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Annotation of coherent/b/lib/libm/i8087/log87.m, revision 1.1.1.1

1.1       root        1: //////////
                      2: / libm 8087
                      3: / log(x), log10(x), ylog2x
                      4: //////////
                      5: 
                      6: #include "larges.h"
                      7: #include "ifno8087.h"
                      8: 
                      9:        .globl  log10_
                     10:        .globl  log_
                     11:        .globl  ylog2x
                     12:        .globl  cfcc87
                     13:        .globl  tstcc
                     14:        .globl  edomain
                     15: 
                     16: //////////
                     17: / double
                     18: / log(x)
                     19: / double x;
                     20: /
                     21: / double
                     22: / log10(x)
                     23: / double x;
                     24: /
                     25: / ylog2x does the work for log functions and pow().
                     26: / Passed args x, y on the 8087 stack,
                     27: / it pops both and returns y * log base 2 of x on the 8087 stack.
                     28: /
                     29: / Useful identity:
                     30: /       log base n of x = log base n of 2 * log base 2 of x.
                     31: / Uses fyl2x, which requires x > 0.
                     32: //////////
                     33: 
                     34: x      =       RASIZE          / arg offset
                     35: 
                     36: log10_:
                     37:        ifno8087(_log10_)
                     38:        fldlg2                  / Load y = log base 10 of 2.
                     39:        jmp     1f
                     40: 
                     41: log_:
                     42:        ifno8087(_log_)
                     43:        fldln2                  / Load y = log base e of 2.
                     44: 
                     45: 1:     mov     bx, sp
                     46:        fdld    Pss x(bx)       / Load argument x.
                     47: 
                     48: ylog2x:                                / x, y
                     49:        Gcall   tstcc           / Check sign of x.
                     50:        ja      2f              / x > 0., ok.
                     51:        fstp    st              / x <= 0, pop y.
                     52:        Gjmp    edomain         / Issue EDOM error and return 0.
                     53: 
                     54: 2:
                     55: //////////
                     56: / The code below uses
                     57: /      fyl2x   if d <= 1 or d >= 2-sqrt(2)/2,
                     58: /      fyl2xp1 if 1 < d < 2-sqrt(2)/2.
                     59: / Since the precision fyl2xp1 purports to save is already lost,
                     60: / this appears to make no difference in the result.
                     61: / The code is commented out for now and should be removed eventually.
                     62: /      fdcom   cs:near1
                     63: /      Gcall   cfcc87          / Compare d to 2+sqrt(2)/2.
                     64: /      jae     4f              / d >= 2+sqrt(2)/2, use fyl2x.
                     65: /      fld1                    / Load 1.
                     66: /      fcom
                     67: /      Gcall   cfcc87          / Compare 1 to d.
                     68: /      jae     3f              / 1 >= d, use fyl2x.
                     69: /      fsub                    / d = d-1 and pop off the 1.
                     70: /      fyl2xp1                 / result = y * log base 2 of d+1.
                     71: /      Gret
                     72: /
                     73: /3:    fstp    st              / Pop off the 1.
                     74: /
                     75: /4:
                     76: //////////
                     77:        fyl2x                   / result = y * log base 2 of x.
                     78:        Gret
                     79: 
                     80: //////////
                     81: / Commented out as above...
                     82: /near1:        .byte   0x1A, 0x62, 0xC0, 0xCC, 0xB0, 0xAF, 0xF4, 0x3F  / 2 - sqrt(2)/2
                     83: //////////

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