RFR: JDK-8301202: Port fdlibm log to Java [v3]
Brian Burkhalter
bpb at openjdk.org
Sat Feb 11 00:21:29 UTC 2023
On Thu, 9 Feb 2023 06:46:17 GMT, Joe Darcy <darcy at openjdk.org> wrote:
>> 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.
>
> Joe Darcy has updated the pull request incrementally with two additional commits since the last revision:
>
> - Appease jcheck.
> - Add StrictMath.log vs Math.log differences.
Looks fine.
-------------
Marked as reviewed by bpb (Reviewer).
PR: https://git.openjdk.org/jdk/pull/12465
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