[PATCH] Convert 0.0/0.0 to NAN

Craig Howland howland@LGSInnovations.com
Thu Apr 20 00:25:00 GMT 2017

On 04/19/2017 08:01 PM, Richard Allen wrote:
> Whoops, that was the wrong patch. Attached is correct patch.
> On Wed, Apr 19, 2017 at 6:50 PM, Richard Allen <rsaxvc@rsaxvc.net> wrote:
>> With newlib's default compiler flags(-O2 and whatnot),
>> the GCC I'm using creates calls to __aeabi_(f|d)div.
>> This causes a little extra register copying
>> and an unnecessary branch.
>> Additionally, __aeabi_?div may throw exceptions,
>> which is probably not what we want a
>> simple assignment to do.
>> Compiler version tested:
>> arm-none-eabi-gcc (15:5.4.1+svn241155-1) 5.4.1 20160919
>> -Richard
- exc.retval = 0.0/0.0;
+ exc.retval = NAN;

Well, actually, an exception is exactly why it is coded that way; yes, this is 
what this simple assignment is intended to do.  The 0.0/0.0 construct is used so 
that at runtime the floating point flags gets set and yield the NAN result; a 
floating point exception is supposed to be raised.  (The compiler is not allowed 
to optimize it away because of the side effects--which is why you're seeing the 
divide even with -O2.  Although I would guess that in an implementation which 
does not have the side effects, the compiler would be able to substitute the NAN 
at compile time.)

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