[Issue 13548] wrong FP comparison

Sep 27, 2014

Илья Ярошенко

Oct 02, 2014

hsteoh@quickfur.ath.cx

Feb 08, 2015

yebblies

Feb 14, 2015

Илья Ярошенко

[Issue 13548] wrong CTFE sort

Feb 14, 2015

Илья Ярошенко

[Issue 13548] wrong CTFE sqrt

Feb 14, 2015

Илья Ярошенко

https://issues.dlang.org/show_bug.cgi?id=13548 Илья Ярошенко <ilyayaroshenko@gmail.com> changed: What |Removed |Added ---------------------------------------------------------------------------- Keywords| |wrong-code --

https://issues.dlang.org/show_bug.cgi?id=13548 hsteoh@quickfur.ath.cx changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |hsteoh@quickfur.ath.cx --

https://issues.dlang.org/show_bug.cgi?id=13548 yebblies <yebblies@gmail.com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |yebblies@gmail.com --- Comment #1 from yebblies <yebblies@gmail.com> --- My guess is that this is a similar bug to issue 13474, where ad is truncated by saving it to the stack then reloading it, while cd is kept in a register after being returned with >double precision. --

https://issues.dlang.org/show_bug.cgi?id=13548 --- Comment #2 from Илья Ярошенко <ilyayaroshenko@gmail.com> --- Probably this is reduced example for FreeBSD_32 and Win32: ---- double two = 2.0; assert(sqrt(two) == sort(2.0)); ---- --

https://issues.dlang.org/show_bug.cgi?id=13548 Walter Bright <bugzilla@digitalmars.com> changed: What |Removed |Added ---------------------------------------------------------------------------- Keywords| |backend CC| |bugzilla@digitalmars.com --

August 31, 2020

[Issue 13548] wrong sqrt

Posted by Walter Bright

Permalink

Walter Bright

Permalink

https://issues.dlang.org/show_bug.cgi?id=13548

--- Comment #3 from Walter Bright <bugzilla@digitalmars.com> ---
Code:

  import core.stdc.math;
  void main() {
    double two = 2.0;
    assert(sqrt(two) == sqrt(2.0));
  }

Generates for main():

        enter   014h,0
        fld     qword ptr FLAT:CONST[00h]
        fstp    qword ptr -8[EBP]
        push    dword ptr -4[EBP]
        push    dword ptr -8[EBP]
        call    near ptr _sqrt
        add     ESP,8
        push    dword ptr FLAT:CONST[04h]
        push    dword ptr FLAT:CONST[00h]
        fstp    qword ptr -014h[EBP]
        call    near ptr _sqrt
        add     ESP,8
        fld     qword ptr -014h[EBP]
        fxch    ST(1)
        fucompp ST(1),ST
        fstsw   AX
        sahf
        jp      L40
        je      L4D
L40:    push    6
        mov     EAX,offset FLAT:___a7_74657374322e64
        push    EAX
        call    near ptr __d_assertp
L4D:    xor     EAX,EAX
        leave
        ret

Both calls to sqrt() produce the same result in ST0. But the first one stores and reloads it as a double, thus causing a round-to-double operation. The second does not, so they compare unequal. yebblies is right.

I'm not sure what the right fix would be. I could change sqrt() in DMC's runtime library to store/load the value it returns, but that wouldn't fix things for FreeBSD32.

On Win64 there isn't a problem because the sqrt() result is returned in XMM0
rather than ST0.

--

https://issues.dlang.org/show_bug.cgi?id=13548 --- Comment #4 from Walter Bright <bugzilla@digitalmars.com> --- I fixed DMC's sqrt to round to double. https://github.com/DigitalMars/dmc/commit/29d49d5527fcb3e92f0b6f8620eb1bd9e0de1a14 Not much I can do about FreeBSD32. --

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