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[Bug math/4997] lround(nexafter(0.5,-1)) has incorrect value


------- Additional Comments From rsa at us dot ibm dot com  2007-11-08 18:53 -------
The following asm stub resolves the issue by returning 'zero' if 0.5 > x > -0.5.  

        fsub    fp10,fp0,fp0    /* Compute 0.0.  */
        fabs    fp2, fp1        /* Get the absolute value of x.  */
        fcmpu   cr3, fp1, fp10  /* orig x is negative? x < 0.0  */
        fcmpu   cr6, fp2, fp0   /* if |x| < 0.5  */
        ble-    cr6,.Lretzero
        fadd    fp3,fp2,fp0     /* |x|+=0.5 bias to prepare to round.  */
        bge     cr3,.Lconvert   /* orig x was positive so don't negate ans.  */
        fnabs   fp2,fp3         /* -(|x|+=0.5)  */
.Lconvert:
        fctiwz  fp4,fp2         /* Convert to Integer DW lround toward 0.  */
        stfd    fp4,8(r1)
        nop
        nop
        nop
        lwz     r3,12(r1)       /* Load return as integer.  */
.Lout:
        addi    r1,r1,16
        blr
.Lretzero:              /* 0.5 > x > -0.5  */
        li      r3,0            /* return 0.  */
        b       .Lout

I need to identify all the places where the erroneous rounding is happening,
apply this fix and also apply a check where x > 2^52 for llround.  This may also
be an issue for round().  Patch forthcoming.


-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=4997

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