3 files changed: 391 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12465.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12465/head:pull/12465

PR: https://git.openjdk.org/jdk/pull/12465


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