Bin Xin wrote:
> On Mar 19, 9:02 am, Terje Mathisen <spamt...@[EMAIL PROTECTED]
> wrote:
>> Tim Roberts wrote:
>>> I admit that I lost track part way through, but it looks to me like
you
>>> aren't cleaning up thefloatingpointstack when you are done. If you
>>> leave stuff on the stack, sooner or later the stack will overflow
(there
>>> are only 8 entries, after all), which triggersfloatingpointexceptions.
>> The easy way to check this is to call your my_pow() function in a loop,
>> preferably while inside the debugger, and watch the x87 stack contents.
>>
>
> Thanks for replying.
>
> So I try to follow the stack state after each call to my_pow, it
> looks ok for a few calls (9), then I see a weird thing: when "fld1" is
> executed at some point, it loads a "nan" instead of "1" into st0:
As Terje says, check for FPU stack overflow.
If you are using normal calling conventions (as enforced by
compilers), your FPU stack should be empty upon entering any
function, and upon exit should be either empty (I) or contain
a single entry (II), in the case where the function returns
a floating point number:
void myfunc(...) (I)
double myfunc(...) (II)
To find out how many entries out off the available eight of the
FPU stack are used, inspect the FPU TAGS word with your debugger.
(NB: The TAGS word; NOT the FPU CONTROL OR STATUS word).
The TAGS word should read FFFF upon entry to the function (just
prior to the hardware call instruction) and upon exit should read
FFFF in case (I) and 3FFF in case (II), if I am not mistaken.
This is because the coding is from left to right and is 11 for an
empty register and 00 for a used register, so 3FFF means
0011 1111 1111 1111
which means that the first FPU register "st0" is in use and
st1...st7 are empty. Note that some debuggers display garbage even
in empty registers, so do not rely on the debugger FPU stack display.
This is as much as I know; I am not sure whether the used registers
should be contiguous, or what happens upon stack rotation, I haven't
investigated that.
Jen
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