[Numpy-discussion] Problems testing the floating point flags

[Charles R Harris]

On Sun, Nov 14, 2010 at 1:09 PM, Mark Wiebe <mwwiebe at gmail.com> wrote:

>
>
> On Sun, Nov 14, 2010 at 7:07 AM, Charles R Harris <
> charlesr.harris at gmail.com> wrote:
>
>>
>> Another possible solution is like so:
>>
>> static __attribute__ ((noinline)) int
>> fpecheck(int *status)
>> {
>>     *status = PyUFunc_getfperr();
>>     return 0;
>> }
>>
>> static __attribute__ ((noinline)) int
>> fpeclear(int *status)
>> {
>>     PyUFunc_clearfperr();
>>     return 0;
>> }
>>
>> int myfunc(void)
>> {
>>     int status;
>>
>>    fpeclear(&status);
>>     do {
>>         stuff;
>>     } while (fpecheck(&status));
>>     return status;
>> }
>>
>>
> While this may work in this particular case because of compiler specifics,
> I don't think this creates a reliable ordering dependency because of the
> form of 'stuff'. It will be loop invariant, so the compiler may do as
> follows:
>
>     do {
>         result = arg1 / arg2;
>     } while (fpecheck(&status));
>
> Pulling out the loop-invariant statement:
>
>     result = arg1 / arg2;
>     do {
>     } while (fpecheck(&status));
>
> Since the loop doesn't use result, it may feel free to reorder:
>
>     do {
>     } while (fpecheck(&status));
>     result = arg1 / arg2;
>
> producing the bug again.
>
>
Good point. I was trying to keep the fpeclear in front of the code to be
tested.

Chuck
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