https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113365
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
At -O0, GCC produces a lot of load/stores to the stack and subnormals always
have a penality in HW for x87.
Note double uses SSE while long double uses x87 so the effect there will show
up more.
Note if we use -O2 and change the variable to be a volatile variable (otherwise
the loop is just optimized away), the speed difference is gone because you no
longer have addition happening of a subnormal which is what is causing the slow
down.
Anyways as I mentioned this is not a GCC bug but rather a HW limitation.
There are many other targets where subnormals are slow even for double too.
